Glossary of genetics and evolutionary biology

This glossary of genetics and evolutionary biology is a list of definitions of terms and concepts used in the study of genetics and evolutionary biology, as well as sub-disciplines and related fields, with an emphasis on classical genetics, quantitative genetics, population biology, phylogenetics, speciation, and systematics. Overlapping and related terms can be found in Glossary of cellular and molecular biology, Glossary of ecology, and Glossary of biology.

A
adaptation: The dynamic process by which biological organisms develop characteristics that allow them to survive and reproduce within their environments. The state or condition reached by a during that process. Any character or with a functional role in an individual organism and which has evolved and is maintained through.

adaptationism:

The view that many or most physiological and behavioral traits of organisms are that have evolved for specific functions or for specific reasons (as opposed to being  of the evolution of other traits, consequences of, or the result of random variation).

adaptive radiation: The simultaneous or near-simultaneous of multiple members of a single  into a variety of different forms with different, especially a diversification in the use of resources or habitats.

agamospecies: A species that does not reproduce but rather by cloning. Agamospecies are sometimes represented by that contain some diploid individuals and other apomictic forms—in particular, plant species that can reproduce via agamospermy.

allele: One of multiple alternative versions of an individual, each of which is a viable sequence occupying a given position, or, on a. For example, in humans, one allele of the eye-color gene produces blue eyes and another allele of the eye-color gene produces brown eyes.

allele frequency: The relative frequency with which a particular of a given (as opposed to other alleles of the same gene) occurs at a particular  in the members of a population; more specifically, it is the proportion of all chromosomes within a population that carry a particular allele, expressed as a fraction or percentage. Allele frequency is distinct from, although they are related.

allochronic isolation: The isolation of two populations of a species due to a change in breeding periods. This isolation acts as a precursor to allochronic speciation, a type of which results when two populations of a species become isolated due to differences in reproductive timing. An example is the periodical 13- and 17-year Magicicada species.

allo-parapatric speciation: A mode of where divergence occurs in allopatry and is completed upon secondary contact of the populations--effectively a form of reinforcement.

allometry: The comparative study of the relationship between the size of an organism's body (or of a specific organ, e.g. the brain) and various other biological characteristics, such as body shape, anatomy, physiology, or behavior.

allopatric speciation:

A mode of where the evolution of is caused by the geographic separation of two or more  of a single species.

allopatric taxa: Specific species that are distributed.

allopatry: The phenomenon by which two or more populations of a single species exist in geographic isolation from one another.

allopolyploid: A cell or organism in which the several sets of chromosomes originate from more than one, as in an intraspecific.

allo-sympatric speciation: A mode of where divergence occurs in and is completed upon  of the populations–effectively a form of.

altruism:

anagenesis: Evolutionary change that occurs within a species lineage as opposed to lineage splitting.

analogous structures: A set of structures in different organisms which have similar form or function but were not present in the organisms'. The cladistic term for the same phenomenon is homoplasy.

ancestor:

ancestral trait:

For a given, any trait or feature (e.g. a specific ) that appears in the clade's ; the same trait may also appear in some or all of the lineal descendants included within the clade, indicating that it has undergone little or no significant change during the clade's evolutionary history and thus retained its "primitive" condition. Some but not all subgroups within the clade may contain, in which the ancestral trait has changed significantly over evolutionary time such that the original ancestral condition no longer exists. Both terms are relative: an ancestral trait for one clade may be a derived trait for a different clade. The term "ancestral trait" is often used interchangeably with the more technical term.

anticipation: A phenomenon by which the symptoms of a become apparent (and often more severe) at an earlier age in affected individuals with each generation that inherits the disorder.

apomorphy: A state; i.e. the state or condition of a particular trait or feature (e.g. a specific ) that is distinct from and derivative of an by virtue of its modification over time in one or more lineal descendants of a given. Apomorphies are often viewed as evolutionary "innovations" which set the in which they appear apart from the clade's, as well as from other clades;  apomorphies are used to construct and define clades. The term is relative; a trait considered an apomorphy in one clade may not be considered an apomorphy in a different clade. Contrast.

aptation: Any character or that is currently subject to, whether its origin can be ascribed to selective processes or to processes other than selection or selection for a function that is different from the current function.

area cladogram:

asexual reproduction:

associative overdominance: The phenomenon by which the of a neutral locus to a selectively maintained causes the  of the neutral locus to increase.

assortative mating:

A in which individuals with similar mate with each other more frequently than would be expected in a completely random mating system. Assortative mating usually has the effect of increasing genetic relatedness between members of the mating population. Contrast.

atavism: A modification of a biological structure whereby an suddenly reappears after having been lost through evolutionary change in previous generations. Atavisms can occur in a number of different ways, including by the re-expression of latent genes for ancestral phenotypes as a result of mutation, or by the shortening or prolongation of the time allocated for the of a particular trait during development.

autapomorphy:

autoallopolyploid:

autopolyploid:

autozygote: A cell or organism that is for a at which the two homologous  are, both having been derived from a single gene in a. Contrast.

B
backcrossing:

The breeding of a organism with one of its parents or an individual genetically similar to one of its parents, often intentionally as a type of, with the aim of producing offspring with a genetic identity which is closer to that of the parent. The reproductive event and the resulting progeny are both referred to as a backcross, often abbreviated in genetics shorthand with the symbol BC.

Bateson–Dobzhansky–Muller model:

An evolutionary model of the genetic incompatibility that occurs as a result of negative interactions between two or more or  with different evolutionary histories, which may meet when distinct populations. The incompatible genes or alleles themselves, referred to as Dobzhansky–Muller incompatibilities, may be the result of or neutral mutations, or they may be specific  driven by. By preventing populations from successfully interbreeding, these incompatibilities can reinforce and thereby increase the chance of.

behavioral isolation:

biogeography: The scientific study of the spatial distributions of biological organisms, populations, and species. It includes the study of both extinct and extant organisms.

biological constraints:

biological species concept:

bottleneck: See.

C
canalisation: The ability of a population to consistently produce the same regardless of the variability of its environment or the within its. The concept is most often used in to interpret the observation that developmental pathways are frequently shaped by  such that developing cell lineages are "guided" or "canalized" towards a single, definite fate, becoming progressively more resistant to any minor perturbations that may redirect development of the cells away from their initial course.

carrier: An individual who has a  for a genetic trait or mutation but in whom the trait is not usually expressed or observable in the. Carriers are usually for the recessive allele and therefore still able to pass the allele onto their offspring, where the associated phenotype may reappear if the offspring inherits another copy of the allele. The term is commonly used in in the context of a disease-causing recessive allele.

centrifugal speciation: A variation of in which speciation occurs by geographic isolation, but evolves in the larger population instead of the peripherally isolated population.

character displacement: The phenomenon by which differences between similar species that occupy similar and have partially overlapping geographic distributions are accentuated in regions where the species co-occur but are minimized or lost where the species' distributions do not overlap. This occurs because competition between the similar species for one or more limited resources drives evolutionary change that differentiates the species in the common geographic areas such that they no longer occupy the same niche, thereby allowing them to coexist and avoiding.

chimerism: The presence of two or more populations of cells with distinct within the body of an individual organism, known as a chimera, which has developed from the fusion of cells originating from separate ; each population of cells retains its own genome, such that the organism as a whole is a mixture of genetically non-identical tissues. Genetic chimerism may be inherited (e.g. by the fusion of multiple embryos during pregnancy) or acquired after birth (e.g. by allogeneic transplantation of cells, tissues, or organs from a genetically non-identical donor); in plants, it can result from grafting or errors in cell division. It is similar to but distinct from.

chromosomal speciation:

chronospecies:

clade:

A grouping of organisms that consists of a single and all of its, and which by definition is. The common ancestor may be an individual organism, a, a , or any other ; any and all members of a clade may be or. Clades can be visualized with and are the basis of.

cladistics: An approach to biological classification in which organisms are grouped in defined by ; hypothesized relationships between organisms are typically based on which can be traced to the  and are not present in more distant ancestors or unrelated groups.

