Spatial Variation

Consider a landscape where some habitats contain "source" populations producing an excess of births over deaths, with the excess forced into "sink" populations where deaths exceed births. We expect to observe a relatively good fit between phenotypes and environment in the source habitat, but a poorer fit between phenotypes and environment in the sink habitat. Models of adaptive evolution in sink habitats (e.g., Holt 1996a; Holt and Gomulkiewicz

Figure 11.4. Fitnesses of phenotypes relative to an optimal phenotype. With a flat-topped fitness function, phenotypes far from the optimum may have almost the same fitness as the optimal phenotype; thus, selection acts only weakly against deviations from the optimum. If the fitness function is sharply peaked, phenotypes close to the optimum have fitnesses much lowerthan the optimal phenotype. The text argues that the degree of maladaptation one might observe depends both on the shape of the fitness function and on the demographic context of selection.

Figure 11.4. Fitnesses of phenotypes relative to an optimal phenotype. With a flat-topped fitness function, phenotypes far from the optimum may have almost the same fitness as the optimal phenotype; thus, selection acts only weakly against deviations from the optimum. If the fitness function is sharply peaked, phenotypes close to the optimum have fitnesses much lowerthan the optimal phenotype. The text argues that the degree of maladaptation one might observe depends both on the shape of the fitness function and on the demographic context of selection.

1997a, 1997b; Kawecki 1995) suggest that it may be difficult for selection to sculpt adaptations to sink environments, particularly harsh ones, at least when most mutations have small fitness effects. Recent environmental change (e.g., human-caused habitat degradation) makes this sort of maladaptation likely. When a trade-off exists between performance in the sink and source habitats, foragers will remain adapted to the source habitat, and local maladaptation in the sink can persist indefinitely. Moreover, a behavior may be "maladapted" when examined in one local environment, but "well-adapted" when evaluated over the entire range of environments a population experiences (Brown and Pavlovic 1992).

A similar phenomenon emerges even with ideal free habitat selection, which implies that in a stable environment, no individuals will occupy sink habitats. Assume that in each generation, individuals settle in one oftwo habitats that differ in their carrying capacity. Assume also that they use rules ofthumb that create an ideal free distribution. When the population reaches demographic equilibrium, each habitat will be at its respective carrying capacity, so the fraction of individuals in habitat 1 will be p = K1/K1 + K2). Now imagine that a mutant arises, and that this mutant increases fitness in habitat 1 by a small amount, at a life stage after habitat selection occurs. The probability that an individual bearing this mutant is in habitat 1 is p. If K1 K2, then selection will be very weak, simply because the probability is low that the mutant will reside in the habitat where it can express its fitness advantage. This implies that drift will often eliminate weakly favored mutants that improve fitness in the rare habitat. Similarly, selection acts only weakly against deleterious mutants that reduce fitness in the rare habitat. In effect, selection will emphasize adaptation to the higher-K habitat at the expense of adaptation to the lower-K habitat.

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