Evol Ecol Res 19: 1-13 (2018)     Full PDF if your library subscribes.

Site-specific plasticity and demography determine litter size
in a population of white-footed mice

Douglas W. Morris

Department of Biology, Lakehead University, Thunder Bay, Ontario, Canada

Correspondence: D.W. Morris, Department of Biology, Lakehead University, Thunder Bay, ON P7B 5E1, Canada. email: douglas.morris@lakeheadu.ca


Background: Despite heritable variation and positive selection gradients, the mean number of young that a female produces often fails to increase through time. This paradox can be resolved by mapping fitness as a manifestation of heritable variation, direct and indirect selection on traits, and environmental interactions with demography.

Questions: Does environmental variation in time and space determine the outcome of selection on litter size? Is the pattern of variation consistent with our understanding of the fitness map?

Study organism: White-footed mouse, Peromyscus leucopus, reproducing in nest boxes in southern Ontario, Canada.

Methods: I analysed 35-years’ data on litter-size variation at two nearby study sites and interpreted the results in the context of the fitness-mapping function.

Analyses: Linear regression, mixed models, generalized linear models, model selection, mean-standardized selection gradients.

Results: There was no detectable cost of reproduction. Mean annual litter sizes did not vary at one site where shallow selection gradients were associated with small spring litters. Those litters yielded higher recruitment than larger litters born in autumn. The other site was different: mean litter size increased marginally from year to year and recruitment was higher for large autumn litters than for smaller litters produced in spring.

Conclusions: Spatial variation in apparent selection on white-footed mouse litter size is associated with site-specific, demographically induced seasonal differences in recruitment success. Failure to incorporate such relevant environmental clines can lead to biased interpretations of selection and misunderstanding of the lock-step connection between ecology and evolution.

Keywords: fitness-mapping equation, habitat, litter size, Peromyscus, phenotypic plasticity, predation, reaction norm.

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