cladogenesis: The splitting of a single species within a into multiple lineages.

cladogram:

classical genetics: The branch of based solely on observation of the visible results of reproductive acts, as opposed to that made possible by the modern techniques and methodologies of molecular biology. Contrast.

cline: A measurable spatial gradient in a single biological character or trait of a species or population across its geographic range. The nature of a cline may be genotypic (e.g. variation in ) or phenotypic (e.g. variation in body size or pigmentation), and may show smooth, continuous gradation or abrupt changes between different geographic regions.

cloning: The process of producing, either naturally or artificially, individual organisms or cells which are genetically identical to each other. Clones are the result of all forms of, and cells that undergo produce daughter cells that are clones of the parent cell and of each other. Cloning may also refer to biotechnology methods which artificially create copies of organisms or cells, or, in molecular cloning, copies of DNA fragments or other molecules.

cluster analysis:

clustering:

co-operation:

coadaptation: The mutual of organisms belonging to different populations or species, of different parts of the same organism, or of at different  in the same, especially implying that adaptation in both entities is driven by the same evolutionary force.

codominance:

coevolution: The process by which two or more distinct,, or other groups of organisms, or two or more distinct traits within a species, reciprocally affect each other's through. Each party in a coevolutionary relationship exerts upon the other, leading to the evolution of separate traits in each party.

cohesion species concept:

colonization: The spread of a to a new geographic area.

common ancestor: An organism or (e.g. a ) which is hypothesized to be the lineal progenitor of two or more organisms or taxa which exist at a later point in evolutionary time. The concept of common descent is fundamental to the study of,, and ; for instance, all , by definition, are rooted in a common ancestor. See also.

competitive gametic isolation:

complex trait: See.

congruent clines:

consanguineous: (of two or more individuals) Closely genetically related; sharing a recent (usually no more than three or four distant). The effect of consanguineous mating, also known as, is to increase the probability that the progeny will be at any given pair of genetic.

conservation genetics: An interdisciplinary branch of which applies genetic methods and concepts in an effort to understand the dynamics of genes in populations, with a principal aim of avoiding and preserving and restoring.

convergent evolution:

The independent of similar traits or in two or more different taxa from different periods or epochs in time, creating  structures that have similar form or function but were not present in the  of those taxa; e.g. structures enabling flight evolved independently in at least four distinct lineages: insects, birds, pterosaurs, and bats. In, the same phenomenon is termed. Contrast.

copulatory behavioral isolation:

coupling:

court jester hypothesis:

cospeciation: A type of in which more than two species speciate concurrently due to their ecological associations (e.g. host-parasite interactions).

crossbreeding:

The breeding of parents belonging to two different, varieties, or populations, often intentionally as a type of, with the aim of producing offspring which share traits of both parent lineages or which show. In animal breeding, the progeny of a cross between breeds of the same species is called a crossbreed, whereas the progeny of a cross between different species is called a.

crown group:

cryptic species:

cytoplasmic isolation:

D
Darwinism:

The understanding of biological as developed by the English naturalist Charles Darwin and others, which states that all biological organisms arise and develop through the of small, inherited variations that increase the individual's ability to compete, survive, and reproduce. Colloquially, the term is sometimes used to refer more broadly to as a whole, though in scientific circles distinctions are usually made between Darwin's ideas and later additions to evolutionary biology.

de novo mutation: A spontaneous in the of an individual organism that is new to that organism's, having first appeared in a  of one of the organism's parents or in the fertilized egg that develops into the organism; i.e. a mutation that was not present in either parent's genome.

de-extinction:

derived trait:

For a given, any trait or feature (e.g. a specific ) that is present within one or more subgroups of the clade but not in the clade's. Derived traits show significant differences from the original "primitive" condition of an found in the common ancestor, implying that the trait has undergone extensive during the clade's evolutionary history to reach its derivative condition. Both terms are relative: a derived trait for one clade may be an ancestral trait for a different clade. The term "derived trait" is often used interchangeably with the more technical term.

descendant:

developmental biology:

diploid:

(of a cell or organism) Having two copies of each. Contrast ' and '.

directional selection:

A mode of in which an extreme or  is favored over other phenotypes, causing  to shift over time in the direction of that trait or phenotype. This shift can occur whether or not the alleles governing the extreme phenotype are dominant or recessive.

directional speciation:

disassortative mating:

A in which individuals with dissimilar mate with each other more frequently than would be expected in a completely random mating system. Disassortative mating usually has the effect of decreasing genetic relatedness between members of the mating population. Contrast.

dispersal:

disruptive selection:

A mode of in which the extreme values of a or  within a breeding population are favored over intermediate values, causing  to shift over time away from the intermediate. This causes the variance in the trait to increase and results in the population dividing into two distinct groups, each with trait values at one end of the trait's distribution curve.

divergence-with-gene-flow:

divergent evolution:

The process by which any or distinction emerges between two different  or evolutionary. Divergence may occur by any of a variety of mechanisms but is often especially noticeable after the two lineages have been for many.

diversification:

Dobzhansky–Muller model: See.

dominance: A relationship between the of a in which one allele produces an effect on  that overpowers or "masks" the contribution of another allele at the same ; the first allele and its associated phenotypic  are said to be dominant, and the second allele and its associated trait are said to be. Often, the dominant allele codes for a functional protein while its recessive counterpart does not. Dominance is not an inherent property of any allele or phenotype, but simply describes its relationship to one or more other alleles or phenotypes; it is possible for one allele to be simultaneously dominant over a second allele, recessive to a third, and to a fourth. In genetics shorthand, dominant alleles are often represented by a single uppercase letter (e.g. "A", in contrast to the recessive "a").

dosage compensation: Any mechanism by which organisms neutralize the large difference in caused by the presence of differing numbers of in the different sexes, thereby equalizing the  of sex-linked genes so that the members of each sex receive the same or similar amounts of the  of such genes. An example is in female mammals.

E
ecogeographic isolation:

ecological allopatry:

ecological character displacement:

ecological genetics: The study of as it pertains to the and  of natural populations of living organisms.

ecological isolation:

ecological niche: See.

ecological speciation: A type of in which reproductive isolation is caused by the interaction of individuals of a species with their environment.

ecological species concept:

emergenesis: The quality of genetic that results from a specific configuration of interacting, rather than simply their combination.

endemism: The ecological state of a species being unique to a single geographic location, such as an island, nation, country, or any other clearly defined area, or to a single habitat type.

environmental gradient:

epigenetics:

epistasis: The collective action of multiple genes interacting during. A form of gene action, epistasis can be either additive or multiplicative in its effects on specific.

error catastrophe: The of a of organisms (insofar as the population can be defined by one or more identifiable characteristics) as a result of the excessive accumulation of genetic mutations, such that the population loses self-identity because all of its mutated descendants lack the identifiable characteristics.

ethological isolation:

ethological pollinator isolation:

euploidy: The condition of a cell or organism having an abnormal number of complete sets of, possibly excluding the. Euploidy differs from, in which a cell or organism has an abnormal number of one or more specific individual chromosomes.

evolution: The phenomenon by which the characteristics of biological change over successive. Evolution occurs when processes such as and  act on the  in characteristics that exists between members of a population, resulting in certain characteristics becoming more or less common within the population.

evolutionary arms race: The positive feedback mechanism operating between competing sets of, traits, species, or other taxa which evolve specific and counter-adaptations due to each other's presence, which may be seen as analogous with an "arms race".

evolutionary biology: The discipline of biology that studies the of biological organisms and the processes by which it operates, including,, , and. A core element of the, evolutionary biology integrates concepts from , , , , , and numerous other fields.

evolutionary landscape:

evolutionary species concept:

exaptation:

expressivity: For a given associated with a variable non-binary, the proportion of individuals with that genotype who show or express the phenotype to a specified extent, usually given as a percentage. Because of the many complex interactions that govern, the same may produce a wide variety of possible phenotypes of differing qualities or degrees in different individuals; in such cases, both the phenotype and genotype may be said to show variable expressivity. Expressivity attempts to quantify the range of possible levels of phenotypic variation in a population of individuals expressing the phenotype of interest. Compare.

