If the egg-to-adult survival rates of genotypes A 1 A 1 , A 1 A 2 , and A 2 A 2

Question

If the egg-to-adult survival rates of genotypes A1A1, A1A2, and A2A2 are 90 percent, 85 percent, and 75 percent, and their fecundities are 50, 55, and 70 eggs per female, respectively, what are the absolute fitnesses (W) of these genotypes? Using A1A1 as the fitness reference, what are the relative fitnesses (w)? If the frequency of the A2 allele is p = 0.5, what will be its frequency one generation later? What will be the allele frequency when the population reaches equilibrium (stops evolving)?

Explanation / Answer

Absolute fitness = average fecundity X the probability of survival to adulthood

In the 2-generation examples, I've taken the average number of each offspring for each phenotype as starting point for next generation -- in other words, claculated number of grandchildren as absolute fitness.

The first scenario shows directional selection for A2; homozygote A2 has higher fitness than heterozygote and heterozygote has higher fitness than other homozygote; under this scenario A2 goes to fixation; there's no indication of disruptive selection that would allow A1 to persist.
In the 2-generation-per-year scenario (scenario 2), fitness differences are reversed in the 2nd generation, so this suggests some disruptive selection, which increases the likelihood of maintained genetic variation. However, I've calculated aper annum fitness (i.e., how many descendants at end of year per individual at beginning of year, and the higher fitness of A1 in 2nd generation over-rides the higher fitness of A2 in first, so, all else being equal, it looks like the net result would still be directional selection, in the opposite direction, taking A1 to fixation -- but it would take longer. (Also, of course, all else might NOT be equal -- if, for example, population sizes were different for the two generations, that could change things...)

This isn't changed in third scenario with different fecundities; even though fecundity contributions to fitness are exactly symmetrical now, the survival differences still reduce A2's advantage in first generation and enhance A1's advantage in second -- so probably still goes to A1 fixation, and probably faster

SCENARIO 1 SCENARIO 2 annualized annualized egg-adult ave.adult absolute relative adults ave absolute relative survival fecundity fitness fitness generation2 fecundity fitness fitness A1A1 0.9 50 45 0.86 45*.9=40.5 70 2835 1 A1A2 0.85 55 46.75 0.89 39.74 65 2582.94 0.91 A2A2 0.75 70 52.5 1 39.38 55 2165.63 0.76 SCENARIO 3 annualized annualized egg-adult ave.adult absolute relative adults ave absolute relative survival fecundity fitness fitness generation2 fecundity fitness fitness A1A1 0.9 55 49.5 0.88 44.55 75 3341.25 1 A1A2 0.85 65 55.25 0.98 46.96 65 3052.56 0.91 A2A2 0.75 75 56.25 1 42.19 55 2320.31 0.69

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