The Origin of Species: Speciation Processes

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The Origin of Species Speciation Processes 6
December, 2004 Text Chapter 24
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Allopatric speciation occurs when two populations
of an ancestral species become geographically
isolated. Each population will then follow its
own evolutionary course, in the absence of gene
flow. Eventually, the two populations may become
two species, unable to interbreed even if the
geographical isolation is removed.
Sympatric speciation occurs when two populations
diverge without geographic isolation. Both of
these modes of speciation can result in
cladogenesis, or branching patterns of speciation.
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Isolated populations that are small are more
likely to diverge rapidly from the ancestral
form. The founder effect in the small splinter
population will lead to relatively large initial
differences. Until the splinter population
becomes large, these differences will be
magnified by genetic drift. The more different
the environment in which the splinter population
finds itself, the more likely natural selection
will drive additional divergence.
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Adaptive radiation in island chains is an example
of the results of repeated dispersal and
isolation
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Sympatric speciation in plants is often the
result of polyploidy.
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Speciation may be gradual or abrupt, but
punctuated equilibrium may be more important
gradualism in most radiations. Remember that
even abrupt events still require hundreds to
thousands of generations.
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Homeobox (Hox) genes control many of the steps in
animal development. Duplication and divergence
of these gene clusters allowed vertebrates to
evolve more complex developmental pathways.
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Patterns of speciation may be complex.
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