extant: Currently living or existing; still in existence and not. The term is generally used to refer to the present-day state of existence of a particular (such as a family,,, etc.).

extended evolutionary synthesis:

extinction:

extrinsic hybrid inviability:

extrinsic postzygotic isolation:

F
fitness: The reproductive success, or propensity to produce offspring, during the lifetime of an individual

fixation: The process by which a single for a particular with multiple alleles increases in  in a given population such that it becomes permanently established as the only allele at that  within the population's. How long fixation takes depends on and  in allele frequencies.

floral isolation:

flowering asynchrony:

founder effect: The loss of that occurs when a new, physically isolated is established by a very small number of individuals who have migrated from a larger population and are not fully representative of the larger population's genetic diversity. As a result, the new population is often distinctively different, both and, from the parent population. Besides migration, can also result in a type of founder effect; extreme founder effects can lead to.

founder event:

founder-flush-crash:

founder takes all: A hypothesis that describes the evolutionary advantages of the first-arriving lineages in a new ecosystem. An example could be when a species becomes on an island, as in.

fugitive species: A that only temporarily occupies environments or habitats (either because its members frequently migrate or because its environments frequently change) and so does not persist for many at any one site.

G
gametic isolation:

gene: Any segment or set of segments of a nucleic acid molecule that contains the information necessary to produce a functional RNA transcript in a controlled manner. Genes are often considered the fundamental of and are typically encoded in DNA. A particular gene can have multiple different versions, or, and a single gene may influence many different.

gene dosage: The number of copies of a particular present in a. Gene dosage directly influences the amount of a cell is able to express, though a variety of controls have evolved which tightly. Changes in gene dosage caused by mutations include.

gene drive:

gene duplication:

A type of defined as any of a region of  that contains a. Compare.

gene expression: The set of processes by which the information encoded in a is used in the synthesis of a, such as a protein or non-coding RNA, or otherwise made available to influence one or more. Expression at the molecular level canonically consists of transcription and translation, though the information contained within a DNA sequence need not necessarily be transcribed and translated to exert an influence on molecular events, however; broader definitions encompass a huge variety of other ways in which genetic information can be expressed.

gene flow: The transfer of from one to another, by any available means, e.g. by,, or retroviral.

gene pool: The sum of all of the various alleles shared by the members of a single.

gene product: Any of the biochemical material resulting from the of a, most commonly interpreted as the functional mRNA transcript produced by transcription of the gene or the fully constructed protein produced by translation of the transcript, though non-coding RNA molecules such as transfer RNAs may also be considered gene products.

genealogical species concept:

generation: In any given organism, a single reproductive cycle, or the phase between two consecutive reproductive events, i.e. between an individual organism's reproduction and that of the of that reproduction; or the actual or average length of time required to complete a single reproductive cycle, either for a particular or for a population or species as a whole. In a given population, those individuals (often but not necessarily living contemporaneously) who are equally removed from a given by virtue of the same number of reproductive events having occurred between them and the ancestor.

genetic association: The co-occurrence within a population of one or more or with a particular  more often than might be expected by chance alone; such statistical correlation may be used to infer that the alleles or genotypes are responsible for producing the given phenotype.

genetic bottleneck: See.

genetic counseling: The process of advising individuals or families who are affected by or at risk of developing in order to help them understand and adapt to the physiological, psychological, and familial implications of genetic contributions to disease. Genetic counseling integrates,, and.

genetic disorder: Any illness, disease, or other health problem directly caused by one or more abnormalities in an organism's which are congenital (present at birth) and not acquired later in life. Causes may include a to one or more, or a  such as an  of a particular chromosome. The mutation responsible during embryonic development or may be  from one or both parents, in which case the genetic disorder is also classified as a. Though the abnormality itself is present before birth, the actual disease it causes may not develop until much later in life; some genetic disorders do not necessarily guarantee eventual disease but simply of the organism developing it.

genetic distance: A measure of the genetic divergence between species, populations within a species, or individuals, used especially in to express either the time elapsed since the existence of a or the degree of differentiation in the  comprising the  of each population or individual.

genetic diversity:

The total number of genetic or characteristics in the genetic make-up of a population, species, or other group of organisms. It is often used as a measure of the adaptability of a group to changing environments. Genetic diversity is similar to, though distinct from,.

genetic drift:

A change in the with which an existing allele occurs in a population due to random variation in the distribution of alleles from one to the next. It is often interpreted as the role that random chance plays in determining whether a given allele becomes more or less common with each generation, irrespective of the influence of. Genetic drift may cause certain alleles, even otherwise advantageous ones, to disappear completely from the, thereby reducing, or it may cause initially rare alleles, even neutral or deleterious ones, to become much more frequent or even.

genetic epidemiology: The study of the role played by genetic factors in determining health and disease, in particular through the interaction of genetic factors with environmental factors, and typically as observed in genetically related individuals, often families or lineages but also populations and subpopulations.

genetic erosion:

genetic genealogy: The use of genealogical DNA testing in combination with traditional methods to infer the level and type of genetic relationships between individuals, to find ancestors, and to construct family trees, genograms, or other genealogical charts.

genetic hitchhiking:

A type of selection by which the positive selection of an undergoing a  causes alleles for different genes at nearby  to change  as well, allowing them to "hitchhike" to  along with the positively selected allele. If selection at the first locus is strong enough, neutral or even slightly deleterious alleles within the same linkage group may undergo the same positive selection because the physical distance between the nearby loci is small enough that a event is unlikely to occur between them. Genetic hitchhiking is often considered the opposite of.

genetic load: Any reduction in the mean of a owing to the existence of one or more  with lower fitness than that of the most fit genotype.

genetic testing:

A broad class of procedures used to identify features of an individual's particular chromosomes, genes, or proteins in order to determine parentage or, diagnose vulnerabilities to heritable diseases, or detect alleles associated with increased risks of developing. Genetic testing is widely used in human medicine, agriculture, and biological research.

genetic variation: The genetic differences both within and between,, or other groups of organisms. It is often visualized as the variety of different in the of different populations.

genetic variability:

The formation or the presence of individuals differing in within a population or other group of organisms, as opposed to individuals with environmentally induced differences, which cause only temporary, non-heritable changes in. Barring other limitations, a population with high genetic variability has a greater potential for successful adaptation to changing environmental conditions than a population with low genetic variability. Genetic variability is similar to, though distinct from,.

genetic variation:

The genetic differences both within and between populations, species, or other groups of organisms. It is often visualized as the variety of different in the of different populations.

genetics: The field of biology that studies,, and in living organisms.

genic balance: A mechanism of sex determination that depends upon the ratio of the number of (X) to the number of sets of (A). Males develop when the X/A ratio is 0.5 or less, females when it is 1.0 or more, and an intersex develops when it is between 0.5 and 1.0.

genic selection: A type of that occurs at the level of individual or, in which the  of an allele within a breeding  is determined by its  averaged over the variety of  in which it occurs; the differential propagation of different alleles within a population as a consequence of properties borne by the alleles themselves, rather than by the genotypes in which they are found.

genic speciation:

genome: The entire complement of genetic material contained within the of an organism, organelle, or virus. The term is also used to refer to the collective set of genetic shared by every member of a population or species, regardless of the different  that may be present at these loci in different individuals.

genomics: An interdisciplinary field that studies the structure, function, evolution, mapping, and editing of entire, as opposed to individual.

genotype: The entire complement of present in a particular individual's, which gives rise to the individual's.

genotype frequency: The frequency or proportion of a population having a given. Compare.

genotypic cluster species:

geographic speciation:

germ cell: Any cell that gives rise to the of an organism that reproduces. Germ cells are the vessels for the genetic material which will ultimately be passed on to the organism's descendants and are usually distinguished from, which are entirely separate from the.

