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As the guppies became more abundant in the low-predation habitat, they faced more competition. It turns out that high guppy densities—and the subsequent depletion of resources—is the main driver of guppy evolution, not the higher probability of survival in the low-predation communities (Reznick et al., in review). Moreover, as the guppies evolved, they further influenced the environment, affecting not only individuals in their own population, but all the other organisms in the community, which were also evolving.