Stevens, E. E., and R. H. Wiley. 1995. Genetic
consequences of restricted dispersal and incest avoidance in a
cooperatively breeding wren. Journal of Theoretical
Biology 175: 423-436.
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
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
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.,
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
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
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
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