germ line: In multicellular organisms, the population of cells which are capable of passing on their genetic material to the organism's progeny and are therefore (at least theoretically) distinct from, which cannot pass on their genetic material except to their own immediate daughter cells. Cells of the germ line are called. The of germ cells, spanning many generations, that contains the genetic material which has been passed on to an individual from its ancestors.

grade:

gradualism: Continuous evolutionary change within a species lineage. See also.

green-beard effect:

H
habitat isolation:

Haldane's rule: A rule formulated by J.B.S. Haldane which states that if one sex of the offspring resulting from a cross between two is inviable or sterile, that sex is more likely to be the heterogametic sex (i.e. the one with two different sex chromosomes).

haplodiploidy: A sex-determination system in which the sex of an individual organism is determined by the of it possesses: offspring which develop from fertilized eggs are  and female, while offspring which develop from unfertilized eggs are  and male, with half as many chromosomes as the females. Haplodiploidy is common to all members of the insect order Hymenoptera and several other insect taxa.

haplogroup:

haploid:

(of a cell or organism) Having one copy of each, with each copy not being part of a pair. Contrast ' and '.

haploinsufficiency:

haplotype: A set of in an individual organism that were inherited together from a single parent.

Hardy–Weinberg principle: A principle of population genetics which states that and frequencies of a population will remain constant from  to generation in the absence of other evolutionary influences. In the simplest case of a randomly mating population of diploid organisms possessing a single locus with two alleles, A and a, with frequencies $f (A) = p$ and $f (a) = q$, respectively, the expected genotype frequencies are $f (AA) = p^{2}$ for AA, $f (aa) = q^{2}$ for aa homozygotes, and $f (Aa) = 2pq$ for. In the absence of evolutionary forces such as, mutation, , , and , p and q will remain constant between generations, such that the population is said to be in Hardy–Weinberg equilibrium with respect to the locus in question.

hemizygous: In a organism, having just one at a given  (where there would ordinarily be two). Hemizygosity may be observed when only one copy of a is present in a normally diploid cell or organism, or when a segment of a chromosome containing one copy of an allele is, or when a gene is located on a  in the heterogametic sex (in which the sex chromosomes do not exist in matching pairs); for example, in human males with normal chromosomes, almost all  genes are said to be hemizygous because there is only one  and few of the same genes exist on the.

heredity:

The storage, transfer, and expression of genetic information in biological organisms, as manifested by the passing on of from parents to their, either through or. Offspring cells or organisms are said to inherit the genetic information of their parents.

heritability: The ability to be. A statistic used in that estimates the proportion of variation within a given that is due to  between individuals in a particular population. Heritability is estimated by comparing the individual of closely related individuals in the population.

heteropatric speciation:

heterosis:

The improved or increased function or quality of any biological in a, with respect to the same trait in its genetically distinct parents. If any one or more of the parents' traits are noticeably enhanced in the offspring as a result of the mixing of the parents' genetic contributions, the offspring is said to be heterotic.

heterozygous: In a organism, having two different at a given. In genetics shorthand, heterozygous are represented by a pair of non-matching letters or symbols, often an uppercase letter (indicating a  allele) and a lowercase letter (indicating a  allele), such as "Aa" or "Bb". Contrast.

homoallele: A mutant having a different at the same site as another allele. Intragenic between homoalleles cannot produce a functional.

homologous chromosomes:

A set of two matching, one maternal and one paternal, which pair up with each other inside the nucleus during. They have the same at the same, but may have different.

homology: A similarity between a pair of structures, traits, or DNA sequences in different taxa that is due to.

homoplasy:

homoploid recombinational speciation:

homozygous: In a organism, having two identical at a given. In genetics shorthand, homozygous are represented by a pair of matching letters or symbols, such as "AA" or "aa". Contrast.

horizontal gene transfer (HGT):

Any process by which genetic material is transferred between unicellular and/or multicellular organisms other than by vertical transmission from parent to offspring, e.g. bacterial conjugation.

host race:

host-specific parasite:

host-specific species:

hybrid: The that results from combining the qualities of two organisms of different,, , or varieties through. Hybrids may occur naturally or artificially, as during selective breeding of domesticated animals and plants. Reproductive barriers typically prevent hybridization between distantly related organisms, or at least ensure that hybrid offspring are sterile, but fertile hybrids may result in.

hybrid breakdown:

hybrid incompatibility:

hybrid inviability:

hybrid speciation:

hybrid sterility:

hybrid swarm:

hybrid zone: A geographic area in which the ranges of two interbreeding species or populations overlap, allowing them to cross-fertilize and generate offspring. The formation of a hybrid zone is one of the four outcomes of between divergent genetic lineages.

hybridization: The process by which a organism is produced from two parents of different genera, species, breeds, or varieties.

hypermorphosis: The exaggeration of one or more features of a descendant organism compared to those of its ancestors due to an increase in the duration of development over evolutionary history.

I
identical ancestors point:

identical by descent (IBD): (of a gene or allele) Traceable back through an arbitrary number of without mutation to a of the group of descendant organisms that carries the gene or allele. A gene or allele present in a group of descendant organisms is said to be identical by descent to a gene or allele in a common ancestor of the group if both sequences are identical, indicating that the sequence has been passed down unmodified from the common ancestor to its descendants.

inbred line: Any of a particular species in which individuals are nearly or completely genetically identical to each other due to a long history of repeated, either by natural or artificial means. Lineages are typically considered inbred after at least 20 of inbreeding (e.g. by self-fertilization or sib mating), at which point nearly all  across the genome are  and all individuals can therefore effectively be treated as  (despite the fact that individuals are still produced by sexual reproduction).

inbreeding:

Sexual reproduction between breeds or individuals that are closely. Inbreeding results in, which can increase both the probability of offspring being affected by deleterious traits and the probability of fixing beneficial traits within the breeding population. The reproductive event and the resulting progeny may both be referred to as an incross, and the progeny is said to be inbred. Contrast.

inbreeding depression:

inclusive fitness: The number of equivalents that an individual organism rears, rescues, or otherwise supports through its behavior, regardless of whether or not the individual is actually a biological parent of the offspring equivalents. Inclusive fitness is one of two metrics of evolutionary success as defined by W.D. Hamilton in 1964, the other being.

incidence: The frequency of new occurrence of a (or more broadly any genetic condition or, deleterious or otherwise) among the members of a particular population and within a particular period of time.

incomplete dominance:

incomplete speciation:

incipient species: Any that is in an early stage of.

inheritance: See.

interbreeding:

intercross: A in which both the male and female parents are at a particular.

intrinsic postzygotic isolation:

introgression:

The of a from the  of one population or species into that of another population by the repeated  of  of the two populations with one of the parent populations. Introgression is a ubiquitous and important source of in natural populations, but may also be practiced intentionally in the cultivation of domesticated plants and animals.

inviability:

isolating mechanism:

isolation:

isolation by distance:

isomeric genes: Two or more that are equivalent and redundant in the sense that, despite coding for distinct, they each result in the same when set within the same. If several isomeric genes are present in a single they may be either cumulative or non-cumulative in their contributions to the phenotype.

iterative evolution: The repeated of similar characteristics or traits in different organisms at different times during the evolutionary history of a, a phenomenon which can result in the seeming  of an organism previously considered.

iteroparity: A reproductive strategy characterized by multiple reproductive cycles during an individual organism's lifetime. Organisms that use such a strategy are said to be iteroparous. Iteroparity is usually contrasted with.

J
Jordan's Law:

K
K-strategist:

Ka/Ks ratio:

Kaneshiro model: A model of developed by Kenneth Y. Kanneshiro where a sexual species experiences a —that is, when the genetic variation is reduced due to small population size—mating discrimination among females may be altered by the decrease in courtship behaviors or displays of males. This allows to give rise to novel sexual traits in the new population.

kin:

kin recognition:

kin selection: A form of by which differ in their rates of propagation by influencing the survival or reproductive success of individuals who carry the same alleles by  (their ).

koinophilia: An evolutionary hypothesis which proposes that during, organisms preferentially seek mates with a minimum of unusual or, e.g. in terms of functionality, appearance, or behavior. The hypothesis attempts to explain the clustering of sexual organisms into distinct and other issues described by.

