Individuals may differ in fitness because of their underlying genotype

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Presentation transcript:

Individuals may differ in fitness because of their underlying genotype Incorporating Selection Individuals may differ in fitness because of their underlying genotype Genotype A1A1 A1A2 A2A2 Frequency p2 2pq q2 Fitness w11 w12 w22 Average fitness of the whole population: w = p2w11 + 2pqw12 + q2w22

Given variable fitness, frequencies after selection: Genotype A1A1 A1A2 A2A2 Freq p2 w11 2pq w12 q2 w22 w w w New allele frequencies after mating: p2 w11 pq w12 pq w12 q2w22 + + w w New Frequency of A1 New Frequency of A2

Fitness: Probability that one’s genes will be represented in future generations. Hard to measure. Often, fitness is indirectly measured: (e.g. survival probability given a particular genotype) WAA WAa Waa 1 1 1 + s Fitness is often stated in relative terms Selection coefficient gives the selection differential

Persistent Selection Changes Allele Frequencies (i.e. Evolution by Natural Selection) Strength of selection is given by the magnitude of the selection differential

Selection Experiments Show Changes in Allele Frequencies HW Cavener and Clegg (1981) Food spiked with ethanol

Selection can drive genotype frequencies away from Hardy Weinberg Expectations

Predicted change in allele frequencies at CCR5 High frequency (Europe) High selection/transmisson (Africa) Predicted change in allele frequencies at CCR5 High frequency (Europe) Low selection/transmisson (Europe) Low frequency (Africa) High selection/transmisson (Africa)

What is the frequency of A1 in the next generation? p2w11 + pqw12 pt + 1 = p2w11 + 2pqw12 + q2w22 What is the change in frequency of A2 per generation? After some algebra Dp = pt + 1 - pt = p / w (pw11 + qw12 - w ) With this equation we can substitute values for relative fitness and analyze various cases of selection.