Facultative sex ratio adjustment in natural populations of wasps: cues of local mate competition and the precision of adaptation.

Sex ratio theory offers excellent opportunities to examine the extent to which individuals adaptively adjust their behavior in response to local conditions. Hamilton's theory of local mate competition, which predicts female-biased sex ratios in structured populations, has been extended in numerous directions to predict individual behavior in response to factors such as relative fecundity, time of oviposition, and relatedness between cofoundresses and between mates. These extended models assume that foundresses use different sources of information, and they have generally been untested or have only been tested in the laboratory. We use microsatellite markers to describe the wild oviposition behavior of individual foundresses in natural populations of the parasitoid wasp Nasonia vitripennis, and we use the data collected to test these various models. The offspring sex ratio produced by a foundress on a particular host reflected the number of eggs that were laid on that host relative to the number of eggs that were laid on that host by other foundresses. In contrast, the offspring sex ratio was not directly influenced by other potentially important factors, such as the number of foundresses laying eggs on that patch, relative fecundity at the patch level, or relatedness to either a mate or other foundresses on the patch.

Cooperation and the scale of competition in humans.

Explaining cooperation is one of the greatest challenges for evolutionary biology. It is particularly a problem in species such as humans, where there is cooperation between nonrelatives. Numerous possible solutions have been suggested for the problem of cooperation between nonrelatives, including punishment, policing, and various forms of reciprocity. Here, we suggest that local competition for resources can pose a problem for these hypotheses, analogous to how it can select against cooperation between relatives. We extend the prisoner's dilemma (PD) game to show that local competition between interacting individuals can reduce selection for cooperation between nonrelatives. This is because, with local competition, fitness is relative to social partners, and cooperation benefits social partners. We then test whether nonrelated humans adjust their level of cooperation facultatively in response to the scale of competition when playing the PD for cash prizes. As predicted, we found that individuals were less likely to cooperate when competition was relatively local. Cooperation between humans will therefore be most likely when repeated interactions take place on a local scale between small numbers of people, and competition for resources takes place on a more global scale among large numbers of people.

Evolution: revenge of the clones!

Recent work on ants shows both extraordinary patterns of reproduction and a new type of sexual conflict, leading to the remarkable scenario where females have no father and males have no mother.