How important is geographical isolation in speciation?
promoting reproductive isolation between allopatric populations that and geographic isolation influence hummingbird speciation: genetic. geographic isolation prevents cross breeding between the populations that have been separated by the isolation. Allopatric speciation involves geographical isolation of a population from other As conditions became warmer, barriers between the populations broke down; . and western Victoria developed, without a comparable eastern connection.
Why do species exist? Why is not all nature in confusion instead of the species being, as we see them, well defined?
How important is geographical isolation in speciation?
Darwin pointed out that by the theory of natural selection "innumerable transitional forms must have existed," and wondered "why do we not find them embedded in countless numbers in the crust of the earth. If, on a resource gradient, a large number of separate species evolve, each exquisitely adapted to a very narrow band on that gradient, each species will, of necessity, consist of very few members.
Finding a mate under these circumstances may present difficulties when many of the individuals in the neighborhood belong to other species. The members of the neighboring species, whose population sizes have decreased, experience greater difficulty in finding mates, and therefore form pairs less frequently than the larger species. This has a snowball effect, with large species growing at the expense of the smaller, rarer species, eventually driving them to extinction.
Eventually, only a few species remain, each distinctly different from the other. African pygmy kingfishershowing coloration shared by all adults of that species to a high degree of fidelity. Rare and unusual features are very seldom advantageous. In most instances, they indicate a non-silent mutationwhich is almost certain to be deleterious.
It therefore behooves sexual creatures to avoid mates sporting rare or unusual features koinophilia. This uniformity of all the adult members of a sexual species has stimulated the proliferation of field guides on birds, mammals, reptiles, insects, and many other taxain which a species can be described with a single illustration or two, in the case of sexual dimorphism.
Once a population has become as homogeneous in appearance as is typical of most species and is illustrated in the photograph of the African pygmy kingfisherits members will avoid mating with members of other populations that look different from themselves.
Thus, asexual organisms very frequently show the continuous variation in form often in many different directions that Darwin expected evolution to produce, making their classification into "species" more correctly, morphospecies very difficult.
Over an estimated 10, generations, the sticklebacks show structural differences that are greater than those seen between different genera of fish including variations in fins, changes in the number or size of their bony plates, variable jaw structure, and color differences.
This masks possible initial divergence in allopatry and can indicate a "mixed-mode" speciation event—exhibiting both allopatric and sympatric speciation processes.
From this, if a recently separated population comes in contact again, low fitness hybrids may form, but reinforcement acts to complete the speciation process. Mathematical models[ edit ] Developed in the context of the genetic basis of reproductive isolation, mathematical scenarios model both prezygotic and postzygotic isolation with respect to the effects of genetic drift, selection, sexual selectionor various combinations of the three.
Masatoshi Nei and colleagues were the first to develop a neutralstochastic model of speciation by genetic drift alone. Both selection and drift can lead to postzygotic isolation, supporting the fact that two geographically separated populations can evolve reproductive isolation : Mathematical models concerning reproductive isolation-by distance have shown that populations can experience increasing reproductive isolation that correlates directly with physical, geographical distance.
Other models[ edit ] Various alternative models have been developed concerning allopatric speciation. Special cases of vicariant speciation have been studied in great detail, one of which is peripatric speciation, whereby a small subset of a species population becomes isolated geographically; and centrifugal speciation, an alternative model of peripatric speciation concerning expansion and contraction of a species range.
Peripatric[ edit ] In peripatric speciation, a small, isolated population on the periphery of a central population evolves reproductive isolation due to the reduction or elimination of gene flow between the two. Peripatric speciation Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population. Given both geographic separation and enough time, speciation can result as a byproduct.
Speciation - Wikipedia
This promotes divergence due to strong selective pressures, leading to the rapid fixation of an allele within the descendant population. This gives rise to the potential for genetic incompatibilities to evolve. These incompatibilities cause reproductive isolation, giving rise to rapid speciation events.
Oceanic islands and archipelagos provide the strongest empirical evidence that peripatric speciation occurs.
This model contrasts with peripatric speciation by virtue of the origin of the genetic novelty that leads to reproductive isolation. These isolated populations will contain samples of the genetic variation from the larger parent population. This variation leads to a higher likelihood of ecological niche specialization and the evolution of reproductive isolation. Microallopatry refers to allopatric speciation occurring on a small geographic scale.
Rico and Turner found intralacustrine allopatric divergence of Pseudotropheus callainos Maylandia callainos within Lake Malawi separated only by 35 meters. Smith to describe a level of geographic resolution.
A sympatric population may exist in low resolution, whereas viewed with a higher resolution i. Ecological speciation Ecological speciation can occur allopatrically, sympatrically, or parapatrically; the only requirement being that it occurs as a result of adaptation to different ecological or micro-ecological conditions.
A cladogram of species in the Charis cleonus group superimposed over a map of South America showing the biogeographic ranges or each species. As allopatric speciation is widely accepted as a common mode of speciation, the scientific literature is abundant with studies documenting its existence. The biologist Ernst Mayr was the first to summarize the contemporary literature of the time in and Allen Orr 's publication Speciation. They list six mainstream arguments that lend support to the concept of vicariant speciation: