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Irrespective of population size. For many populations, more than a single migrant individual per generation is quite likely. Human populations (even isolated tribal populations) have a higher migration rate than this minimum value, and, as a result, no locus is known in humans for which one allele is fixed in some populations and an alternative allele is fixed in others. The effects of selection are more variable than those of random genetic drift because selection may or may not push a population toward homozygosity. Directional selection pushes a population toward homozygosity, rejecting most new mutations as they are introduced but occasionally (if the mutation is advantageous) spreading a new allele through the population to create a new homozygous state. 

Whether such directional selection promotes the differentiation of populations depends on the environment and on chance events. Two populations living in very similar environments may be kept genetically similar by directional selection, but, if there are environmental differences, selection may direct the populations toward different compositions. Selection favoring heterozygotes (balancing selection) will, for the most part, maintain more or less similar polymorphisms in different populations. However, again, if the environments are different enough, then the populations will show some divergence. The opposite of balancing selection is selection against heterozygotes, which produces unstable equilibria. 

Such selection will cause homozygosity and divergence between populations. 21.2 Multiple adaptive peaks We must avoid taking an overly simplified view of the consequences of selection. At the level of the gene—or even at the level of the partial phenotype—there is more than one possible outcome of selection for a trait in a given environment. Selection to alter a trait (say, to increase size) may be successful in a number of ways. In 1952, F. Robertson and E. Reeve successfully selected to change the wing size in two different populations of Drosophila. However, in one case, the number of cells in the wing changed, whereas, in the other case, the size of the wing cells changed. Two different genotypes had been selected, both causing a change in wing size. The initial state of the population at the outset of selection determined which of these selections occurred.

A simple hypothetical case illustrates how the same selection can lead to different outcomes. Suppose that the variation of two loci (there will usually be many more) influences a character and that (in a particular environment) intermediate phenotypes have the highest fitness. (For example, newborn babies have a higher chance of surviving birth if they are neither too big nor too small.) If the alleles act in a simple way in influenc- ing the phenotype, then the three genetic constitutions AB/ab, Ab/Ab, and aB/aB will produce a high fitness because they will all be intermediate in phenotype. On the other hand, very low fitness will characterize the double homozygotes AB/AB and ab/ab. What will the result of the selection be? We can predict the result by using the mean fitness of a population. 

As previously discussed, selection acts in most simple cases to increase. Therefore, if we calculate every possible combination of gene frequencies at the two loci, we can determine which combinations yield high values of the genome. Then we should be able to predict the course of selection by following a curve of increasing W. The surface of mean fitness for all possible combinations of allelic frequency is called an adaptive surface or an adaptive landscape (Figure 21-6). The figure is like a topographic map. The frequency of allele A at one locus is plotted on one axis, and the frequency of allele B at the other locus is plotted on the other axis. The height above the plane (represented by topographic lines) is the value that the population would have for a particular combination of frequencies of A and B. According to the rule of increasing fitness, selection should carry the population from a low-fitness “valley” to a high-fitness “peak.”