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Journal Article

The Adaptive Demography of Four Freshwater Pulmonate Snails

Kenneth M. Brown
Evolution
Vol. 33, No. 1, Part 2 (Mar., 1979), pp. 417-432
DOI: 10.2307/2407631
Stable URL: http://www.jstor.org/stable/2407631
Page Count: 16
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
The Adaptive Demography of Four Freshwater Pulmonate Snails
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Abstract

To understand the adaptive demography of four fresh water pulmonate snails, their population dynamics were followed for two years, field fecundity and survival rates were estimated, and water temperature and adult densities were manipulated in the laboratory. The species in temporary habitats have evolved flexibility in age at first reproduction in one case (L. palustris) and show many of the correlates of r-selection in the other (P. integra). The species in the more permanent habitat have evolved high fecundities (both L. stagnalis and P. gyrina) and long reproductive intervals with fecundity increasing with age (P. gyrina). I suggest that these higher fecundities may be adaptations to the lower survival rates indicated in the permanent habitat, due possibly to higher predation or parasitism rates. These contrasts among species in life history patterns remained under common laboratory conditions, suggesting both a genetic basis for differences in life history patterns and differing selection differentials among habitats. The laboratory experiments also indicated that increased water temperatures affected the life histories of all species except P. integra. Water temperature differences among habitats were not large enough, however, to explain among species variation in life history patterns. Laboratory density increments caused monotonic decreases in fecundity in L. stagnalis and P. integra, but maximal fecundities occurred at intermediate densities in L. palustris and P gyrina. Field adult densities were high and clumped dispersions present, however, only in the temporary habitats. Iteroparity has not evolved in these fresh water snails in spite of high juvenile mortality rates, implying that early, fecund reproduction is more adaptive.

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