Natural Selection

The theory of evolution by natural selection has three main components: phenotypic variation, differential reproductive success, and inheritance. Phenotypic variation and inheritance were discussed in the previous section. Consider now some rules governing natural selection.

Natural selection is usually identified with reproductive success—the ability of organisms to reproduce and pass their genes onto the next generation. This is referred to as Darwinian or direct fitness because it involves maximization of direct descent—offspring. Notice that natural selection is not defined here in terms of survival value, as in the famous but misleading phrase, “survival of the fittest” (Spencer, 1855). Survival is generally regarded as secondary in importance to reproductive success. However, other things being equal and within a given species, animals that live longer tend to bear more offspring than animals that live shorter lives (Stearns, 1992).

William D. Hamilton (1926-2000) proposed the view that there are ways of “passing on” genes across generations that do not involve direct descent (Hamilton, 1964). For example, helping behavior that increases the reproductive success of a genetically related organism (a relative) can lead to the spread of the helper’s genes, even if the helper’s own direct fitness is reduced (termed indirect fitness'). Examples of helping behavior directed at relatives include the breeding behavior of scrub jays and the antipredator behavior of California ground squirrels. Because indirect fitness implies interaction among conspecifics, it is important to understand social evolution, and this will be described in detail in Chapter 11 (see references to kin selection).

An organism’s inclusive fitness is equal to the sum of its direct fitness (number of offspring) plus its indirect fitness (offspring of relatives surviving because of the organism’s helping behavior weighed by the degree of genetic relatedness). For the moment, however, let us concentrate on direct fitness.

Types of Direct Fitness

According to Darwin’s theory of evolution by natural selection, the reproductive success of an organism is measured in terms of the number of offspring it can contribute to the next generation, compared to the offspring produced by conspecifics. Because individual organisms cannot reproduce ad libitum but are constrained by limited resources, fitness is always understood in relative terms. Alternative traits (presumably based on alternative alleles of the same gene), expressed in different organisms within a population, can convey differential fitness. Such relative differential fitness determines the evolutionary fate of alternative traits— the one correlated with a greater fitness spreads in the population over many generations, while the one correlated with lower fitness decreases in frequency or entirely disappears. Because most organisms reproduce either continuously or during several breeding seasons, their direct fitness must be measured not just in terms of a single breeding episode, but in relation to their entire reproductive life—lifetime reproductive success (LRS).

Three different types of direct fitness can be recognized: (1) traits that contribute to LRS by improving survival; (2) traits that may not contribute directly to LRS, but are correlated with traits that enhance LRS (these two cases are covered later in this section); and (3) traits that evolved through sexual selection. Sexual selection refers to the evolution of traits that contribute to LRS either by improving intra-sexual competition (e.g., male-male competition for access to females) or by affecting the choice of sexual partner (see Chapter 11).