Adaptations such as reproductive rates and longevity can best be understood from the perspective of individual selection. Life histo-ry traits are components of the fitness of individual genotypes, which is the basis for natural selection.
The major components of fitness are the age-specific values of survival, female fecundity, and male mating success. Natural selec-tion on morphological and other phenotypic characters results chiefly from the effects of those characters on these life history traits.
An organism allocates energy and resources among several functions, such as reproduction and survival. The trade-off between reproduction and survival, or cost of reproduction, prevents organisms from evolving indefinitely long life spans and infinite fecun-dity.
The effect on fitness of changes in survival (lx) or fecundity (mx) depends on the age at which such changes are expressed and de-clines with age. Hence selection for reproduction and survival at advanced ages is weak.
Consequently, senescence (physiological aging) evolves. Senescence appears to be a result, in part, of the negative pleiotropic ef-fects on later age classes of genes that have advantageous effects on earlier age classes. In addition, more deleterious alleles are ex-pressed at later ages.
Reproduction at a later age may maximize fitness if juveniles have high mortality, adults have high survival, and large body size greatly increases fecundity. Under these conditions, there can be selection for long life. In populations that are frequently growing in number, selection favors early reproduction and a short generation time. The life history of many species lies on a fast–slow con-tinuum, ranging from rapid maturation, short life, and numerous small offspring to delayed maturation, long life, and fewer but larger offspring.
The optimal number of offspring is affected by a trade-off between number and the size (mass) of each offspring, and by the opti-mal reproductive effort at that age—the parent’s allocation to reproduction versus continued survival.
Because lower fecundity and delayed reproduction can evolve, the intrinsic rate of population increase—the maximum rate of in-crease, which occurs at low density, may evolve to be lower. These features often evolve, especially in stable populations that are limited by resources and are not increasing anyway.
In addition to survival and reproduction, the life history of a species includes its ecological niche. Species vary in niche width—the range of conditions they tolerate or resources they use. Broad tolerance, often enabled by phenotypic plasticity, has some costs, such as lowered efficiency because of trade-offs between functions. Specialization may evolve because it increases efficiency or be-cause of relaxed selection for fitness in a relatively rare environment or habitat. Mutations that disable features adapted to rare envi-ronments may increase by genetic drift.