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)?

Answer 1

Here are my calculations (I've calculated fitness on an 'egg to egg' basis â how many next-generation eggs is each egg likely to produce; multiply probability of survival to adulthood by average fecundity to get a per egg absolute fitness... Again, you could take somewhat different approaches). NOTE ALSO; 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, calculated number of grandchildren as absolute fitness. You could do this differently, too (e.g., calculate fitness for each generation separately and average them). Finally, I've essentially assumed clonal reprod, which you wouldn't have to do with some approaches...

SCENARIO 1

	egg-adult	average adult	absolute	relative
	survival	fecundity	fitness	fitness
A1A1	0.9	50	45	0.86
A1A2	.85	55	46.75	.89
A2A2	.75	70	52.5	1

SCENARIO 2

	adults	ave	absolute	relative
	generation2	fecundity	fitness	fitness
A1A1	45*.9=40.5	70	2835	1
A1A2	39.74	65	2582.94	0.91
A2A2	39.38	55	2165.63	0.76

SCENARIO 3

	egg-adult	ave.adult	absolute	relative
	survival	fecundity	fitness	fitness
A1A1	.95	55	.49.5	0.88
A1A2	0.85	65	55.25	0.98
A2A2	.75	75	56.25	1

SCENARIO 4

	adults	ave	absolute	relative
	generation 2	fecundity	fitness	fitness
A1A1	44.55	75	3341.25	1
A1A2	46.96	65	55.25	0.91
A2A2	42.19	55	2320.31	0.69

he 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 a per 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.