As it should be clear by now, the fitness of a population or species is highly dependent on its life history strategy in relationship to its surrounding environment. Thus it makes sense that genes that maximize growth, reproduction, and survival would be favored by natural selection, resulting in an optimal life history strategy for that species or population. However, there are *constraints on optimality, resulting in trade-offs in life history traits that put limits on the possible life history outcomes for species.

Trade-offs include allocation of resources to such traits as growth versus reproduction or number of offspring versus their size. Energy devoted to one life history trait can reduce the amount available for others. For example, the growth of Douglas fir trees (Pseudotsuga menziesii) measured using annual growth ring size is negatively correlated with cone production. The more cones the tree produces, the narrower the width of the rings, suggesting that the allocation to reproduction takes energy from growth. Another trade-off that can have dramatic consequences for population growth is that of survival and reproduction. An example of this occurs in the giant Pacific octopus (Enteroctopus dofleini), which lives only 3–5 years and dies soon after significant parental investment in its thousands of offspring.

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As you have seen, much of the variation in life history traits appears to result from an evolutionary response to environmental variability. Generally, unpredictable environments are associated with greater allocation to reproducing quickly, shorter life spans, and correspondingly high per capita growth rates. Conversely, predictable environments, where organisms must compete for high-quality resources, are associated with lower fecundity, longer life spans, and correspondingly lower per capita growth rates. Species whose life history strategies allow for high population growth rates are called r-strategists, and species whose life history strategies allow them to persist at or near the carrying capacity (K) of their environment are called K-strategists (Figure 54.15). Keep in mind, however, that these categories are extremes; most species fall along a continuum between these two strategies.

For r-strategists, life is uncertain. Individuals tend to reproduce only once and to produce large numbers of offspring, most of which are expected to die early in life (that is, they have a type III survivorship curve; see Figure 54.9C). They can generally use a wide variety of resources and tolerate a wide range of conditions. K-strategists are adapted to predictable conditions, are long-lived, and reproduce several times; their smaller numbers of offspring have a high probability of surviving to adulthood (that is, they have a type I or II survivorship curve; see Figure 54.9A, B). K-strategists tend to be good at enduring competition for resources, but this sometimes leads to trade-offs in reproduction.

That life history trade-offs can evolve is suggested by genetic correlations among suites of life history traits. Such genetic correlations imply either simultaneous selection on two or more life history traits or *linkages among the genes that code for those traits. Across Drosophila melanogaster strains, for example, a high per capita rate of growth is correlated with the ability to reproduce under starvation conditions and with the ability to develop on a variety of media in the laboratory—both of which are consistent with the r strategy of tolerating a wide range of resources and conditions.