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huelsenbeck10notes - Lecture 10 Friday Reproductive...

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Lecture 10 Friday, March 20, 2009 Reproductive isolating mechanisms Prezygotic barriers: Anything that prevents mating and fertilization is a prezygotic mechanism. Habitat isolation, behavioral isolation, temporal isolation, mechanical isolation and gametic isolation are all examples of prezygotic isolating mechanisms. Some species of fruit flies in the genus Rhagoletis provide an example of habitat and behavioral isolating mechanisms. Different species are reproductively isolated because each species lays its eggs on different host species. Adults return to lay eggs on the hosts from which they emerged. Some species of fruit flies in the genus Drosophila are reproductively isolated because of mechanical incompatibility of their genitalia. Postzygotic barriers: Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult. The mule is a typical example. Reduced viability or fertility of hybrid individuals or reduced viability or fertility of the offspring of hybrid individuals are evidence of postzygotic reproductive isolation. Differences in chromosome number or arrangement of genes on chromosomes usually result in postzygotic isolation because chromosomes may not pair normally during mitosis or meiosis. The formation of new species requires some initial reduction of gene flow Two processes of speciation are thought to be important: allopatric speciation and sympatric speciation. Allopatric speciation: The first step in allopatric speciation is the restriction of dispersal between two or more populations that would otherwise freely interbreed. One possibility is that a geographic barrier such as a river or desert forms. The separation of two populations by such a barrier is called a vicariant event. A second possibility is that individuals from a source population colonize a new geographic area that is separated from the source population by a nearly complete barrier to gene flow. The establishment of a new population is a founder event. Both vicariant and founder events may reduce gene flow sufficiently that reproductive isolating mechanisms can evolve afterwards. Whether a geographic barrier leads to allopatric speciation or not depends on dispersal ability. A barrier may lead to speciation in some groups but not in others. For example, a river may be a barrier for a snake but not a bird.
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