American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“The population genetics of evolutionary rescue in diploids: X chromosomal vs. autosomal rescue”

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Robert L. Unckless and H. Allen Orr (Mar 2020)

Evolutionary rescue alters the dynamics of adaptation on the autosomes vs. sex chromosomes

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Probability of evolutionary rescue from new mutation on autosomes (black) and the X-chromosome (red); lines are analytical results, points are from simulation; vertical dotted line represents the dominance at which rescue from autosomes is more likely than from the X-chromosome; initial population size = 10,000, benefit of new mutation = 0.02, rate of population decline = 0.005,  mutation rate to beneficial allele = 10<span style="font-size:70%;   position:relative;   top:-0.3em;">−6</span>, 100,000 realizations.<br />(Credit: Robert L. Unckless and H. Allen Orr)
Probability of evolutionary rescue from new mutation on autosomes (black) and the X-chromosome (red); lines are analytical results, points are from simulation; vertical dotted line represents the dominance at which rescue from autosomes is more likely than from the X-chromosome; initial population size = 10,000, benefit of new mutation = 0.02, rate of population decline = 0.005, mutation rate to beneficial allele = 10−6, 100,000 realizations.
(Credit: Robert L. Unckless and H. Allen Orr)

Extinction is eventually inevitable for all species. However, when challenged by environmental changes (pesticides, climate change, new parasites, etc.), populations may be able to prevent extinction by adapting to these new conditions. This is essentially a race – the population must adapt before it goes extinct. Evolutionary rescue describes those cases in which adaptation wins the race and a population survives. Unckless and Orr explore the likelihood that evolutionary rescue occurs because of mutations on the sex chromosomes versus non-sex chromosomes. They find that the X chromosome is more likely to contribute to evolutionary rescue, all else being equal, than non-sex chromosomes under more conditions than when populations are not threatened by extinction. Unckless and Orr use both mathematical models and computer simulations to explore this problem.


Abstract

Most population genetic theory assumes that populations adapt to an environmental change without a change in population size. However, environmental changes might be so severe that populations decline in size and, without adaptation, go extinct. This “evolutionary rescue” scenario differs from traditional models of adaptation in that rescue involves a race between adaptation and extinction. While most previous work usually focused on models of evolutionary rescue in haploids, here we consider diploids. In many species, diploidy introduces a novel feature into adaptation: adaptive evolution might occur either on sex chromosomes or on autosomes. Previous studies of non-rescue adaptation revealed that the relative rates of adaptation on the X chromosome vs. autosomes depend on the dominance of beneficial mutations, reflecting differences in effective population size and the efficacy of selection. Here, we extend these results to evolutionary rescue and find that, given equal-sized chromosomes, there is greater parameter space in which the X is more likely to contribute to adaptation than the autosomes relative to standard non-rescue models. We also discuss how subtle effects of dominance can increase the chance of evolutionary rescue in diploids when absolute heterozygote fitness is close to one. These effects do not arise in standard non-rescue models.