L
last universal common ancestor (LUCA):

The most recent of organisms from which all organisms on Earth share a common descent; i.e. the  of all organisms now living. LUCA is not thought to have been the earliest life on Earth, but rather the only organism of its time to still have living descendants. Its existence is not known from any specific fossil record but is inferred from comparisons of modern organisms, all of which are its descendants.

lateral gene transfer (LGT): See.

Law of Dominance and Uniformity: One of three fundamental principles of, which states that different of the same may be  or  relative to others, and that an organism with at least one dominant allele will uniformly display the  associated with the dominant allele.

Law of Independent Assortment: One of three fundamental principles of, which states that genes responsible for different phenotypic traits are segregated independently during. genes are a notable exception to this rule.

Law of Segregation: One of three fundamental principles of, which states that during, the of each  from each other such that each resulting  carries only one allele of each gene.

lethal equivalent value: The average number of deleterious genes existing in the condition that is carried by a member of a population of  organisms, multiplied by the average probability that each such gene will cause premature  when. For example, an organism carrying eight recessive alleles, each of which produces only a 50% probability of premature death when homozygous, is said to carry a genetic burden of four "lethal equivalents".

lethal mutation: Any that results in the premature death of the organism carrying it. lethal mutations are fatal only to, whereas lethals are fatal even in.

lineage:

A linear evolutionary sequence connecting an ancestral cell, organism, or species to a particular descendant cell, organism, or species, including all intermediate organisms and spanning any number of ; the direct progression of reproductive events (i.e. the line of descent) between two individuals, including vertically related individuals, e.g. parent(s) and offspring, but usually excluding horizontally related individuals who did not themselves directly contribute genetic material to any of the included individuals, e.g. siblings.

lineage-splitting:

When between two populations is completely eliminated.

linkage: The tendency of DNA sequences which are physically near to each other on the same chromosome to be together during. Because the physical distance between them is relatively small, the chance that any two nearby parts of a DNA sequence (often or ) will be separated on to different chromatids during  is statistically very low; such loci are then said to be more linked than loci that are farther apart. Loci that exist on entirely different chromosomes are said to be perfectly unlinked. The standard unit for measuring genetic linkage is the (cM).

linkage disequilibrium:

locus:

A specific, fixed position on a where a particular or  resides.

lyonization: See.

M
macroevolution: change as it occurs at a relatively large scale, at or above the level of, as opposed to, which occurs at a smaller scale. Macroevolution is often thought of as the compounded effects of microevolution.

map unit (m.u.): See.

maternal effect: Any nongenetic effect of the mother on the of her, owing to factors such as cytoplasmic inheritance, transmission of congenital disease, and the sharing of nutritional conditions.

mating system:

mating system isolation:

matroclinous: (of an ) Resembling the female parent, genotypically or phenotypically, more closely than the male parent; derived from the mother. Contrast.

maximum parsimony: See.

mechanical isolation:

mechanical pollinator isolation:

medical genetics: The branch of medicine and medical science that involves the study, diagnosis, and management of, and more broadly the application of knowledge about human to medical care.

meiosis: A specialized type of cell division that occurs exclusively in eukaryotes, during which DNA replication is followed by two consecutive rounds of division to ultimately produce four genetically unique daughter cells, each with half the number of  as the original  parent cell. Meiosis only occurs in cells of the sex organs, and serves the purpose of generating haploid such as sperm, eggs, or spores, which are later fused during fertilization. The two meiotic divisions, known as Meiosis I and Meiosis II, may also include various events between.

Mendelian inheritance: A theory of biological based on a set of principles originally proposed by Gregor Mendel in 1865 and 1866. Mendel derived three generalized laws about the genetic basis of inheritance which, together with several theories developed by later scientists, are considered the foundation of. Contrast.

meristic trait: A discretely varying, countable trait, e.g. number of digits.

metagenomics:

The study of genetic material recovered directly from environmental samples, as opposed to organisms cultivated in laboratory cultures.

microallopatric: occurring on a small geographic scale.

microevolution: change as it occurs at a relatively small scale, typically within a particular or, as opposed to, which occurs at a larger scale. Because of the convenience of observing and modeling small-scale changes in within discrete populations, the principles of  are often conceptualized at microevolutionary scales.

microspecies:

midparent value: The mean of the two parental values for a in an individual or in a specific.

migration:

mimicry: The process by which an organism evolves to resemble another object, often an organism of another species. Mimicry can also occur between individuals of the same species. A type of adaptive signaling, mimicry evolves when a signal-receiver, known as the dupe, perceives the similarity between the mimic and the object or organism it is mimicking, known as the model, and as a result changes its behavior in a way that provides a advantage to the mimic; the model may also benefit from the shared resemblance, in which case there is a, or the mimicry may be to the model's detriment, making it or competitive. The evolved resemblance may be visual, acoustic, chemical, tactile, or electrical, or any combination of sensory modalities. There are many varieties of mimicry, such as, , and.

minor allele frequency (MAF):

missense mutation: A type of which results in a that codes for a different  than in the unmutated sequence. Compare.

mitochondrial Eve:

mitosis: In eukaryotic cells, the part of the cell cycle during which the division of the nucleus takes place and are separated into two distinct nuclei. Mitosis is generally preceded by the S stage of, when the cell's is replicated, and either occurs simultaneously with or is followed by cytokinesis, when the cytoplasm and cell membrane are divided into two new daughter cells.

modern synthesis:

modes of speciation: A classification scheme of processes based on the level of between two populations. The traditional terms for the three modes—,, and —are based on the spatial distributions of a species population.

molecular genetics: A branch of that employs methods and techniques of to study the structure and function of  and  at the molecular level. Contrast.

monophyly:

morphological species concept:

mosaic evolution: The evolutionary change of certain adaptive structures, traits, or other components of the phenotype at different times or different rates than others, either within a single species or between different species.

mosaic hybrid zone: A zone in which two speciating lineages occur together in a patchy distribution–either by chance, random colonization, or low hybrid fitness.

mosaic sympatry: A case of in which two populations overlapping in geographic distribution exhibit habitat specializations.

mosaicism: The presence of two or more populations of cells with different in an individual organism which has developed from a single fertilized egg. A mosaic organism can result from many kinds of genetic phenomena, including of chromosomes,, or mutations in individual stem cell lineages during the early development of the embryo. Mosaicism is similar to but distinct from.

most recent common ancestor (MRCA):

Muller's ratchet:

multifurcation: See.

mutant: An organism,, or phenotypic resulting from a, of a type that would not be observed naturally in specimens.

mutation: Any permanent change in the of a strand of or, or in the  sequence of a. Mutations play a role in both normal and abnormal biological processes; their natural occurrence is integral to the process of. They can result from errors in, chemical damage, exposure to high-energy radiation, or manipulations by. mechanisms have evolved in many organisms to correct them. By understanding the effect that a mutation has on, it is possible to establish the function of the or sequence in which it occurs.

mutation distance: The smallest number of required to derive one particular gene, sequence, or phenotype from another; the minimum number of nucleobase insertions, deletions, or substitutions necessary to change one sequence into another.

mutation event: The actual origin of a particular in time and space; the instance of its original introduction into a genome, as opposed to that of its phenotypic manifestation, which may only occur after the fact.

mutation rate: The frequency of new at a particular or in a particular,, , or organism over a specified period of time, e.g. during a single. Mutation rates may be calculated for a specific class of mutation or for all types collectively; they vary widely by organism and with an organism's environment.

mutational hot spot:

mutational load: See.

mutational meltdown:

mutationism:

mutator gene: Any mutant or sequence that increases the spontaneous of one or more other genes or sequences. Mutators are often, or may be mutant such as those that encode  or proteins involved in.

