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lecture_notes_11_16_07 - • Traits that affect an...

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1 Johanne Brunet Biocore 301, fall 2007, Population Genetics LECTURE summary for November 16, 2007 Lecture 3: Learning objectives Continue impact of non-random mating on gene and genotype frequencies (see lecture notes for lecture 2 section on non random mating). Introduction to Selection Definitions Evolution: change in gene frequencies over time Selection: a mechanism by which evolution can occur Adaptation: change in a population with favorable characteristics accumulating over generations Selection Selection acts on phenotypes; Response to selection depends on additive genetic variance; Narrow sense heritability matters for selection; Artificial and natural selection; Conditions necessary before natural selection can occur : - Limited resources - struggle for existence: organisms produce more offspring than environment can support; Variation in traits among individuals; Heritable traits;
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Unformatted text preview: • Traits that affect an individual’s survival and/or reproduction. Natural selection • Natural selection is the differential success in reproduction and survival that results from the interaction between individuals that vary in heritable traits and their environment • Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions than others. Modes of selection • Directional selection: selection for one of the extreme of a trait; changes the mean of a trait. • Selection for higher crop yield • Disruptive selection: selects for both extremes. Changes mean and increases the variance; • Selection for beak lengths in Darwin finches • Stabilizing selection: does not change the mean, decreases the variance of a trait. • Selection for baby body weight in humans...
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