The paradox of the visibly irrelevant

Natural History, Dec-Jan, 1997 by Stephen Jay Gould

1. Guppies from Trinidad. In many drainage systems on the island of Trinidad, populations of guppies live in downstream pools, where several species of fish can feed upon them. "Some of these species prey preferentially on large, mature-size classes of guppies." (I take all quotes from the primary technical article that inspired later press accounts. "Evaluation of the Rate of Evolution in Natural Populations of Guppies [Poecilia reticulata]," by D. N. Reznick, F. H. Shaw, F. H. Rodd, and R. G. Shaw, published in Science, vol. 275, 1977.) Other populations of the same species live in "upstream portions of each drainage," where most "predators are excluded ... by rapids or waterfalls, yielding low-predation communities."

In studying both kinds of populations, Reznick and colleagues found that "guppies from high-predation sites experience significantly higher mortality rates than those from low-predation sites." They then reared both kinds of guppies under uniform conditions in the laboratory and found that fishes from high-predation sites in lower drainages matured earlier and at a smaller size. "They also devote more resources to each litter, produce more, smaller offspring per litter, and produce litters more frequently than guppies from low-predation localities."

This combination of observations from nature and the laboratory yields two important inferences. First, the differences make adaptive sense, for guppies subjected to greater predation would fare better if they could grow up fast and reproduce both copiously and quickly before the potential boom falls -- a piscine equivalent of the old motto for electoral politics in Boston: vote early and vote often. On the other hand, guppies in little danger of being eaten might do better to bide their time and grow big and strong before engaging their fellows in any reproductive competition. Second, since these differences persist when both kinds of guppies are reared in identical laboratory environments, they must be based upon evolved and inherited distinctions between the populations.

In 1981, Peznick transferred some guppies from high-predation downstream pools into low-predation upstream waters then devoid of guppies. These transplanted populations evolved rapidly to adopt the reproductive strategy favored by indigenous populations in neighboring upstream environments: delayed sexual maturity at a larger size, and longer life. Moreover, Reznick and colleagues made the interesting observation that males evolved considerably more rapidly in this favored direction. In one experiment, males reached their fun extent of change within four years, while females were still evolving after eleven years. Since the laboratory populations had shown higher heritability for these traits in males than in females, these results make good sense. (Heritability is, roughly, the correlation between traits in parent and offspring due to genetic differences. The greater the heritable basis of a trait, the faster it can evolve by natural selection.)