User:Ziqixie/Hybrid zone

Models and theories
Various models and theories have been developed by the researchers of hybrid zones. Major models can be largely categorized into four types: ephemeral hybrid zone theory, bounded hybrid superiority model , mosaic hybrid zone model and tension zone model. In each model, different evolutionary forces are attributed different levels of importance. The different models result largely from evolutionary biologists’ different perspectives on natural hybridization and species concepts. The four major models operate mostly under a general framework of either a balance between natural selection and dispersal or interaction between genotypes and environment. Different hybrid zones may fit different models and no single theory or model serves to explain all the hybrid zones found in nature.

Ephemeral hybrid zone theory
Some early evolutionary biologists who preferred a biological species concept, such as Ernst Mayr and Theodosius Dobzhansky, believed that hybrid zones are generally rare and ephemeral, with an eventual fate of either merging of the hybridizing populations or reinforcement, which leads to a speciation event. The extinction of one of the hybridizing populations through introgression is sometimes termed “waves of advance”. (Although this term can also refer to the spreading of advantageous allele across a reproductive barrier ) The ephemerality of hybrid zone has been countered by the discovery of many hybrid zones that has lasted for a long period of time, up to 100,000 years found between the iguanid lizards, Sceloporus woodi and S. undulatus undulatus.

Bounded hybrid superiority
The bounded hybrid superiority model predicts that hybrids have higher fitness in a habitat that is intermediate between those of their parental populations. The hybrid habitat occurs usually, but not necessarily, on a narrow ecotone. Clines of a bounded hybrid superiority zone reflect a smooth gradient corresponding to the gradient of differential selection strength in fitness-related characteristics. The bounded superiority model places a high importance on the ecological aspect of the habitat. In fact, botanist Edgar Anderson suggested that hybrid populations are more likely to inhabit ecologically disturbed areas, which often occur under human’s modification of landscapes or geological events that create novel habitat conditions. He argued that hybrid zones are essentially formed via “hybridization of habitats”. Anderson also considered natural hybridization as a positive evolutionary stimulus that allows different populations and lineages to exchange adaptive genetic elements – similar view that place a high evolutionary importance on hybrid zones is more prevalent among botanists, in contrast to zoologists who are more likely see hybrid zones as more of a “natural laboratory” of population genetics. Some criticism for the bounded superiority model suggests that hybrid zones with higher hybrid fitness are theoretically unlikely to be distributed along narrow ecotones ; some also point out that there has not been direct empirical evidence of higher hybrid fitness along an ecological gradient (i.e. ecotones).

Tension zone model
The term “tension zone” was first used by K. H. L. Key to describe an area of hybridizing populations that act like a “semipermeable membrane” in terms of gene exchange. This term was later taken by Nicholas Barton and Godfrey Hewitt to denote a hybrid zone maintained by a balance between selection and dispersal. Similar models have also previously been termed “dynamic equilibrium”.

A tension zone is characterized by a dispersal-dependent cline (in contrast to a dispersal-independent cline such as a bounded hybrid superiority zone) maintained between the “homogenizing effect” of dispersal and some forces of “spatial heterogeneity”, such as differential selection and introgression. Whether a hybrid zone is a tension zone or not is determined by the characteristic scale of selection, l = σ/√s, where σ2 is dispersal rate and s is selection strength. The width w of a dispersal-dependent cline is of the same order as l, whereas a dispersal-independent cline has a much greater w than l . As the hybrids in a tension zone model often exhibit lower fitness compared to the parentals, a “hybrid sink” is maintained through parental gene flow into the tension zone, but rarely hybrid gene flow outwards.

Although tension zones can be restricted by natural barriers to gene flow, they are generally considered to be environment-independent. Therefore, the movement of a tension zone can be described independent of ecological characteristics of the habitat. A tension zone can move geographically by mainly three kinds of forces– the fitness variation among individuals of a population, variation in density or dispersal rate, and gene frequencies that may lead to change in density or dispersal.

Mosaic hybrid zone model
A mosaic hybrid zone is characterized by a “patchy” distribution of genotype frequencies. Richard Harrison suggested that in some hybrid zones the patterns of environmental heterogeneity can be more complex than can be accounted for by a gradient model, as the transition between two environments may not be a continuous gradient but rather a mosaic distribution constituting of patches of varying proportions of the two environments. A hybrid zone demonstrating a mosaic distribution can be hard to detect. If a transect is taken on a mosaic zone, the distribution of a genotypic trait may present itself as a wave with multiple peaks or plateaus, which may be interpreted as clines depending on the size of the geographical study. The detection of patchy distribution depends on whether the mosaic zone is “fine-grained” or “coarse-grained”, i.e. the comparative size of the dispersal distance and the average size of a patch.

