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Developmental drive is characterized by positive biases toward other trajectories or phenotypes. Developmental drive is a type of biased developmental reprogramming of ontogenetic trajectory within a lineage in favor of certain changes. Developmental drive can have a powerful influence on the direction of evolutionary change along with natural selection.

History
Over the last three decades, there has been rapid growth of a new approach to comprehending the evolution of organisms and the effect that development has on the direction of evolution. Evolutionary developmental biology or "evo-devo", is focused on the developmental genetic machinery that lies behind embryological phenotypes, which were all that could be studied in the past, stemming from the work of Von Baer and Ernst Haeckel.

Present day evo-devo erupted out of the discovery of the homeobox in the early 1980s. Proposals that biases (positive and negative) can potentially lead to the direction of evolutionary change being determined by developmental dynamics as well as by population dynamics are in contrast with the historical thrust of Darwinism and Neo-Darwinism, that the direction of change is determined exclusively by selection. Development has long been the missing link of evolutionary theory.

Developmental Reprogramming
Over any period of evolutionary time, the prevailing ontogenetic trajectory within a lineage may either recur unchanged from generation to generation (stasis) or alter (developmental reprogramming). Evolution is portrayed as the action of selection on genetic variation introduced by mutation. This view is incomplete, because mutation only acts on genes but not on phenotypes, whereas selection acts not on genes but on phenotypes. The missing link is how we get from altered gene to new phenotype. This is the process of developmental reprogramming. This term refers to alterations that mutations in developmental genes produce in the course of ontogeny, resulting in variant individuals. If a particular ontogeny is represented by a trajectory through multi-dimensional phenotypic space, then after reprogramming we have a different trajectory.

Recognition of this middle level of evolutionary change is crucial, because it allows us to consider whether the introduction of variants may have inherent ontogenetic directionality, even though the mutation is random.

Developmental reprogramming is a mutation-based, and thus inherited change in the overall genetic/epigenetic/ecological program through which we get from genome to phenome. Reprogramming can be studied at different levels, from the alteration of a developmental gene's product through its ontogenetic consequences to its ultimate effects on the adult phenotype. At any one level, there are four possibilities: changes in timing called heterochrony, changes in spatial distribution called heterotopy, changes in quantity called heterometry and changes in type called heterotypy. A change of one of these at the molecular level may well give rise to other kinds of change at the phenotypic level.

Reprogramming becomes controversial if it is proposed that it can be systematically biased, in that mutation more readily produces changes in certain directions than others, including the extreme case of some directions being prohibited.

Developmental Bias
A bias in developmental reprogramming is reffered to as a mutation bias or a developmental bias (evolution biased by development). The term developmental bias is used for both positive and negative effects.

Constraint vs. Drive
Developmental constraint is a term used solely for negative biases, that is, biases against the production of certain developmental variants. However, a bias against the production of some phenotypes necessarily implies a bias in favor of others. Positive biases have recently been termed developmental drive which is quite distinct from some other processes that have been suggested as having a directional role, namely meiotic drive, molecular drive, and dominance drive.

Evidence for a directional evolutionary role for development bias is limited and difficult to aquire.

Absolute vs. Relative
If a developmental system is incapable of producing a certain variant ontogeny, it is an absolute constraint. If the particular ontogeny x is difficult to produce, in the sense that a small proportion of mutations lead in that direction while many lead elsewhere, it is a relative constraint. The same distinction can be applied to developmental drive (positive biases).

Developmental Drive in the Real World
To show the involvement of developmental drive, it is necessary to compare existent and nonexistent morphologies. This is a difficult step and causes a problem because there must be a decision made about which nonexistent morphologies to compare with the existent. However, there is a broad scope for uninformative comparisons to be made so it must be done with careful consideration.

Centipede Segments
There exists at least 3000 species of centipede with trunk segment numbers that range from 15 to 191. There are two orders of centipedes in which developmental drive is evident. The order Lithobiomorpha consists of about 1100 species. All have 15 trunk segments at their adult stage. However, these centipedes hatch from the egg with fewer than 15 segments. The segments are added with age through a series of moults through which the hatchling grows towards adulthood. Lithobiomorph centipedes with an even number of trunk segments exist, but only as juveniles. However, given the role of heterochrony in evolution, there have been no shifts of relative timing so that reproductive maturity and a cessation of segment addition have occurred in an even-numbered trunk segment juvenile in at least one of the many lithobiomorph species. It is more plausible that there is a drive towards odd-numbered trunk segments than a constraint of even-numbered trunk segments. The order Geophilomorpha consists of about 1000 species with variable trunk segment numbers. The overall range of segment number for the order is from 27 to 191. Within this range almost all odd numbers are represented in individuals whereas all even numbers are completely absent. Despite the larger number of segments in geophilomorphs, all are formed during embryonic development. The tiny hatchling has its full adult complement of segments. There are no segments added in any series of moults in any geophilomorph species in postembryonic growth.

All variant centipede ontogenies, from whatever starting point, are driven into odd-segment-number character states. Therefore, there is absolute drive in this direction, and equivalently absolute constraint regarding even numbers of segments.

Gastropod Shells
Gastropodes are a clade that consist of about 50,000 species which exhibit a huge variety of shell form. All gastropod shells are built, during development, in the same basic way. Each shell is a tube that extends in length and increases in diameter. One possible kind of developmental drive in shell form within a hypothetical gastropod species, the within-population variation in tube length and diameter takes a downward-sloping form, indicating a trade-off between growth in the two directions. Given such a biased range of phenotypes, some local fitness optima may be reachable, others not, on the basis of selection on the existing variation, despite being equally distant from the population mean. The march of the population toward a reachable optimum rather than an unreachable one is a joint result of selection and drive.

It is always possible to go back in evolutionary time to an ancestor in which those characters did not exist. There must have been a proto-gastropod without a typical gastropod shell form, and indeed a proto-mollusc with no shell at all. The mutations of developmental genes that initiated the first shells and opened up whole new areas of morphospace were perhaps based on the first kind of developmental drive, while the manifold wanderings within such morphospace were most likely based on the type of drive that utilizes preexisting within-population variation. But in both cases the important point is that the direction of evolutionary change at the phenotypic level may be as much a product of the within-individual dynamics of development as of the within-population dynamics of natural selection.