Offspring Diversity Hypothesis
KeywordsSperm Competition Unpredictable Environment Multiple Male Offspring Diversity Subsequent Marriage
Females mix sperm from different males in order to increase the genetic diversity of their offspring.
If polyandry is a female strategy, it must benefit the reproductive success of females. By mating with more than one male, females may enhance their fitness through direct or material benefit, such as increased provisioning. The payoff to fitness may also be genetically based. One possible explanation for these genetic benefits is the offspring diversity hypothesis, which states that females mix sperm from different males in order to increase the genetic diversity of their offspring (Williams 1975). Sexually reproducing parents produce genetically variable offspring because recombination of male and female genes produces novel gene combinations (Willams 1975). Genetic contributions from multiple males will result in more novel gene combinations and even greater offspring genetic diversity. Such diversity can improve female fitness either by reducing sibling competition or by serving as a hedge against environmental uncertainty (Loman et al. 1988; Watson 1991). Watson (1991) suggests that females who are employing this strategy should (1) have low quality of information on male viability and (2) choose to mate with the most phenotypically unique males, regardless of their viability.
Benefits of Offspring Diversity
In some species, more genetic diversity in a sibling set decreases between-sibling competition for resources and increases offspring survival (e.g., Aguirre and Marshall 2011). If genetic variation among offspring results in differences in morphology, physiology, or behavior, then genetic variation may also lead to variation in resource use, thus reducing competition for the same resources between members of the same clutch (Case and Taper 1986). Genetically diverse offspring may also result in increased fitness for their mother in unpredictable environments (Yasui 2001). Phenotypic differences will cause offspring to respond with variable success to changes in the environment, and a more diverse brood will increase the chance of at least some offspring surviving. Genetic and phenotypic diversity is less favorable in stable environments, however, which favor broods with the maximum number of individuals fit to the current environment (Yasui 2001).
Offspring Diversity Mechanisms
Some nonhuman females can obtain a diverse sperm pool by mating with multiple males within a short period of time, resulting in those males fertilizing the same clutch (e.g., Ridley 1993; Watson 1991). Other females – nonhuman and human – can also increase genetic diversity in offspring by mating with different males at longer intervals, resulting in subsequent pregnancies of which different males sire individual or clutches of offspring (Jennions and Petrie 2000). Humans achieve multiple mating in one of three ways: divorce and remarriage, extra-pair mating, or polyandry. The relationships between the men and women mate within subsequent marriages or extra-marital affairs have not been studied. However, we know it is very common for men in polyandrous relationships to be closely related to one another and for primary husbands to play a role in deciding who the subsequent husbands will be (Starkweather and Hames 2012). When a woman’s multiple mates are brothers or first cousins, it is unlikely that her offspring will be very genetically diverse.
Evidence in support of the offspring diversity hypothesis is difficult to achieve for all animals, but particularly for humans given ethical constraints (Scelza 2013). So far, no studies have compared the genetic quality of in-pair and extra-pair offspring among humans. Some studies are using proxy measures for quality, such as facial symmetry and major histocompatibility complex (MHC) compatibility, in order to determine if and when these traits are preferred by women. Higher MHC similarity between partners indicates genetic incompatibility, and women report more extra-pair desires and mating when they have greater MHC similarity with their long-term partner (Garver-Apgar et al. 2006). A true test of the offspring diversity hypothesis does not yet exist, however, and would entail comparing the genetic diversity within sibling sets and determining the effect of stable versus unpredictable environments. Given the nature of the husbands’ relationships to one another in most polyandrous, it is unlikely that tests of the offspring diversity hypothesis would find support within polyandrous unions.
- Williams, G. C. (1975). Sex and evolution. Princeton: Princeton University Press.Google Scholar