Chapter 11 - Units and Targets of Natural Selection

Abstract

A unit of selection is a level of genetic organization with sufficient stability across generations to respond to natural selection. It arises from the balance between fitness epistasis and recombination among its components. Most units display close linkage, but unlinked genes can form multilocus units under assortative mating and adaptation to a spatially varying environment coupled with restricted gene flow. The target of selection is the level of biological organization that manifests the phenotype under selection. The individual is often the target, but other targets also exist. Elements in the genome such as transposons can make copies of themselves and can be selected at the genomic level. Transposons also can have deleterious and beneficial effects at the individual level. These two targets of selection of transposons have helped shape the evolution of the human genome. Biased gene conversion, meiotic drive, and trinucleotide meiotic expansions are other targets of selection at the genome and gametic levels that often induce targets at the individual level. Selection can also occur between cells within a single individual as somatic mutations generate the necessary variation for selection. Some somatic mutations that are favored at the cellular level lead to cancer and are deleterious for the individual. The target of selection can also be more than a single individual. Two of the three fitness components, mating success and fertility, arise from interactions among individuals. Selection arising from interactions among individuals is almost always frequency-dependent; that is, the fitness assigned to a specific genotype depends upon the frequencies of interacting genotypes in the population. Frequency-dependent selection has many unusual properties: multiple equilibria, chaotic changes in allele frequency, and no maximization—even locally—of individual fitnesses or of average fitness. Only the average excess gives a reliable guide to the course of selection when interacting individuals are the target.