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Lecture 34: "Social Interactions III"

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Biology 3466B
Yolanda Morbey

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Evolution Lecture No. 34: Social Interactions III th Wednesday December 5 , 2012 Frequency-Dependent Selection: -Frequecny-dependent selection occurs when the fitness of individuals with a particular phenotype depends on their frequency in the population. For example, elderflower orchids blossom as purple and yellow morphs and because neither flower type offers a pollinator reward, bumblebees alternate their visits between flowers of each colour in search of a reward. In a model explaining this, researchers found that relative male and female reproductive success of the morphs, as measured by relative pollinia removal (r = - 0.99), relative pollinia deposition (r = - 0.76), and fruit set (r = - 0.85), were all correlated significantly negatively with morph frequency across experimental arrays. -For all estimates of relative reproductive success, the identity of the morph that has the relative advantage reverses as the frequency of yellow plants increases. At both extreme frequencies, the rare morph has a significantly higher value for all indices of reproductive success considered, compared with the common morph. This shows that negative frequency-dependent selection, mediated through pollinator foraging preference, has the potential to maintain a color polymorphism in natural plant populations. Where the fitness of yellow flowers and purple flowers is equal, the evolutionarily stable strategy (ESS) states that natural selection will push towards this point (no individual can do better by adopting another strategy). The Prisoner’s Dilemma: -Assuming that T > R > P > S (as 5 > 3 > 1 > 0), the Prisoner’s Dilemma conveys that 2 players would do best if both cooperated (both would receive payoff R). However, this is vulnerable to invasion (not evolutionarily stable) by a defector, who would receive payoff T. If A assumes B will cooperate, the defection will give payoff T (vs. R); 5 vs. 3, and if A assumes B will defect, the defection will give payoff P (vs. S); 1 vs. 0. In a single encounter, the payoff for accusing the other person will always be greater than staying quiet. The
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