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Bio lecture notes 2

Bio lecture notes 2 - Bio March 4 2008 Mechanical Isolation...

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Bio March 4, 2008 Mechanical Isolation - Size and shape of reproductive organs may be crucial - Insects: great diversity in, particularly among males - Flowering plant morphology, small differences can prevent bees from transferring pollen from another plant Gametic Isolation - gamete = haploid (reproductive cell) - sperm of one species not attracted to eggs of another species - appropriate chemicals not released to allow sperm to penetrate egg - sperm might function poorly in reproductive tract of another species Behavioral isolation - lack of sexual attraction between sexes - unwelcome or “unattractive” courtship rituals - could be based on chemical signals - pheromones in humans, for example??? Types of postzygotic mechanisms - those that operate after mating has occurred - hybrid zygote abnormality - hybrid infertility - low hybrid viability Hybrid zygote abnormality - zygotes fail to mature normally - abnormalities may develop that prevent successful mating - may die during development—lethal genes Hybrid infertility - hybrids may mature normally, but be infertile when they attempt to mate - Recall example of mule: offspring of a female horse and a male donkey. - Mules are vigorous but sterile: produce no offspring. Low hybrid viability - Offspring have tougher time surviving - May do well during 1 st generation, but not in subsequent generations. Incomplete Reproductive Isolation - Contact is reestablished before genetic differences accumulate - Thus can interbreed freely and be perfectly successful - Gene pools can quickly combine and no new species are formed
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Geographical Hybrid Zones - Zone where species interbreed only when in contact in one area. - Example of warblers - Townsend warblers and hermit warblers hybridize - Both breed in conifer forests and no boundaries t hybrid zone - Narrow zone because of selection against hybrids - Hybrid zone shifting southward—Townsend warblers replacing hermit warblers —Why? - Townsend warblers: more aggressive - Townsend Warblers: better at attracting mates - Townsend warblers: ability to maintain territory Rates of evolution - rates vary from group to group, and within groups - mammals evolve slowly - Lungfish evolve even more slowly: show little change in the last 150 million years - Rates also vary from one period to another throughout history Why do rates vary?—1 - 1. Species richness: the larger the number of species in a lineage, the greater opportunities for new species to form - 2. Species range: the larger the geographical range, the great the likelihood that a physical barrier will divide it - 3. mobility differences: species with poor dispersal abilities are unlikely to move across borders and establish new populations Along a Texas street - even a street can act as a barrier to slowing the movement of a species why do rates vary?—2 - 4. Complex discriminations: organisms that make complex mate choice discriminations are more likely to speciate - use of multiple criteria: size, shape, general appearance, behavior - this is significant because mate selection is a major cause of evolutionary change
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Bio lecture notes 2 - Bio March 4 2008 Mechanical Isolation...

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