ABSTRACT
Can restricted dispersal in a family-structured population produce conditions that favor the evolution of helping behavior by kin selection? To address this question we developed a numerical model of a population of cooperatively breeding stripe-backed wrens (Campylorhynchus nuchalis). The model incorporated the demography and social dynamics of a natural population under long-term study in Venezuela. Replicated simulations followed uniquely labelled copies of alleles for 100 annual cycles, in order to estimate coefficients of kinship between mates and between helpers and the young they raised in relation to overall levels of inbreeding in the population. We investigated the consequences of restricted dispersal and incest avoidance, under different rates of migration. The results indicated that close incest occurred too infrequently to influence genetic structure significantly. Restricted dispersal, on the other hand, even in combination with relatively high rates of immigration (10-20%), like those observed in the natural population, tended to produce genetically viscous populations.		Relatedness between mates was significantly greater than 0, and relatedness among members of different groups decreased with distance. The relatedness between mates tended to increase both the relatedness of individuals to their own progeny and the relatedness of helpers to the young they raised. The latter effect predominated, so the genetic consequences of restricted dispersal, in relation to the overall level of inbreeding, tended to favor the evolution of helping by kin selection. Mutually reinforcing consequences of short-range dispersal, on the one hand, and delayed dispersal, on the other, could thus result in accelerating evolution of cooperative breeding.
CONCLUSIONS
The lessons from our simulations pertain to two wider evolutionary issues: possibilities for group selection and for rapid evolution through positive feedback. Selection at the level of groups of individuals has remained controversial, despite several theoretical treatments (Levin & Kilmer, 1974; Wilson, 1977, 1983; Uyenoyama & Feldman, 1981; Michod, 1982), because it remains uncertain whether the population structure required by these models occurs in nature (Wade & Bredon, 1987; Chepko-Sade et al., 1987). Most natural populations have higher immigration rates and larger population sizes than are consistent with these models (Templeton, 1987). Our simulations, however, show that significant genetic structure can result from social behavior even when immigration rates are high. The long-term perspective provided by our simulations also suggests that the genetic and social structures of populations might evolve in concert. In the case analyzed here, short-range dispersal generates conditions that favor the evolution of helping and a concomitant delay in dispersal. The crucial ecological constraint on the evolution of helping by kin selection is the marginal gain in a group's reproductive success as a result of the presence of helpers. By altering the genetic relatedness of individuals to their genealogical relatives, short-range dispersal reduces the marginal gain sufficient for the evolution of helping and delayed dispersal.		The converse also holds: delayed dispersal produces conditions that favor the evolution of short-range dispersal. Among stripe-backed wrens, the disappearance of a breeding female from a large group triggers intense competition among dispersing females to fill the vacancy. It is the oldest female auxiliaries from nearby territories, those that have delayed dispersal, that compete most effectively for these vacancies (Zack & Rabenold, 1989; Zack, 1990). If nearby females have an advantage over distant rivals, then delayed dispersal creates conditions that favor short-range dispersal. These reciprocal consequences of short-range dispersal and delayed dispersal are mutually reinforcing and thus could produce "runaway social evolution" (Wade & Bredon, 1987). Once started in this direction, evolution might quickly lead to socially complex and genetically viscous populations.

[ENTIRE ARTICLE (.pdf file)]