bild 3 lecture 12

bild 3 lecture 12 - accumulate as a result of chance...

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accumulate as a result of chance mutations, random drift, and selective pressures that are unrelated to the speciation process. It seems that, at least in Drosophila, this is a slower process than the accumulation of prezygotic mechanisms that are driven by selection for reinforcement. Note that in the Mimulus species we examined earlier, prezygotic isolation is provided by the preferences of the bees and hummingbirds. The two species are still interfertile, showing that postzygotic mechanisms have not accumulated. Is this because speciation has happened swiftly, as a result of rapid changes in pollinator preferences? Genetic investigation of the degree of DNA divergence between the species may provide the answer. Types of Speciation 1) Allopatric Speciation The term allopatric speciation comes from the Greek for “other country.” This type of speciation will occur when the environment is divided into geographically separate regions and the organisms in the different regions diverge from their common ancestor . Many Drosophila species on the Hawaiian islands have evolved from a few immigrants that have landed on new islands as they formed. The Hawaiian chain is continuing to form as the North Pacific tectonic plate moves westward over a hot plume of magma rising from the Earth’s mantle. Most of the islands have eroded down to sea level, but the Hawaiian Drosophila have migrated from island to island as old ones disappear and new ones appear.
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The picture below shows a picture-wing fly, which is very different from the Drosophila with which many of you are familiar (green arrow).
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The picture-wing fly above, compared with a more common Drosophila, is typical of the 800 species of diverse and behaviorally varied Drosophila that are found on the islands. The map below shows the pattern of migration of picture-wing flies among the islands. Once they arrived they began to evolve in different directions from the ancestral species on the older islands
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1 1 7 MAUI AND ITS NEARBY ISLANDS 2) Parapatric Speciation If there is partial but not complete separation of the gene pools in different parts of a species range, resulting from environmental differences, a second important type of speciation, parapatric (“next country”) speciation , can take place. Parapatric speciation is speciation that can occur in adjacent
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subpopulations even though there is still some gene flow between the gene pools of the subpopulations. The gene flow is not so great that it prevents the subpopulations from diverging from each other.
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This note was uploaded on 11/05/2008 for the course BILD BILD 3 taught by Professor Woodruff during the Spring '08 term at UCSD.

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bild 3 lecture 12 - accumulate as a result of chance...

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