Natural selection on fecundity variance in subdivided populations: kin selection meets bet hedging

Abstract

In a series of seminal articles in 1974, 1975, and 1977, J. H. Gillespie challenged the notion that the "fittest" individuals are those that produce on average the highest number of offspring. He showed that in small populations, the variance in fecundity can determine fitness as much as mean fecundity. One likely reason why Gillespie's concept of within-generation bet hedging has been largely ignored is the general consensus that natural populations are of large size. As a consequence, essentially no work has investigated the role of the fecundity variance on the evolutionary stable state of life-history strategies. While typically large, natural populations also tend to be subdivided in local demes connected by migration. Here, we integrate Gillespie's measure of selection for within-generation bet hedging into the inclusive fitness and game theoretic measure of selection for structured populations. The resulting framework demonstrates that selection against high variance in offspring number is a potent force in large, but structured populations. More generally, the results highlight that variance in offspring number will directly affect various life-history strategies, especially those involving kin interaction. The selective pressures on three key traits are directly investigated here, namely within-generation bet hedging, helping behaviors, and the evolutionary stable dispersal rate. The evolutionary dynamics of all three traits are markedly affected by variance in offspring number, although to a different extent and under different demographic conditions.

Figure 1.—

Negative-binomial fecundity probability distribution for the number J of juveniles produced by an individual. The mean is given by f and the variance by f + f²/α. For all curves we have a mean of f = 10, while α = ∞ for the curve with the highest peak (i.e., Poisson distribution), α = 5 for the curve with the intermediate peak, and α = 1.5 for the curve with the lowest peak.

Figure 2.—

Evolutionary stable dispersal rate graphed as a function of deme size N with survival rate set to s = 0.9. The solid line is the classical ES dispersal rate obtained by assuming that the progeny distribution is Poisson or has an infinite mean. The dotted line (curve between solid and dashed curves) corresponds to the ES dispersal rate when while the dashed line corresponds to the ES dispersal rate when Fecundity distributions corresponding to such variances are given in Figure 1.