The second trade-off is between quantity and quality of offspring, where quality is a function of parental investment in offspring and reflects its ability to survive and reproduce. The general expectation is that natural selection on offspring number and investment per offspring will tend to maximize the long-term production of descendents; this may be estimated by the number of offspring that survive to reproduce themselves during an organism’s lifetime (Smith and Fretwell, 1974) or if fertility affects the production and survival of grandchildren, by more distant effects.

Sexual reproduction, which most probably evolved as a means of increasing variability among offspring through the sharing of parents’ genetic material, complicates the trade-off between quantity and quality of offspring. This is because offspring share roughly equal amounts of their parents’ genetic material, yet parents may contribute unequally to their viability. In this sense, offspring may be considered as “public goods,” with each parent profiting from the investments of the other and having an incentive to divert resources to the production of additional offspring. This public goods problem tends to create conflicts of interest between the sexes (see Gangestad, this volume, for a treatment of such conflicts).

In fact, an almost universal by-product of sexual reproduction is the divergent evolution of the two sexes. Sex is defined by gamete size, and the sex with the larger gametes is called female. Larger gametes represent greater initial energetic investment in offspring. With increased investment beyond energy in gametes, the divergence between the two sexes is often exaggerated but may also balance or even reverse. For example, females provide all investment to offspring in greater than 95 percent of mammalian species, but males provide similar amounts or more total investments among most altricial birds, male brooding fish, and some insects, such as katydids (see Clutton-Brock and Parker, 1992, for a review).

To the extent that one sex invests more in offspring than the other, the one that does more investing sex is in short supply resulting in operational sex ratios greater than unity and competition for mates among members of the sex that does less investing. This public goods problem generates the third major trade-off: that between mating and parental effort. Sexual reproduction involves two components: finding a mate and achieving a mating, on the one hand, and investing in the resulting offspring to increase its viability, on the other.

To the extent that there are gains from specialization in the two components, one sex will evolve to produce many small highly mobile gametes specialized to mating, and another will evolve to produce fewer larger gametes, specialized for energetic investments in offspring. Trivers (1972) recognized that these differences in relative parental investment affect the structure of mating markets and the characteristics of the more and less investing sexes. The more investing sex is selected to be choosy about when