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Modern Genetic Analysis Chapter 17. Population and Evolutionary Genetics
Multiple Adaptive Peaks

We must avoid taking an overly simplified view of the consequences of selection. At the level of the geneor even at the level of the partial phenotypethe outcome of selection for a trait in a given environment is not unique. 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 wing size in Drosophila in two different populations. 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.

The way in which the same selection can lead to different outcomes can be most easily illustrated by a simple hypothetical case. 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 influencing 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 selection be? We can predict the result by using the mean fitness

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