Evolutionary Consequences of Asymmetric Dispersal Rates

Abstract

We study the consequences of asymmetric dispersal rates (e.g., due to wind or current) for adaptive evolution in a system of two habitat patches. Asymmetric dispersal rates can lead to overcrowding of the "downstream" habitat, resulting in a source-sink population structure in the absence of intrinsic quality differences between habitats or can even cause an intrinsically better habitat to function as a sink. Source-sink population structure due to asymmetric dispersal rates has similar consequences for adaptive evolution as a source-sink structure due to habitat quality differences: natural selection tends to be biased toward the source habitat. We demonstrate this for two models of adaptive evolution: invasion of a rare allele that improves fitness in one habitat but reduces it in the other and antagonistic selection on a quantitative trait determined by five additive loci. If a habitat can sustain a population without immigration, the conditions for adaptation to that habitat are most favorable if there is little or no immigration from the other habitat; the influence of emigration depends on the magnitude of the allelic effects involved and other parameters. If, however, the population is initially unable to persist in a given habitat without immigration, our model predicts that the population will be most likely to adapt to that habitat if the dispersal rates in both directions are high. Our results highlight the general message that the effect of gene flow upon local adaptation should depend profoundly on the demographic context of selection.

Does colonization asymmetry matter in metapopulations?

“…Models and concepts related to asymmetric dispersal arise mainly from evolutionary and genetic fields (Morris 1991;Kawecki 1995;Dias 1996;Case & Taper 2000;Lebreton et al 2000;Fraser et al 2001;Kawecki & Holt 2002). Methods providing indirect estimates of the number of migrants when gene flow is asymmetric are currently available (Beerli & Felsenstein 2001) and commonly applied (Imbert & Lefèvre 2003;Fraser et al 2004).…”

“…For the asymmetric case with only one-way connections, we classified patches into several types: isolated patches, recipient-only, donoronly, recipient-donor, donor-recipient, equally donorrecipient patches and isolated pairs of patches (figure 1). Note that we do not define these patches according to their ability to sustain a population or their demographic properties (Pulliam 1988;Morris 1991;Watkinson & Sutherland 1995;Kawecki & Holt 2002), but according to the kinds of connections entering and leaving a patch. Isolated patches have no connections in or out and do not count towards the count of patches, M, in the metapopulation.…”

Between migration load and evolutionary rescue: dispersal, adaptation and the response of spatially structured populations to environmental change

“…The two alleles at each locus take continuous values and are assumed to be co-dominant, with no epistatic or pleiotropic effects. Heritability is assumed to be 1; thus the value of the alleles directly determines an individual's phenotype, each making a small and equal contribution (as in [45,46] Offspring are generated by the mating of two individuals selected from the same patch, with a small probability of self-fertilization (1/local sub-population size). The number of offspring produced by each mated pair is drawn randomly from a Poisson distribution with mean R. An offspring's genotype is composed of a maternal and a paternal gamete, each of which is produced following recombination (outlined above).…”

Population sinks resulting from degraded habitats of an obligate life-history pathway

“…For sink populations, local reproduction is insufficient to balance mortality and populations only persist through immigration from more productive source populations (Pulliam 1988). Source-sink dynamics, therefore, can critically underpin the ecological and evolutionary feedbacks among populations (Kawecki and Holt 2002). The selection and use of poor habitats can be maladaptive and lead to ecological traps (Battin 2004), whereby a sink habitat is chosen over better alternatives, even when population densities are low (Gilroy and Sutherland 2007).…”