Earthquakes due to other causes.
History of speciation In addressing the question of the origin of species, there are two key issues: Since Charles Darwin's time, efforts to understand the nature of species have primarily focused on the first aspect, and it is now widely agreed that the critical factor behind the origin of new species is reproductive isolation.
Why do species exist? Why is not all nature in confusion instead of the species being, as we see them, well defined? 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.
The isolated populations then undergo genotypic or phenotypic divergence as: When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging genes.
Island genetics is the term associated with the tendency of small, isolated genetic pools to produce unusual traits. Examples include insular dwarfism and the radical changes among certain famous island chains, for example on Komodo.
Though the finches were less important for Darwin, more recent research has shown the birds now known as Darwin's finches to be a classic case of adaptive evolutionary radiation.
Peripatric speciation In peripatric speciation, a subform of allopatric speciation, new species are formed in isolated, smaller peripheral populations that are prevented from exchanging genes with the main population.
It is related to the concept of a founder effectsince small populations often undergo bottlenecks. Genetic drift is often proposed to play a significant role in peripatric speciation. Parapatric speciation In parapatric speciation, there is only partial separation of the zones of two diverging populations afforded by geography; individuals of each species may come in contact or cross habitats from time to time, but reduced fitness of the heterozygote leads to selection for behaviours or mechanisms that prevent their interbreeding.
Parapatric speciation is modelled on continuous variation within a "single," connected habitat acting as a source of natural selection rather than the effects of isolation of habitats produced in peripatric and allopatric speciation.
Even if there is a gene flow between two populations, strong differential selection may impede assimilation and different species may eventually develop. Caucasian rock lizards Darevskia rudis, D.
A niche must be available in order for a new species to be successful. Ring species such as Larus gulls have been claimed to illustrate speciation in progress, though the situation may be more complex.View this research paper on Geology of the Great Lakes.
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