Hybrid zone in conservation biology
Hybrid zones involving a rare species and a more common one can be at risk for outbreeding depression that reduces the fitness of the rare species. Another risk that can arise is the assimilation of the rare species through loss of rare genotypes or phenotypes. This risk is especially high when an invasive species hybrids with an endemic species on an island. However, hybridization can also serve to introduce genetic diversity into small, inbred populations, such as the case with the Florida panther. In this respect, conservation policies based on taxa instead of genetic structure can be disadvantageous to rare species experiencing inbreeding depression.

Hybrid subpopulations formed through sympatric or parapatric speciation at the geographical periphery of larger populations can be important targets for conservation, as they may be sites of future speciation events that lead to higher biodiversity.

Hybrid zones can be a good indicator in the study of climate change. Monitoring the range of hybrid zones through genetic methods such as geographical cline analysis of genotype distribution can tell us the populations’ response to historical as well as ongoing changing environments. Examining the exchange of adaptive alleles or genomic regions can also be useful in mapping the populations’ adaptation to climatic niches.

Louisiana Irises
[add images of the species mentioned here]

Hybridization of Louisiana irises of the Mississippi Delta have been extensively studied since the 1930s. In illustration of his introgressive hybridization theory (1949), Edgar Anderson used the hybrid zone between Iris fulva and Iris hexagona var. giganti-caeulea (HGC) as an example, based on a previous study done by Herbert Riley (1938). I. fulva was a wide-range species that can be found in wet clay soil (bayou system) from Ohio River valleys to the lower Delta of the Mississippi, where the smaller-range HGC grew in alkaline marshes. The two species came into contact in the lower Mississippi Delta between New Orleans and the sea, which is characterized by branching rivers with constantly changing courses and lots of agricultural development by small farms in between the rivers and bayous. (Viosca 1935) I. fulva is characterized by its small, red flowers, while HGC has large flowers with white to blue colors and a patch of bright yellow. In the early 1900s, irises of various colors grown in the natural areas of this region attracted the attention of lots of local gardeners. Later, John Small illustrated and described these diverse irises as newly discovered species (Small 1927, Small and Alexander 1931). Subsequent investigations by other botanists (Visosca 1935, Foster 1937) raised questions as to how many of these iris variations could be defined as species. Riley’s study (1938) took place in an abandoned deltaic stream with two levees, on top of one of which was small farms and public roads. One of the bayous of the river cut across these levees and formed a wide marsh, where HGC populations inhabit. Along the edge of the stream are sporadically growing I. fulva for several miles. The hybrids were mainly found in two groups (H-1 and H-2), where individuals bearing intermediate characteristics in terms of flower size, color and assortment were found. Riley recorded a series of morphological traits such as color of sepal blade, sepal length and petal shape as well as pollen fertility of the two parental populations and the two putative hybrid populations (1938). He then performed a hybrid index analysis developed by Anderson (1936) by arbitrarily assigning values to the morphological characters, with one of the parental species individuals associated with a higher value and the other associated with a lower value. According to the results, a number of hybrids were identified from both of the two putative hybrid habitats. Noting the limited range of the H-1 habitat, Anderson used the Louisiana iris hybrid zones as a case of “hybridization of the habitat” (1948, 1949), where either an intermediate habitat has to exist for the F1 hybrids to establish, or some F1 hybrids have to be similar enough to one of the parents in order to sustain in the parental habitat.

While the morphological biogeography of hybrid zones between I. fulva and HGC was used by Anderson (1949) as an illustration for introgressive hybridization, as a lot of other putative hybrid populations and habitats demonstrated similar patterns. A later cytological study denied the presence of genetic material exchange, and hence introgression, between the two species (Randolph et al. 1967). As the original study site of Riley (1938) has vanished due to increased human settlement, other putative hybrid populations in Southern Louisiana were chosen for study. The new study site contained three species, I. fulva, I. brevicaulis, and I. giganticaerulea (HGC) [add description of each of their habitats]. Natural hybrids of I. fulva and I. brevicaulis, and of I. fulva and I. giganticaerulea were identified based on the intermediate flower color and size they demonstrated. The researchers also sampled from allopatric populations of all three species in Southwestern Louisiana. [more to write about the result analysis]

[Arnold et al. (1990) showed through genetic marker that introgressive hybridization did take place between the populations studied by Randolph et al. 1967]

[Cruzan and Arnold (1993): non-random association between hybrid genotype and parental habitat]

The continued research done on Louisiana iris hybrid zones across the century illustrates how biologist’s understandings of natural hybridization events have developed alongside advancements in molecular technologies.

Terrestrial animal hybrid zone
[Plenty examples, find a paper or two and summarize]