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The Origin of Species:
Speciation Processes
6 December, 2004
Text Chapter 24
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
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.
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.
Adaptive radiation in island chains is
an example of the results of repeated
dispersal and isolation
Sympatric speciation in
plants is often the result
of polyploidy.
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.
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.
Patterns of speciation may be complex.
Biology, Psychobiology, Biochemistry majors: Keep your text and
portfolio, they will be useful in other biology classes!