N

which can be simplified by utilizing equations 6.2 and 6.3:

to represent each genotype by its proportion of the total population at any time t. When w = 1.0 the two genotypes have identical growth rates and the proportion of each genotype remains constant in the population through time. If w > 1.0 then the genotype in the numerator grows faster than the genotype in the denominator and it will represent a larger proportion of the population over time. Conversely, if w < 1.0 then the genotype in the numerator grows less rapidly than the genotype in the denominator and it will represent a shrinking proportion of the population over time. Using the A genotype as the standard of comparison and the absolute fitness values from Fig. 6.1, wA = 1.03/1.03 = 1.0 and wB = 1.01/1.03 = 0.981 and so the frequency of the A genotype is expected to increase over time.

The relative fitness can be used to determine the change in frequency of a genotype over time, as shown in Table 6.1. The change in genotype frequency is the

Table 6.1 The expected frequencies of two genotypes after natural selection, for the case of clonal reproduction. The top section of the table gives expressions for the general case. The bottom part of the table uses absolute and relative fitness values identical to Fig. 6.1 to show the change in genotype proportions for the first generation of natural selection. The absolute fitness of the A genotype is highest and is therefore used as the standard of comparison when determining relative fitness.

Genotype

Generation t

Initial frequency

Genotype-specific growth rate (absolute fitness) Relative fitness

Pt qt

Frequency after natural selection Generation t + 1

Initial frequency pt+1

Change in genotype frequency Generation t Initial frequency

Genotype-specific growth rate (absolute fitness) Relative fitness

Frequency after natural selection Generation t + 1

Initial frequency pt+1 Change in genotype frequency

PtwA

PtwA

0 0

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