Patrickarbuthnot
Posts: 21
Joined: Feb. 2010
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| Quote (jeannot @ Mar. 11 2010,08:08) | On Margulis: I have not read her book, but from reviews I can see here and there, I can only concur with her critics. Her view on speciation is not supported by studies on natural populations.
I may expand on this topic, which have been my area of research for the past 4 years. At best, it may inform our friend Patrickarbuthnot and clear some early misconceptions on speciation (for example, the view that speciation rarely involves natural selection).
Contrarily to Margulis' opinion, there is considerable evidence that speciation involves genetic changes (point mutations or other genomic rearrangements) and wherever these changes have been pinpointed, they appeared to have been subject to selection. In many cases divergent adaptation to distinct ecological niches has been shown to contribute to reproductive isolation. Geographical isolation will permit the fixation of these changes, though it is not an absolute requirement if divergent selection is strong.
On the other hand, there very little (or no) evidence that genetic drift and bottlenecks induce reproductive isolation (via the "genetic revolutions" proposed by Mayr, among other models). The various founder-effect speciation models have failed all laboratory tests. These were proposed to explain the frequent events of peripratric speciation (in founding populations), but in the end, it seems that natural selection in the new colonized habitat is the main driver of reproductive isolation (aided by geographical isolation).
Speciation by deleterious chromosomal rearrangements does not appear very common. The "stasipatric" model of speciation has recently been discredited in the biological model where it was first proposed (Australian grasshoppers or something). But chromosomal speciation requires more investigation. It could act in mammals, but the only studied case I know is the house mouse. There may be a couple of other candidate models though.
Speciation by polyploidization is frequent in plants (especially ferns), but less common than the classical model (selected genetic changes in isolated populations) and rare in animals.
Overall, it seems that Margulis gives too much importance to endosymbiosis. Although it certainly enabled the colonization of new niches (in insects in particular), thereby contributing to biodiversity, a direct link to speciation is not demonstrated.
It is sad that Margulis seems to embrace controversial hypotheses without looking carefully at the available data. I am not judging from her book (which I haven't read), but from the recent fiasco at PNAS, where she communicated this ridulous paper advocating the hypothesis that caterpillars came from velvet worms. :( |
If you don't mind I will ask her that very thing in a e-mail and see what she says? I don't necessarily believe her hypothesis is correct I just am interested in her research.
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