LIFE-HISTORY EVOLUTION IN GUPPIES (POECILIA-RETICULATA) .6. DIFFERENTIAL MORTALITY AS A MECHANISM FOR NATURAL-SELECTION

We have previously reported a correlation between the life-history pat terns of guppies and the types of predators with which they coexist. G uppies from localities with an abundance of large predators (high pred ation localities) mature at an earlier age and devote more resources t o reproduction than those found in localities with only a single, smal l species of predator (low predation localities). We also found that w hen guppies were introduced from a high to low predation locality, the guppy life history evolved to resemble what was normally found in thi s low predation locality. The presumed mechanism of natural selection is differences among localities in age/size-specific mortality (the ag e/size-specific mortality hypothesis); in high predation localities we assumed that guppies experienced high adult mortality rates while in the low predation localities we assumed that guppies experienced high juvenile mortality rates. These assumptions were based on stomach cont ent analyses of wild-caught predators and on laboratory experiments. H ere, we evaluate these assumptions by directly estimating the mortalit y rates of guppies in natural populations. We found that guppies from high predation localities experience significantly higher mortality ra tes than their counterparts from low predation localities, but that th ese higher mortality rates are uniformly distributed across all size c lasses, rather than being concentrated in the larger size classes. Thi s result appears to contradict the predictions of the age/size-specifi c predation hypothesis. However, we argue, using additional data on gr owth rates and the probabilities of survival to maturity in each type of locality, that the age-specific mortality hypothesis remains plausi ble. This is because the probability of survival to first reproduction is very similar in each type of locality, but the guppies from high p redation localities have a much lower probability of survival per unit time after maturity. We also argue for the plausibility of two other mechanisms of natural selection. These results thus reveal mortality p atterns that provide a potential cause of natural selection, but expan d, rather than narrow, the number of possible mechanisms responsible f or life-history evolution in guppies.