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

Abstract

Abstract.-We have previously reported a correlation between the life-history patterns of guppies and the types of predators with which they coexist. Guppies from localities with an abundance of large predators (high predation localities) mature at an earlier age and devote more resources to reproduction than those found in localities with only a single, small species of predator (low predation localities). We also found that when guppies were introduced from a high to low predation locality, the guppy life history evolved to resemble what was normally found in this low predation locality. The presumed mechanism of natural selection is differences among localities in age/size-specific mortality (the age/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 content analyses of wild-caught predators and on laboratory experiments. Here, we evaluate these assumptions by directly estimating the mortality rates of guppies in natural populations. We found that guppies from high predation localities experience significantly higher mortality rates than their counterparts from low predation localities, but that these higher mortality rates are uniformly distributed across all size classes, rather than being concentrated in the larger size classes. This result appears to contradict the predictions of the age/size-specific predation hypothesis. However, we argue, using additional data on growth rates and the probabilities of survival to maturity in each type of locality, that the age-specific mortality hypothesis remains plausible. This is because the probability of survival to first reproduction is very similar in each type of locality, but the guppies from high predation 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 patterns that provide a potential cause of natural selection, but expand, rather than narrow, the number of possible mechanisms responsible for life-history evolution in guppies.Key words.-Adaptation, life-history evolution, mark-recapture, mortality, selection, size-selective predation.Received September 26, 1995. Accepted September 26, 1995 Testing evolutionary theories in natural populations requires identifying the agent of selection, evaluating how the agent operates, then manipulating it and observing the resulting dynamics of evolutionary change (Endler 1986). Satisfying these criteria is rarely possible, so inferences are more often made less directly, such as with the combination of comparative studies and genetics that comprise the discipline of ecological genetics (e.g., Ford 1971). In initial studies (Reznick 1982;Reznick and Endler 1982), we found an association between predator fauna and the life-history attributes of their prey. ...

Demographic Consequences of Predators on Prey: Trait and Density Mediated Effects on Mosquito Larvae in Containers

“…However, alterations in prey traits may arise due to other processes that change phenotypes, which might include selection among individuals of different phenotypes [22]. For instance, a predator may alter the distribution of prey traits in a population via selective removal of some prey phenotypes, as demonstrated theoretically [23]–[24] and empirically [25]. Plastic responses may be reversible within an individual's life time (e.g., behavioral changes) whereas selective effects occur across generations if the traits are heritable.…”

Sex-Specific Differences in Shoaling Affect Parasite Transmission in Guppies

“…Guppies are small tropical fish and an important ecological and evolutionary model. They have been widely used to explore host adaptations to natural and sexual selection pressures (e.g., [15], [16], [17]). Guppies are sexually dimorphic, with males being more colourful and smaller than females, and there are marked behavioural differences between the sexes [18].…”

Both Geography and Ecology Contribute to Mating Isolation in Guppies

“…In contrast, the downstream reaches of rivers on the northern slopes of the mountains, which meet the Carribean sea, were colonized by piscivorous fish of marine origins, such as the mountain mullet (Agonostomus monticola) and the goby ( Gobiomorus maculatum ), whereas guppies in upstream sites again coexist with the R. haarti and another predator of minimal effect; freshwater prawns ( Macrobrachium sp.) [16], [17].…”

“…Regardless of the particular predator assemblage, high- and low-predation populations within a river show adaptive divergence in many traits, including male colour, behaviour, and life history [16]–[20]. This adaptive divergence has occurred independently in multiple drainages (i.e., parallel evolution), as inferred from patterns of geographical separation and genetic variation [16], [21], [22].…”

“…This adaptive divergence has occurred independently in multiple drainages (i.e., parallel evolution), as inferred from patterns of geographical separation and genetic variation [16], [21], [22]. Given this strong adaptive divergence, the theory of ecological speciation would predict that high- and low-predation guppies should show positive assortative mating [1].…”