N
natural selection:

neontology: The study of, i.e. those with members that are still living in the present day, as opposed to.

network evolution: See.

neutral mutation: Any of a that is neither beneficial nor detrimental to the ability of an organism to survive and reproduce. Any mutation for which does not affect the spread of the mutation within a population.

nexus hypothesis: The hypothesis that each phenotypic trait is likely to be influenced by more than one gene, and conversely that most genes affect more than one phenotype.

niche: The ecological role of a particular or other in a larger community, generally conceptualized as the multidimensional space, of which the coordinates are the various parameters representing the conditions which are necessary for the existence of the species in every aspect of its present form, to which a species is restricted by the presence of competitor species. A particular environment or environmental condition to which a species is matched; the variety of activities, behaviors, and ecological functions carried out by an organism or population in response to its environmental context, e.g. the distribution of resources and competitors, and the ways in which it in turn alters that same context. The term is sometimes used loosely as an equivalent of microhabitat, in the sense of the physical space occupied by a species. See also ' and '.

niche adaptation:

niche preference:

noncompetitive gametic isolation:

nondisjunction: The failure of or to  properly during cell division. Nondisjunction results in daughter cells that are, containing abnormal numbers of one or more specific. It may be caused by any of a variety of factors.

nongenetic barrier:

nonhomologous recombination:

nonreciprocal recombination: See.

nonsense mutation:

A type of which results in a premature in the   sequence, thereby causing the premature termination of, which results in a truncated, incomplete, and often non-functional.

nonsynonymous mutation:

A type of in which the of one  base for another results, after  and, in an amino acid sequence that is different from that produced by the original unmutated gene. Because nonsynonymous mutations always result in a biological change in the organism, they are often subject to strong. Contrast.

non-geographic speciation:

non-Mendelian inheritance: Any pattern of in which traits do not segregate in accordance with, which describe the readily observable inheritance of discretely variable phenotypic traits influenced by single located on nuclear. Though they correctly explain many basic observations of inheritance, Mendel's laws are useful only in the simplest and most general cases; there exist numerous genetic processes and phenomena, both normal and abnormal, which violate them, such as, , , interactions and , , , , and many  phenomena.

non-random segregation of chromosomes:

norm of reaction: See.

null allele: Any made non-functional by way of a genetic. The mutation may result in the complete failure to produce a or a gene product that does not function properly; in either case, the allele may be considered non-functional.

nullisomy: The condition of a cell or organism lacking all of the copies of a particular that are normal for its level; e.g. in a  organism, lacking both members of the normal pair. Nullisomy is frequently lethal early in development.

nullizygous:

O
offspring:

ontogeny:

The origination and of an organism within its own lifetime, as opposed to, which refers to the evolutionary history of the organism's ancestors. In sexually reproducing organisms, ontogeny is the study of the development of an organism from the time of fertilization to the organism's reproductively mature form; the term may also be used to refer to the study of an organism's entire lifespan.

operational taxonomic unit (OTU):

orphan gene: A for which there are no known functional homologs outside of a given species or, and whose evolutionary history is therefore obscure.

orphon: Any coding or non-coding DNA sequence which is derived from a or  but is physically isolated from the other genes in the family because it is dispersed to a distant  in the genome. Orphons are usually non-functional with highly.

orthogenesis:

ortholog: One of a set of (or more generally any DNA sequences showing ) which are present in different but are descended from the same ancestral sequence, i.e. they are directly related to one another by vertical descent from a single gene or sequence in the  of those genomes. Such genes or sequences are said to be orthologous. Orthologs can be inferred to be related to each other based on the similarity of their sequences; though they may have evolved independently within separate genomes by mutation and natural selection, their may still retain similar structures, functions, or levels of  across species and populations. The identification of orthologs has proven important in inferring relationships between organisms. Contrast.

outbreeding:

between different breeds or individuals, which has the potential to increase by introducing unrelated genetic material into a breeding population. The reproductive event and the resulting progeny may both be referred to as an outcross, and the progeny is said to be outbred. Contrast.

outgroup:

P
paleontology:

paleopolyploidy:

para-allopatric speciation: A mode of speciation in which divergence begins in but is completed in.

paracentric inversion: A in which the inverted segment does not include the chromosome's. Contrast.

parallel evolution: The independent evolution of similar or identical or characters in related, thought usually to be based on similar modifications of common developmental pathways. Contrast.

parallel speciation:

paralog: One of a set of (or, more generally, any DNA sequences showing ) which are directly related to each other via one or more genetic events; such genes or sequences are said to be paralogous. Paralogs result from the duplication of a single sequence within a single and then the subsequent  of the duplicated sequences by mutation and natural selection (either within the original genome, or, during, in different genomes). Contrast.

parapatric speciation:



paraphyly:

parasitic DNA: See.

parsimony:

The principle of accounting for empirical observations by whichever hypothesis requires the fewest or the simplest assumptions for which there is limited or no evidence. In biological systematics, maximum parsimony is an optimality criterion which invokes a minimum of evolutionary changes to infer relationships; i.e. the that minimizes the total number of character-state changes is to be preferred.

parthenogenesis: A type of in which the growth and development of embryos occurs without fertilization. In animals which reproduce by parthenogenesis, an unfertilized gamete of the female parent is capable of developing into an adult without any contribution from a male parent, resulting in offspring possessing only the mother's genetic material (the exact proportion of which depends on the parthenogenetic mechanism, of which there are numerous varieties). Some species reproduce exclusively by parthenogenesis, while others can switch between and parthenogenesis under certain environmental conditions.

partial dominance: See.

particulate inheritance: One of the defining ideas of, which holds that are or can be via the passing of "" from  to generation. These particles may not have detectable effects in every generation but nevertheless retain their ability to be in subsequent generations.

patroclinous: (of an ) Resembling the male parent, genotypically or phenotypically, more closely than the female parent; derived from the father. Contrast.

peak shift model:

pedigree chart:

penetrance: The proportion of individuals with a given who express the associated, usually given as a percentage. Because of the many complex interactions that govern, the same may produce an observable phenotype in one individual but not in another. If less than 100% of the individuals in a population carrying the genotype of interest also express the associated phenotype, both the genotype and phenotype may be said to show incomplete penetrance. Penetrance quantifies the probability that an allele will result in the expression of its associated phenotype in any form, i.e. to any extent that makes an individual different from individuals without the allele. Compare.

pericentric: (of a gene or region of a chromosome) Positioned near the of the.

pericentric inversion: A in which the inverted segment includes the chromosome's. Contrast.

peripatric speciation: A variation of allopatric speciation where a new species forms from a small, peripheral isolated population. It is sometimes referred to as centripetal speciation in contrast to centrifugal speciation.

phenetic: Pertaining to similarity, e.g. a phenetic classification.

phenome: The complete set of that are or can be expressed by a, cell, tissue, organism, or species; the sum of all of its manifest chemical, morphological, and behavioral characteristics or traits.

phenomic lag: A delay in the of a genetic  owing to the time required for the manifestation of changes in the affected biochemical pathways.

phenotype: The composite of the observable morphological, physiological, and behavioral of an organism that result from the of the organism's  as well as the influence of environmental factors and the interactions between the two.

phyletic gradualism: A model of evolution which theorizes that most speciation occurs slowly, uniformly, and gradually, and that there is seldom a clear line of demarcation between ancestral species and descendant species unless there is a sudden split which reproductively isolates members of the same population. The theory is often contrasted with.

phylogenetic bracketing: A method used to infer the likelihood of specific traits being present in organisms whose phenotypes are incomplete or unknown based on their positions in a relative to ancestors, descendants, or contemporaneous organisms with more completely understood phenotypes. A major application of this method is in, where extinct organisms known only from fossils are compared to their closest known relatives in order to infer the presence or absence of certain traits for which fossils provide limited or no evidence, such as soft tissues, integumentary structures, and physiological and behavioral traits, though the method is extremely sensitive to confounds from.

