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Abstract

Taking an experimental approach to paleoecology, we evaluated the prey portion of Van Valen's Red Queen hypothesis by retrieving diapausing eggs from lake sediments for laboratory tests of evolutionary responses. The sediments consisted of core samples from Portage Lake (Michigan), which were dated by a combination of varve counts and radioisotope (Cs-137 and Pb-210) techniques. The sedimentary record of Portage Lake documented several major environmental changes (mining, channeling, post-1950s eutrophication) over the past century. Remains of Leptodora, Chaoborus, and Polyphemus from sediments and historic fish surveys suggested a changing balance of invertebrate and vertebrate (fish) predators. Daphnia retrocurva was present in Portage Lake for over 80 yr and replaced Daphnia dentifera during the eutrophication period. Against the background of environmental change, we tested a part of the Red Queen hypothesis (i.e., that the prey species D. retrocurva must be continually evolving relative to its primary invertebrate predator, just to remain in place). We examined D. retrocurva morphology and genetics through time by retrieving and hatching diapausing eggs from, different sediment strata for genetic characterization and by common garden experiments (i.e., isolates reared under the same environmental conditions). Mitochondrial DNA 12S/16S analyses and allozyme electrophoresis were used to characterize hatchlings from various levels. Allele frequencies at one allozyme (Pgi) locus in D. retrocurva differed over the eutrophication phase, suggesting founder effects, natural selection, or both. Common garden experiments with D. retrocurva hatchlings documented significant microevolutionary adjustments in both helmet and spine lengths, supporting the hypothesis of continual evolution in prey against a changing balance of predation.