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The Limits to Life History Evolution in Daphnia

Michael Lynch
Evolution
Vol. 38, No. 3 (May, 1984), pp. 465-482
DOI: 10.2307/2408697
Stable URL: http://www.jstor.org/stable/2408697
Page Count: 18
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Abstract

Using quantitative genetic techniques, the components of phenotypic variance and covariance for fitness traits were periodically determined for an intermittent population of Daphnia pulex and applied to phenotypic selection models to determine the limits to the response to selection. Levels of expressed genetic variance in this population are extremely high early in the year as a consequence of the release of hidden genetic variance via sex in the previous year. However, <25 generations of continuous clonal selection are required to depress the expressed genetic variance to levels expected under obligate parthenogenesis. A minimum of 10-20% selective mortality per generation is required to account for this erosion in genetic variance. Since Daphnia pulex populations generally consist of several closely related clonal groups that are distinct with respect to both phenotypic means and genetic covariance structure, selection between groups supplements clonal selection within groups as a mechanism for phenotypic evolution. Of particular interest in the study population is the coexistence of an obligately unisexual race with two cyclically parthenogenetic clonal groups. Despite the fact that nearly all members of the obligately unisexual race are electrophoretically identical, it contains substantial genetic variance for polygenic characters and clearly does not constitute an evolutionary dead end. The results of this study suggest that rates of phenotypic evolution in excess of ∼2% of the mean phenotype/generation are unlikely to occur in most cladoceran populations, especially those that have foregone sex for more than a few generations, without a substantial reduction in population size. This rules out the possibility of close tracking of seasonal variation in optimal phenotypes by clonal selection, and provides an evolutionary genetic explanation for the widespread use of cyclomorphosis in cladocerans as well as for the extreme sensitivity of lake plankton communities to novel perturbations. On the other hand, it is argued that populations that exhibit the greatest degree of evolutionary stability because of a lack of sex will also experience the most dramatic responses to selection following a rare recombinational event because the amount of hidden genetic variance converted to expressed genetic variance is proportional to the time between sexual phases.

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