Another series of calculations will allow you to quantify the effects of selection on the population. To perform these calculations you need the frequencies of each genotype both before and after selection. The values before selection can be actual counts or the expected values based on the Hardy-Weinberg formula. The values after selection are counts of those organisms left in the population after removal of the less well adapted individuals. As an example, consider the following population with two alleles (M and m).

	MM	Mm	mm
Before Selection:	100	150	50
After Selection:	80	130	30

First calculate the survival rate (w). This is the proportion of organisms of each genotype that survive after selection. Divide the number present after selection by the number that were present before selection for each genotype:

w_MM=80/100=0.80; w_Mm=130/150=0.87; w_mm=30/50=0.60.

Next compute the estimated fitness (l). Fitness compares each genotype to that with the greatest survival rate (w_Mm). Divide each of the genotype's survival rate by the maximum survival rate:

l_MM = w_MM / w_Mm = .80/.87 = 0.92

l_Mm = w_Mm / w_Mm = .87/.87 = 1.00

l_mm = w_mm / w_Mm = .60/.87 = 0.69

These values show that the most fit genotype is Mm, followed by MM, with the lowest fitness held by the mm genotype.

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