This chapter’s question: Does natural selection or genetic drift play a larger role in the origin of species?
A lovely table directly from the book summarizes definitions nicely (p. 384):
- Selection
- Direct: direct natural selection for reproductive isolation. Direct selection characterizes models of sympatric speciation and reinforcement. — Definitely not ubiquitous, especially in allopatric speciation.
- Indirect: Reproductive isolation arises as a pleiotrophic side effect of natural or sexual selection. –Probably more common than direct.
- Primary: Selection acts on a character and the same character ultimately causes reproductive isolation
- Secondary: Selection acts on a character and the genes involved pleiotrophically affect another character that ultimately causes reproductive isolation.
- Tertiary: Selection acts on a character and the linked genes that hitchhike along with those under selection ultimately cause reproductive isolation.
- Genetic Drift
- Neutral: reproductive isolation results from genes whose divergence was strictly or nearly neutral. Further explanation later in chapter: all alleles are equally fit and selection acts only in those rare instances when alleles that differ by two mutational steps segregate together in a population. Males and females that differ by 0 or 1 steps can interbreed, but those that differ by 2+ steps cannot. – Probably not common
- Peak shift: Reproductive isolation involves a period of maladaptive evolution, during which genetic drift must overcome selection. This class of model often involves founder-effects
Conclusions: Selection plays a much larger role in speciation than does drift. Firm evidence for genetic drift speciation is very rare and direct laboratory tests provide little or no support for founder-effect speciation. This makes sense for me. The modern theory of evolution *is* called evolution by natural selection, after all. It seems that natural selection would be much stronger and more common than genetic drift.
