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Divergent Evolution
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Divergent evolution, as represented in the Origin of Species
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Divergent evolution, as described in traditional evolutionary theory, is a relative phenomenon in which initially similar populations accumulate differences over evolutionary time, and so become increasingly distinct (i.e., they "diverge"). This process also known simply as "divergence," was described in the Origin of Species (1859) and was the subject of the single illustration (see figure right) contained in that book. Even before Darwin, Alfred Russel Wallace (1858) wrote of the "many lines of divergence from a central type."
In the views of both Darwin and Wallace, and thus of traditional evolutionary theory, divergence serves two purposes.
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It allows a given type of organism to survive in modified form by utilizing new niches;
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This increase in diversity supposedly boosts a habitat's carrying capacity.
But these are hypothetical propositions that would be difficult, if not impossible to demonstrate experimentally.
When evolution of this type is the sole focus — as has traditionally been the case within the context of standard evolutionary theory — evolutionary change is pictured as treelike, as in the figure above. There is a branching into distinct types, and the branches do not reunite.
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Molecular Divergence
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Within the context of molecular biology,
divergence is the percentage of nucleotides
differing between two related DNA segments
(or the percentage of amino acids differing
between two related polypeptides).
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However, when not only divergence, but also fusion events, such as hybridization and horizontal transfer, are taken into account, the topology of evolution becomes that of a web. Since evolutionary biologists now know that such fusion events do occur with significant frequency, the representation of evolutionary relationships as simple trees has been called more and more into question in recent years. There is a growing consensus that evolution has often been a reticulate process that involves the rejoining of divergent limbs.
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