Traits Contributing to Survival

Multicellular organisms must grow from a fertilized egg before they are capable of reproduction. Therefore, many events have to occur for a healthy organism to develop and survive until reproductive age. Natural selection works intensely on the evolution of traits expressed before sexual maturity. Changes that affect genes expressed early in development can be eliminated if they lead to nonviable organisms. This differential survival and death of pre-reproductive organisms has resulted in the evolution of complex developmental programs involving the coordinated action of thousands of genes and intricate gene-environment interactions that produce viable individuals. However, as emphasized in this chapter, survival is important to the extent that it increases reproductive opportunities for the organism.

A classic example illustrating the evolution of traits that contribute to survival involves changes in protective coloration of peppered moths, Biston betularia, in England (Bishop & Cook, 1980). Light-colored individuals predominated until the tree trunks in some regions were darkened by industrial pollution. As light-colored individuals settled on the trunks, they were easily detected by birds that normally feed on these moths. Predatory pressures then led to a shift in the relative frequencies of these two traits and also of their underlying alleles, such that in industrial areas the dark-colored variety eventually predominated over the light-colored one, but in less industrialized areas the light-colored individuals continued to predominate. When air pollution came under the control of environmentally protective legislation, trees recovered their original coloration and light-colored individuals again spread in the population. Notice that this example illustrates only changes in allele frequencies, not the evolution of a new species. The process was driven by predatory pressures and involved an insufficient number of generations to lead to the evolution of different species.

Correlated Traits

A gene can exert its effects on several traits during development—this is called pleiotropy. If a trait dependent on this gene contributes significantly to LRS and is, thus, under strong positive selection, this could cause correlated traits also to evolve even if they do not themselves have adaptive significance. Darwin noted, for example, that pigeons with long beaks tend to have small feet, concluding that “if man goes on selecting, and thus augmenting any peculiarity, he will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of correlation” (Darwin, 1993/1859). It is commonly observed that animals artificially selected for a particular trait also exhibit changes in other traits across generations. For example, selection for increased developmental rate in body size causes correlated changes in the number and size of brain and liver cells in mice (Atchley,

Wei, & Crenshaw, 2000). Similar effects have been documented in natural populations. In the Galapagos ground finches, beak depth and beak width mentioned previously in this chapter are an example of correlated traits. Therefore, not every trait evolves by the action of natural selection on that particular trait.

Trait correlations imply that evolution can occur by altering entire sets of traits, rather than one trait at a time. Traits that spread because of correlation may provide some benefit when environmental conditions change. Such a trait may then be co-opted by selective pressures into playing a new function. The accumulation of a pigment, such as melanin, could result from strong selection of an enzyme that regulates some metabolic process. If external coloration becomes a factor in terms of LRS, as in the case of peppered moths cited previously, then coloration itself would come under the control of selective pressures. Traits that are available for selection in this manner are referred to as preadaptations and show the opportunistic nature of evolution.

Changes in entire patterns of traits can have important behavioral implications. Consider a simple example. If natural selection were to favor the evolution of increased body size in a mammalian lineage, perhaps because of reduced predatory pressure, this might trigger a variety of developmental changes. Since it would take longer to achieve a larger body size, patterns of parental care, social interactions, and the relative importance of environmental experience could change consequently. The entire adaptive pattern is called a life-history strategy; the word “strategy” suggests that traits evolve in a coordinated fashion. A number of life-history correlations have been demonstrated in mammals, although they can be taken only as broad trends (Eisenberg, 1981). For example, in mammals, increased body size is positively correlated with longevity and negatively correlated with the number of offspring per birth. The size of the brain also increases directly with body size. Consequently, larger-brained mammals tend to produce infants that remain attached to their parents for relatively longer periods.