phylogenetic species concept:

phylogenetic tree: A graphical representation of a, consisting of a branching, tree-like diagram showing the evolutionary relationships between biological or other as inferred from similarities and differences in their morphological or genetic characteristics, and how they have all descended from a.

phylogenetics: The study of the evolutionary history of and relationships between individuals or groups of organisms, such as species or populations, through methods that evaluate similarities and differences between observed, including morphological features and. The graphical presentation of data from such analyses is known as a or.

phylogeny:

phylogeography:

pleiotropy:

The phenomenon by which one influences two or more seemingly unrelated, by any of several distinct but potentially overlapping mechanisms.

plesiomorphy: An state; i.e. the state or condition of a particular trait or feature (e.g. a specific ) that is present in the of a given. Plesiomorphies may or may not be by some or all descendants within the clade. The term is relative; a trait considered a plesiomorphy in one clade may not be considered a plesiomorphy in a different clade. Contrast.

ploidy: The number of complete sets of in a cell, and hence the number of possible present within the cell at any given.

pollinator isolation:

polygene:

polygenic trait: Any trait which is under the direct control of more than one. Polygenic traits are often.

polymorphism: The regular and simultaneous occurrence in the same population of two or more (or ) at the same at  that cannot be accounted for by recurrent mutation alone (generally at least 1%), implying that the multiple alleles are being stably inherited by members of the population.

polypheny: See.

polyphyly: The grouping of organisms which do not share an immediate ; such groups are said to be polyphyletic. The term is often applied to groups of organisms that share characteristics which appear to be similar but are not actually closely related, frequently as a result of. The avoidance of polyphyletic groupings is often a stimulus for major revisions of biological classification schemes. Contrast ' and '.

polyploid: (of a cell or organism) Having more than two copies of each. Polyploidy may occur as a normal condition of chromosomes in certain cells or even entire organisms, or it may result from abnormal cell division or a mutation causing the of the entire chromosome set. Contrast ' and '; see also.

polytomy:

popular sire effect:

population: A group of organisms of the same which occupies a more or less well-defined geographic region and which exhibits reproductive continuity from to generation. It is generally presumed that ecological and reproductive interactions occur more frequently among the members of the group than between them and members of other populations of the same species.

population bottleneck:

A sharp, often sudden reduction in the size of a biological, often due to a major environmental event such as a flood, fire, volcanic eruption, drought, famine, or disease. Because only a small population with a narrower range of remains afterward to pass on genes to future generations, such events tend to reduce the in the population's, and often lead to new and distinct populations through. Diversity increases again only when from another population occurs, or very slowly over time as random mutations accumulate.

population genetics: A subfield of and that studies genetic differences within and between  of organisms.

position effect: Any effect on the or functionality of a or sequence that is a consequence of its location or position within a  or other DNA molecule. A sequence's precise location relative to other sequences and structures tends to strongly influence its activity and other properties, because different on the same molecule can have substantially different  and physical/chemical environments, which may also change over time. For example, the of a gene located very close to a,, or  is often  or entirely prevented because the proteins that make up these structures block access to the DNA by , while the same gene is transcribed at a much higher rate when located in. Proximity to, , and other , as well as to regions of frequent by , can also directly affect expression; being located near the end of a chromosomal arm or to common  points may affect when  occurs and the likelihood of. Position effects are a major focus of research in the field of.

positive selection: See.

postmating barrier:

postmating prezygotic isolation:

postzygotic isolation:

preadaptation: Possession of the necessary properties to permit a shift into a new or habitat. A structure is said to be preadapted if it can before it itself becomes modified by.

premating barrier:

premating isolation:

prezygotic isolation:

proband:

A term used in and to denote a particular subject being studied or reported on.

progenesis: The precocious or accelerated sexual maturation of an organism that is still at a morphologically juvenile stage.

progeny: A genetic descendant or group of descendants; the of a single reproductive event, either sexual or asexual.

progressive selection: See.

protosexual: Of or pertaining to organisms that achieve by conjugation, transduction, or lysogenization. Compare ' and '.

protospecies: An.

protype:

In, a complete specimen that replaces a fragmentary.

pseudoallele: Any of two or more different genes or sequences which have the same or similar contributions to, and thus appear to be genuine, but are not actually structurally allelic (i.e. they do not occupy homologous on ).

pseudogene: A non-functional sequence of DNA that resembles a functional. Pseudogenes are typically superfluous copies of functional genes which have been by natural processes, except that they lack sequences necessary for proper  or  or contain other defects such as, premature, or missing.

pseudopolyploidy: The condition in which the number of in a chromosome set is (or tripled, etc.) but without a corresponding increase in the actual amount of genetic material (i.e. the ). This occurs when the chromosomes of a normal chromosome complement (e.g. ) become fragmented into smaller pieces, increasing the total number of individual chromosomes but not creating additional homologous copies of those chromosomes (such that the cell remains diploid). Any numerical relationship between chromosome sets in groups of related organisms which suggests that some of those organisms are of others when in fact they are not.

punctuated equilibrium:

Punnett square: A tabular diagram used to predict the possible that can be inherited by the of a particular  or breeding experiment by summarizing all of the various combinations of maternal  with paternal alleles. The resulting table can then be used to determine the probabilities that the offspring will have a particular genotype. The usefulness of Punnett squares is limited to discrete inherited according to simple  patterns.

purebred:

putative gene: A specific suspected to be a functional based on the identification of its. The gene is said to be "putative" in the sense that no function has yet been described for its.

Q
quantitative genetics: A branch of which studies that vary continuously (such as height or mass) as opposed to those that fall into discretely identifiable categories (such as eye color or the presence or absence of a particular ). Quantitative genetics employs statistical methods and concepts to link continuously distributed phenotypic values to specific and.

quantitative trait:

quantitative trait locus (QTL):

quantum evolution: A rapid evolutionary shift in a to a state that is distinctly unlike the.

quantum speciation: A chromosomal model of speciation that occurs rapidly when a cross-fertilizing plant species buds off from a larger population on the periphery, experiencing interbreeding and strong genetic drift that results in a new species. The model is similar to that of Ernst Mayr's.

R
r/K selection: The of combinations of traits in organisms or species which appear to involve a trade-off between quantity and quality of, whereby an organism or species may evolve to make use of either of two different reproductive strategies: tend to produce many, low-quality offspring, yielding large numbers of progeny during their lifespan but investing little or no energy in nurturing or protecting them, whereas  tend to produce few, high-quality offspring, yielding small numbers of progeny but with a corresponding increase in parental investment. Which strategy evolves depends on which one results in greater, which itself often depends on the stability of the organism's environment. In an unstable environment, where the probability that any individual offspring will survive to maturity is low, investment in parental care may not be sensible, and the parent may be more likely to pass on its genetic material if it dedicates its metabolic energy to simply producing as many offspring as possible rather than to parenting. Conversely, in more stable environments where survival to maturity is relatively common, the parent may find greater success if it dedicates more time and energy to parental care, improving each individual offspring's likelihood of reproducing successfully. The different strategies are often accompanied by characteristic anatomical or physiological traits, e.g. r-selected species often have small body size, rapid development, and short lifespans.

reaction norm: The pattern or set of of a given across a variety of different environmental conditions.

recapitulation: The passage of an organism's features through stages that resemble the adult features of the organism's ancestors.

recessiveness: A relationship between the of a in which one allele produces an effect on  that is overpowered or "masked" by the contribution of another allele at the same ; the first allele and its associated phenotypic  are said to be recessive, and the second allele and its associated trait are said to be. Often, recessive alleles code for inefficient or dysfunctional proteins. Like dominance, recessiveness is not an inherent property of any allele or phenotype, but simply describes its relationship to one or more other alleles or phenotypes. In genetics shorthand, recessive alleles are often represented by a lowercase letter (e.g. "a", in contrast to the dominant "A").

reciprocal cross: A experiment designed to test whether parental sex influences the inheritance of a particular trait. In crosses where the parents differ in or  or both, and hence only one of the parents (either the male or female parent) expresses the trait of interest, the reciprocal cross is the inverse, in which the parent of the other sex expresses the trait of interest instead. For example, if in the first cross a male expressing the trait is crossed with a female not expressing it, then in the reciprocal cross a female expressing the trait is crossed with a male not expressing it. By observing the resulting from each cross, geneticists can make inferences about which, if either, influences the trait's expression.

reciprocal hybrid: A resulting from a  between parents differing in genotype or phenotype or both.

reciprocal translocation: A type of by which there is a reciprocal exchange of chromosome segments between two or more non-. When the exchange of material is evenly balanced, reciprocal translocations are usually harmless.

recognition species concept:

recombinational speciation:

recurrent evolution: The repeated of a particular trait or character, for whatever reason, whether by or.

