COMPARATIVE LIFE HISTORY EVOLUTION IN THE WATER STRIDER, GERRIS REMIGIS (QUANTITATIVE GENETICS, DEMOGRAPHICS, PENNSYLVANIA)
Demographic theories of life history evolution predict that population mortality patterns can affect the evolution of reproductive traits. High levels of adult mortality have been predicted to select for high levels of reproductive effort (RE). I tested for correlations between adult mortality and RE by comparing two populations of the water strider Gerris remigis (Hemiptera: Gerridae), Study populations occur within 200 km of each other in Pennsylvania, USA. In central PA, striders complete a single generation per year (univoltine, site UV), in eastern PA, striders complete two generations each year (bivoltine, site BV). UV and BV are at the same latitude and environmental conditions during the reproductive season are similar. Voltinism differences result from the relatively long winters at UV.^ Mark-recapture studies (five years at UV, three years at BV) quantified demographic and survivorship differences. BV adults experienced higher levels of age-specific mortality with summer reproductives having the shortest life expectancies. Differences were assumed to reflect conditions in effect over evolutionary time. Thus, the primary assumption of the theory was satisfied.^ Predictions were tested using common-garden, field-enclosure rearing experiments. Parental striders were collected from both sites and mated within their respective demes. Experimental animals were conceived, reared and compared under common, field-simulated conditions. Reproductive differences that persisted in this common-garden were therefore genetically based.^ Populations were genetically different with respect to development time, age at sexual maturity, egg size, rate of egg production, total fecundity, length of reproductive season, and propensity to become reproductive under a variety of ecological conditions. Further, all differences were in the same direction; BV striders demonstrated a greater reproductive commitment to all traits, under all conditions. The predicted correlation between adult survival and relative levels of RE was demonstrated. Results were also consistent with the predicted role of food availability in life history evolution.^ Quantitative genetics techniques revealed the existence of significant levels of genetic variation within the BV, but not the UV population. Voltinism differences observed in the field were found to be genetically based, but mediated by food availability and photoperiodic cues. ^
MICHAEL J FIRKO,
"COMPARATIVE LIFE HISTORY EVOLUTION IN THE WATER STRIDER, GERRIS REMIGIS (QUANTITATIVE GENETICS, DEMOGRAPHICS, PENNSYLVANIA)"
(January 1, 1986).
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