Summary of Chapter Concepts

Reproduction can be sexual or asexual. Asexual reproduction can either be through vegetative reproduction or through parthenogenesis. Compared to asexual reproduction, sexual reproduction results in fewer copies of a parent’s genes in the next generation. This can be offset by adopting a hermaphrodite sexual strategy or by providing parental care that results in raising twice as many offspring. The benefits of sexual reproduction include purging harmful mutations and creating genetic variation to help offspring deal with future environmental variation.

Organisms can evolve as separate sexes or as hermaphrodites. If an individual possessing only male or female function can add a large amount of the other sexual function while giving up only a small amount of its current sexual function, selection will favor the evolution of hermaphrodites. If not, selection will favor the evolution of separate sexes. To avoid inbreeding depression, hermaphrodites have evolved adaptations to prevent selfing and mixed strategies for when selfing is the best option.

Sex ratios of offspring are typically balanced, but they can be modified by natural selection. Depending on the species, sex may be largely determined by genetics or by the environment. In many species, females have the ability to manipulate the sex ratio by controlling which sperm are used to fertilize the eggs or which sex chromosomes end up in the eggs, or by selective abortion of the fertilized embryos. In most organisms, the sex ratio is approximately one to one due to frequency-dependent selection. When offspring are isolated from the rest of the population and are subjected to local mate competition, highly skewed offspring sex ratios can be adaptive.

Mating systems describe the pattern of mating between males and females. While many species are socially monogamous, recent studies have demonstrated that many individuals participate in extra-pair copulations. As a result of this infidelity, species have evolved a variety of mate guarding behaviors to prevent a reduction in their fitness.

Sexual selection favors traits that facilitate reproduction. The difference in the energetic costs of gametes and the costs of parental care typically causes female fitness to be a function of mate quality and male fitness to be a function of mate quantity. As a result, females are typically selective in choosing mates whereas males compete strongly with each other to mate as often as possible. Male competition for mates has favored the evolution of sexually dimorphic traits including body size, ornaments, coloration, and courtship behaviors. Females choose particular males to obtain material benefits, such as nesting sites or food, or for nonmaterial benefits, such as good genes or good health. The best reproductive choices of males and females are often not reciprocal, which can cause conflicts between the sexes.