title: Does variation in the intensity and duration of predation drive evolutionary changes in senescence?
abstract: The evolutionary theory of senescence predicts that increased rates of extrinsic mortality select for faster declines in fertility and survival with age. One predicted mechanism is that increased mortality favours alleles that enhance fitness early in life at the expense of survival or reproduction later in life (antagonistic pleiotropy). We tested these predictions in natural populations of Daphnia ambigua from lakes that vary in the severity and duration of fish predation. Daphnia are found in lakes with (i) anadromous alewife (Alosa pseudoharengus) that migrate between marine and freshwater, (ii) permanent landlocked alewife and (iii) no alewife. Daphnia are rare year-round in 'landlocked lakes' and are seasonally eliminated from the water column in 'anadromous lakes' due to the very strong predatory impact of anadromous alewife on populations of zooplankton. Previous work has also shown that intense seasonal bouts of predation by anadromous alewife has selected for increased allocation towards growth and reproduction in Daphnia found in lakes with anadromous alewife. Such variation in the risk of mortality and the expression of life-history traits early in life provides the raw materials to test the evolutionary theory of ageing. We reared replicate populations of Daphnia from all three lake types and quantified lifetime rates of offspring production and intrinsic survival. We found no differences in age-related declines in fertility or survival. Daphnia from anadromous lakes produced significantly more offspring throughout their lifetime despite no differences in life span or in the number of reproductive bouts compared with Daphnia from lakes with landlocked and no alewife. The lack of divergence in ageing contradicts the prediction that elevated mortality rates drive evolutionary shifts in ageing. We reconcile these results by considering the predictions of theoretical frameworks that incorporate feedbacks associated with increased mortality such as density- and condition-dependent processes. Our results, which are better explained by a consideration of these processes, thus call for a greater consideration of models that more explicitly consider the ecology of focal organisms.
authors.affiliation: Department of Biology, University of Texas at Arlington, Arlington, TX, 76019, USA.
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Walsh
authors.affiliation: Department of Biology, University of Texas at Arlington, Arlington, TX, 76019, USA.
authors.forename: Deirdre
authors.initials: D
authors.lastname: Whittington
authors.affiliation: Department of Biology, University of Texas at Arlington, Arlington, TX, 76019, USA.
authors.forename: Melissa J
authors.initials: MJ
authors.lastname: Walsh
mesh_terms: D000375:Aging; D000818:Animals; D005075:Biological Evolution; D003237:Connecticut; D003621:Daphnia / Q000502:physiology*; D005298:Fertility; D005399:Fishes / Q000502:physiology*; D020387:Food Chain; D060106:Lakes; D008136:Longevity; D011235:Predatory Behavior; D012641:Selection, Genetic
keywords: Daphnia; alewife; antagonistic pleiotropy; life‐history evolution; senescence; trade‐off