Red Queen hypothesis:

refugium:

A geographic location (or, more narrowly, a ) in which one or more species has persisted while becoming elsewhere.

reinforcement: A process of by which increases the  between two  of a species as a result of selection acting against the production of  individuals of low. See also Evidence of speciation by reinforcement.

relict: A species or population that is the last surviving representative of an otherwise group,,, or , or which has been left behind in a locality after extinction throughout most of a formerly larger geographic distribution.

repression: See.

reproduction curve: A graphical representation of the relationship between the number of individuals at a given stage of one and the number of individuals at the same stage in a previous generation.

reproductive character displacement:

reproductive effort: The proportion of an individual's total metabolic resources that is devoted to reproduction.

reproductive isolating barriers: The set of evolutionary mechanisms, behaviors, and physiological processes responsible for the of two or more populations.

reproductive isolation: The condition in which between two or more of organisms is prevented by intrinsic factors, such that the members of one population cannot  with the members of another population and produce fertile. The evolution of reproductive isolation between members of different populations is usually considered the first step in the process of, because it effectively prevents between the populations and thereby allows each to evolve independently; hence the existence of reproductive barriers is often used as a criterion by which to define species in various. Isolation may occur when the populations are physically separated by environmental changes or migration such that members of the other population are simply inaccessible, or it may occur when anatomical or genetic differences make copulation between members of different populations impossible or at least ensure that any offspring that happen to develop are sterile, even though the populations are not physically separated from each other. Isolating mechanisms are typically classified as (isolating barriers occurring before the formation of a ) and  (isolating barriers occurring after the formation of a zygote).

reproductive success: The successful production of by an individual, often quantified as the number of offspring produced by the individual per reproductive event or during the individual's entire lifespan, or as the number of an individual's offspring that survive to reproductive maturity themselves or that are surviving at a given time.

reproductivity effect: The decrease in the rate of reproduction of new individuals per colony member as colony size increases.

reticulate evolution:

The union of different of a by.

reverse genetics: An experimental approach in in which a researcher starts with a known and attempts to determine its function or its effect on phenotype by any of a variety of laboratory techniques, commonly by deliberately mutating the gene's nucleic acid sequence or by repressing or silencing its expression and then  the mutated organisms for obvious changes in phenotype. When the gene of interest is the only one in the genome whose expression has been manipulated, any observed phenotypic changes are assumed to be influenced by it. This is the opposite of, in which a known phenotype is linked to one or more unknown genes.

reverse mutation:

Any in a gene or DNA sequence which restores or the original function or phenotype that was altered or destroyed by a previous mutation in the same sequence. Contrast '; see also '.

revertant: A gene or allele in which a occurs, or an organism bearing such a gene or allele.

ring species: Connected of the same, each of which can interbreed with closely sited, closely related populations, but for which there exist at least two "end" populations in the series which are too distantly related to interbreed.

Robertsonian translocation (ROB): A type of by which at or near the  of two   cause a reciprocal exchange of segments that gives rise to one large  chromosome (composed of the ) and one extremely small chromosome (composed of the ), the latter of which is often subsequently lost from the cell with little effect because it contains very few genes. The resulting shows one fewer than the expected total number of chromosomes, because two previously distinct chromosomes have essentially fused together. of Robertsonian translocations are generally not associated with any phenotypic abnormalities, but do have an increased risk of generating meiotically unbalanced.

robustness: The persistence of a certain phenotypic trait or characteristic in a biological system despite perturbations or conditions of uncertainty. Robustness is achieved through the combination of many genetic and molecular mechanisms which effectively preserve the integrity of a particular, and can evolve by direct or indirect selection.

runaway selection:

S
saltation: A sudden and large mutational change from one to the next which is sufficient to cause rapid or immediate. Various forms of saltation, such as by in plants, have often historically been interpreted as evidence for certain theories of, in contrast to.

secondary contact: The process by which two distributed populations of a species are geographically reunited. Contact between divergent populations may renew the potential for between them, depending upon how  the populations have become.

selection: The non-random differential survival or reproduction of classes of different entities. Selection may occur or may be induced. Selection is often studied in different modes (as with and ) or from the perspective of distinct  (as with  and ).

selection coefficient: The difference between the mean relative of individuals of a given and those of a reference genotype.

selective pressure:

selective sweep: The process by which strong of a new and beneficial within a population causes the mutation to reach  so quickly that nearby  DNA sequences also become fixed via, thereby reducing or eliminating the  of nearby loci within the population.

semelparity: A reproductive strategy characterized by a single reproductive episode during an individual organism's lifetime, especially one in which the programmed death of the organism immediately after the reproductive event constitutes part of an overall strategy that includes putting all available resources into maximizing the probability of reproductive success, at the expense of the organism's future life. Organisms that use such a strategy are said to be semelparous. Semelparity is usually contrasted with.

semi-geographic speciation:

semipermeable species boundary: The idea that gene flow can occur between two species but that certain alleles at particular loci can exchange whereas others cannot. It is often used to describe hybrid zones and has also been referred to as porous.

semispecies: One of several groups of that are partially but not entirely from each other by biological isolating mechanisms, and which are therefore neither easily definable as belonging to the same  nor to separate species. The taxon of species itself is.

sexual reproduction:

sexual selection:

spandrel:

speciation: The process by which populations evolve to become distinct.

speciation experiment: An experiment that attempts to replicate in nature in a scientifically controlled, laboratory setting.

speciation in the fossil record: Speciation that can be detected as occurring in fossilized organisms.

speciation rate:

species: A basic unit of biological classification, traditionally interpreted according to the as the members in aggregate of a group of of organisms which interbreed or potentially interbreed with each other under natural conditions; a basic  to which individual specimens are assigned and which often but not always corresponds to the definition of a biological species; and a fundamental unit used to interpret and measure biodiversity in ecological contexts. The concept of species is notoriously complex and often to define precisely; many different  of what is or should be meant by the term have been defined in scientific literature.

species complex:

species concept:

species problem: The difficulty in precisely defining what a is and in determining the placement of an organism within a particular species.

stasipatric speciation:

stasis: A species lineage that experiences little or change over time.

stepping-stone speciation:

sterility:

subspecies: A named geographic, or a set of of the same which share one or more distinctive features and occupy an area that is geographically separate from other subspecies. Not all species are formally divided into subspecies, and the taxon of species itself is.

survival of the fittest:

suture zone: A geographic region that exhibits a significant number of, contact zones between populations, and phylogeographic breaks.

swamping effect:

sympatric speciation:

sympatry:

symplesiomorphy:

synapomorphy:

T
taxon:

taxonomy:

teleonomy:

temporal isolation:

tension zone:

type:

type species:

U
unit of selection:

V
vicariance biogeography: A approach to species distributions that uses their phylogenetic histories—patterns resulting from events in the past.

vicariant speciation: A biogeographic term meaning the geographic isolation of two species populations (as in ).

W
Wahlund effect: A phenomenon by which a reduction of at a particular genetic locus within a population as a whole is observed when two or more subpopulations have different at that locus, even if the subpopulations themselves are each in.

Wallace effect:

Y
Y-chromosomal Adam:

Z
zygosity:

zygote: