Background on Guppy Research
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The Northern Range Mountains of Trinidad offer a natural laboratory for studying evolution in action. The rivers draining these mountains flow over steep gradients punctuated by waterfalls that separate fish communities. Species diversity decreases as waterfalls block the upstream dispersal of some species. The succession of communities is repeated in many, parallel drainages, providing us with natural replicates. Guppies are found in a diversity of habitats throughout this succession of communities. In the downstream localities they occur with a diversity of predators, which prey on adult size classes of guppies (high predation, or HP). Waterfalls often exclude predators but not guppies, so guppies found above waterfalls have greatly reduced risks of predation and increased life expectancy (low predation, or LP). The only other fish found in these localities rarely preys on guppies and, when it does, preys on small, immature size classes (Haskins, Haskins et al. 1961, Endler 1978).
Mark-recapture studies on natural populations revealed that HP guppies experience substantially higher mortality rates than LP guppies (Reznick, Butler et al. 1996). Predator-induced mortality represents a likely form of selection for the differences in life history, phenotype and behavior. Life history theory predicts that, in high predation environments natural selection should favor those individuals that begin to reproduce at an earlier age and devote more resources to reproduction. My first goal was to see if there were differences in the life histories of guppies from HP and LP environments that were consistent with these predictions.
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A HP site on the Paria River
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Wolf fish, Hoplias malabaricus, a guppy predator
Guppies and a killifish from the Naranjo River
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I characterized how guppies adapt to either HP or LP communities, with paired comparisons between guppies from HP and LP environments in different river drainages. HP guppies mature at an earlier age and devote more resources to reproduction, as predicted by life history theory. In addition, they produce more offspring per litter and produce significantly smaller offspring than LP guppies (Reznick and Endler 1982, Reznick, Rodd et al. 1996). There is a diversity of other adaptive differences between guppies from HP and LP environments. HP and LP guppies differ in male coloration (Endler 1978), courtship behavior (Houde 1997), schooling behavior (Seghers 1974, Seghers and Magurran 1995), morphology (Langerhans and DeWitt 2004), swimming performance (Ghalambor, Reznick et al. 2004), and diet (Zandonà, Auer et al. 2011). Laboratory studies confirm that all of the life history differences between HP and LP guppies have a genetic basis (Reznick 1982, Reznick and Travis 1996). Genetic analyses imply that HP guppies invade guppy free environments then evolve into LP phenotypes and that each river represents an independent replicate of this process (Alexander, Taylor et al. 2006).
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Rivers can be treated like giant test tubes, since fish can be introduced into portions of stream bracketed by waterfalls, creating in situ experiments (Endler 1978, Endler 1980). When guppies were transplanted from HP environments below a barrier waterfall to previously guppy-free environments above a waterfall, delayed maturation and reduced reproductive allocation evolved, with some changes happening in four years or less (Reznick and Bryga 1987, Reznick, Bryga et al. 1990, Reznick, Shaw et al. 1997). Other attributes, including male coloration (Endler 1980) and behavior (O'Steen, Cullum et al. 2002) also rapidly evolved. These results argue that the presence or absence of predators imposes intense selection on many features of guppy phenotypes. They also show that evolution by natural selection happens on a time frame that is similar to ecological interactions. This very rapid evolution adds some justification for considering Pimentel’s perspective of the relationship between ecology and evolution.
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Early mark-recapture work on an LP tributary to the Quare River
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However, differences in predation are confounded with differences in population biology. Guppy HP populations are found at lower population densities, are dominated by small, young fish, and have higher individual growth rates than LP populations. These differences in population structure are most likely attributable to indirect effects of predators, which reduce guppy population densities, and increase food availability to the survivors. For these and other reasons, it became reasonable to ask whether the way guppies adapted to LP environments might be in part adaptation to their impact on the structure of the environment.
Male guppies from the Paria River
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We have also obtained some results that are not consistent with traditional life history theory and suggest that ecological interactions might be important. One such result was our finding that high predation guppies do not begin senescence at an earlier age and have shorter life spans than low predation guppies, as one might predict based on their early life history (Reznick, Bryant et al. 2004). HP guppies are younger at maturity, have higher levels of reproductive effort throughout their lives, have lower mortality rates and longer lifespans than low predation guppies when the two are compared in a uniform laboratory environment. Such results suggest that the HP phenotype is unconditionally superior to the LP phenotype and that the LP phenotype should never evolve, yet it does so predictably when guppies are transplanted from HP to previously guppy free LP localities. The only models for the evolution of senescence that can be reconciled with such results are ones that include density regulation and/or indirect effects of predation (Abrams 1993, Charlesworth 1994, Williams and Day 2003).
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