Experimentally induced life-history evolution in a natural population

Abstract

LIFE-HISTORY theory predicts that reduced adult survival will select for earlier maturation and increased reproductive effort; conversely, reduced juvenile survival will select the opposite^1#150;5. This is supported by laboratory studies^6–10 and comparative data from natural populations^11–15. Laboratory studies may support a theory, but cannot assess its importance in natural populations, and comparative studies reveal correlations, not causation¹⁶. Long-term perturbation experiments on natural populations resolve both problems. Here we report the findings of a long-term study of guppies (Poecilia reticulata), in which the predictions of life-history theory are supported. Life-history differences among populations of guppies are closely associated with predator species with which guppies live^13,17–21. The predators apparently alter age-specific survival because they are size-specific in their choice of prey^21–23. Crenicichla alta (a cichlid), the main predator at one class of localities, preys predominantly on large, sexually mature size classes of guppies^22–24. Rivulus hartii(a killifish), the main predator at another class of localities, preys predominantly on small, immature size classes. Guppies from localities with Crenicichla mature at an earlier age, have higher reproductive effort, and have more and smaller offspring per brood than those from localities with just Rivulus. These differences are heritable, and correspond with theoretical predictions^17–19. To prove that predation caused this pattern, we perturbed a natural population of guppies by changing predation against adults to predation against juveniles. This resulted in significant life-history evolution in the predicted direction after 11 years, or 30–60 generations.