The evolutionary theory of senescence predicts that high extrinsic mortality in natural populations should select for accelerated reproductive investment and shortened lifespan. I examined the theory with natural populations of the Daphnia pulex-pulicaria species complex, a group of common freshwater crustaceans that spans an environmental gradient of habitat permanence. I document substantial genetic variation in life history traits among populations of this complex. Populations from temporary ponds have shorter lifespans, earlier and faster increases of intrinsic mortality risk and earlier and steeper declines in fecundity than populations from permanent lakes. I also examine the age-specific contribution to fitness, which declines faster in populations from ponds than those from lakes. Pond Daphnia also exhibit faster juvenile growth and higher early fitness, measured as population growth rate (r). I observed negative genetic correlations between r and indices of life history timing, further suggesting tradeoffs between early- and late-life performance.
Understanding the evolution of senescence in nature requires knowledge about genetic variation and ecological relevance of senescence plasticity because senescence is responds to factors such as food and temperature. Therefore, I also examined plasticity of senescence and variation of that plasticity within and between two species complexes of Daphnia. I quantified senescence in four environments (crossed high and low temperatures and food levels), to evaluate the plasticity of mortality and fecundity with respect to the ecology of the species’ habitats. Senescence was highly plastic, but species were plastic to different degrees. Population growth rate (r) was most responsive to food, but senescence was most responsive to temperature. Temperature strongly influenced the degree to which genetic differentiation within complexes was expressed, with little differentiation under temperature stress. In the D. pulex-pulicaria complex, whose species use markedly different habitats, genetic variation of senescence was strong and robust across environments. Species of the D. mendotae-dentifera complex, which inhabit stratified lakes, had similar senescence patterns. Most genetic variation of senescence occurred within, rather than between, species complexes, indicating that divergence of senescence in Daphnia is recent.
I further tested for a relationship between extrinsic mortality risk and ecological distribution of senescence variation by measuring natural demographic rates (growth, birth and death) in twelve populations of D. pulex-pulicaria. This work showed a negative relationship between ecological mortality risk and investment in late-life fitness. By constructing a phylogeny of these populations based on mtDNA, I was able to show that variation of senescence is not easily attributed to simple genetic history. My results cannot be explained by a tradeoff between survival and fecundity, nor by non-evolutionary theories of senescence. Instead, the data support the evolutionary theory of senescence because the genetic variation in life histories we observed is congruent variation of death rates experienced in the wild.