Competition theory predicts that population fluctuations can promote genetic diversity when combined with density-dependent selection1,2. However, this stabilizing mechanism has rarely been tested, and was recently rejected as an explanation for maintaining diversity in natural populations of the freshwater herbivore Daphnia pulex3. The primary limitation of competition theory is its failure to account for the alternative types of population cycles that are caused by size- or stage-dependent population vital rates—even though such structure both explains the fluctuating dynamics of many species4 and may alter the outcome of competition5. Here we provide the first experimental test of whether alternative types of cycles affect natural selection in predator–prey systems. Using competing Daphnia genotypes, we show that internally generated, stage-structured cycles substantially reduce the magnitude of selection (thereby contributing to the maintenance of genetic diversity), whereas externally forced cycles show rapid competitive exclusion. The change in selection is ecologically significant, spanning the observed range in natural populations3. We argue that structured cycles reduce selection through a combination of stalled juvenile development and stage-specific mortality. This potentially general fitness-equalizing mechanism may reduce the need for strong stabilizing mechanisms to explain the maintenance of genetic diversity in natural systems.
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We thank P. Hebert for his electrophoresis expertise and J. Fox for constructive comments on the original manuscript. The research was supported by grants from NSERC to E.M.
The authors declare that they have no competing financial interests.
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Nelson, W., McCauley, E. & Wrona, F. Stage-structured cycles promote genetic diversity in a predator–prey system of Daphnia and algae. Nature 433, 413–417 (2005). https://doi.org/10.1038/nature03212
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