BIOB51H3 Lecture 20: Lecture 20.docx
14 Dec 2012
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Lecture 20- modes of selection and speciation: modes of selection. Directional, stabilizing, disruptive: species and speciation. Phylogenies branching from a common ancestor: directional selection. Fitness increases or decrease with changes in trait value. Measured using the selection differentials we looked at. Eg: alpine skypilote flower in the tundra and finch beak size during draught. Selection within one generation (not over several generation: stabilizing selection. The trait values in the middle have the highest fitness. The mean trait value has no change because it (the average) is what is being selected for. Eg: birth weight in humans mortality is lowest (so fitness is highest) when looking at the center of the distribution (average: disruptive selection. Selection on both tails of the distribution. Fitness is lowest at the average for the population. So after selection, the average (mean) stays the same but the population distribution is widened increase in variance. Or can lead to bimodal distribution (two different distributions)
