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Yet another way to maintain variation by selection is through

# Yet another way to maintain variation by selection is through

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Yet another way to maintain variation by selection is through frequency dependent selection . If an allele's fitness is not constant but increases as it gets rare this will drive the allele back to higher frequency. See figure 5.9, pg. 121. Example: allele may give a new or distinct phenotype that predators ignore because they search for food using a "search image" (e.g., I like the green ones). Most (by no means all) evolutionary biologists believe that selection plays a major role in shaping organic diversity, but it is often difficult to "see" selection. One reason is that selection coefficients can be quite small (1-s ~1) so the response to selection is small. When selection coefficients are large p can be large, but the problem here is that with directional selection fixation is reached in a few generations and we still
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Unformatted text preview: can't "see" selection unless we are lucky enough to catch a population in the middle of the period of rapid change. What affects the rate of change under selection? Recall that ∆ p = p t+1- p t ∆ p = [(w AA p2 + w Aa pq)/(w AA p2 + w Aa 2pq + w aa q2)] - p . With some simple algebra we can rearrange this equation to: ∆ p = (pq[p(w AA- w Aa ) + q(w Aa- w aa )])/(w AA p2 + w Aa 2pq + w aa q2) Note that ∆ p will be proportional to the value of pq. This value (pq) will be largest when p=q=0.5 or, in English, when the variance in allele frequency is greatest. This is a simplified version of the main point of the fundamental theorem of natural selection modestly presented by R. A. Fisher....
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