Divergent evolution is the accumulation of differences between groups which can lead to the formation of new species, usually a result of diffusion of the same species to different and isolated environments which blocks the gene flow among the distinct populations allowing differentiated fixation of characteristics through genetic drift and natural selection. Primarily diffusion, the basis of molecular division, can be seen in some higher-level characters of structure and function that are readily observable in organisms. For example, the vertebrate limb is one example of divergent evolution. The limb in many different species has a common origin, but has diverged somewhat in overall structure and function.
Alternatively, "divergent evolution" can be applied to molecular biology characteristics. This could apply to a pathway in two or more organisms or cell types, for example. This can apply to genes and proteins, such as nucleotide sequences or protein sequences that derive from two or more homologous genes. Both orthologous genes (resulting from a speciation event) and paralogous genes (resulting from gene duplication within a population) can be said to display divergent evolution. Because of the latter, it is possible for divergent evolution to occur between two genes within a species.
In the case of divergent evolution, similarity is due to the common origin, such as divergence from a common ancestral structure or function has not yet completely obscured the underlying similarity. In contrast, convergent evolution arises when there are some sort of ecological or physical drivers toward a similar solution, even though the structure or function has arisen independently, such as different characters converging on a common, similar solution from different points of origin. This includes analogous structures.
J. T. Gulick (1832-1923) founded the usage of the term "divergent evolution" and of other related terms, which can vary slightly in usage from one researcher to the next. Furthermore, the actual relationships might be more complex than the simple definitions of these terms might imply. "Divergent evolution" most commonly applies when someone invokes evolutionary relationships; whereas "convergent evolution" refers to cases which evolve independently but result in similar structures and functions. Some scholars use the term "parallel evolution" to describe the appearance of a similar structure in closely related species, whereas "convergent evolution" primarily refers to similar structures in much more distantly related clades. For example, some might regard the modification of the vertebrate limb to become a wing in bats and birds as an example of parallel evolution. Vertebrate forelimbs have a common origin and thus, in general, show divergent evolution. However, the modification to the specific structure and function of a wing evolved independently and in parallel within several different vertebrate clades.
In complex structures, cases may occur where some aspects of the structures result from divergence and other aspects have their origins in convergence or in parallelism. In the case of the eye, researchers initially thought that different clades had different origins of the eye, but some now question this interpretation. It is possible that induction of the light-sensing eye during development might have diverged from a common ancestor across many clades, but the details of how the eye is constructed—and in particular of the structures that focus light in cephalopods and vertebrates, for example—might have some convergent or parallel aspects to it, as well.
Darwin's finches provide a good example of divergent evolution. They have over 80 varieties which all diverged from one original species of finch.[need quotation to verify] For another example of divergent evolution, note the organisms having 5-digit pentadactyle limbs - like humans, bats, and whales. They have evolved from a common ancestor but have become different due to environmental pressures.
The divergent evolution of wolves and domesticated dogs from a common ancestor - presumably the grey wolf - provides another example. Recent studies of the mitochondrial DNA of wolves and of domesticated dogs have found great divergence while also supporting the hypothesis that dogs descend from wolves.
Divergent species are a consequence of divergent evolution. The divergence of one species into two or more descendant species can occur in four major ways:
- Allopatric speciation occurs when a population becomes separated into two entirely isolated subpopulations. Once the separation occurs, natural selection and genetic drift operate on each subpopulation independently, producing different evolutionary outcomes.
- Peripatric speciation is somewhat similar to allopatric speciation, but specifically occurs when a very small subpopulation becomes isolated from a much larger majority. Because the isolated subpopulation is so small, divergence can happen relatively rapidly due to the founder effect, in which small populations are more sensitive to genetic drift and natural selection acts on a small gene pool.
- Parapatric speciation occurs when a small subpopulation remains within the habitat of an original population but enters a different niche. Effects other than physical separation prevent interbreeding between the two separated populations. Because one of the genetically isolated populations is so small, however, the founder effect can still play a role in speciation.
- Sympatric speciation, the rarest and most controversial form of speciation, occurs with no form of isolation (physical or otherwise) between two populations.
Species can diverge when a part of the species is separated from the main population by a reproductive barrier. In the cases of allopatric and peripatric speciation, the reproductive barrier is the result of a physical barrier (e.g. flood waters, mountain range, deserts). Once separated, the species begins to adapt to their new environment via genetic drift and natural selection. After many generations and continual evolution of the separated species, the population eventually becomes two separate species to such an extent where they are no longer able to interbreed with one another. One particular cause of divergent species is adaptive radiation.
An example of divergent species is the apple maggot fly. The apple maggot fly once infested the fruit of a native Australian hawthorn. In the 1860s some maggot flies began to infest apples. They multiplied rapidly because they were able to make use of an abundant food supply. Now there are two distinct species, one that reproduces when the apples are ripe, and another that continues to infest the native hawthorn. Furthermore, they have not only evolved different reproductive timing, but also now have distinctive physical characteristics.
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