In WEIT, I describe several studies showing that natural selection can change species over periods shorter than a human lifetime. These studies have been important in convincing skeptics (although not creationists, who will never be convinced) that natural selection is more than just a speculation, but a force that can really mold animal behaviors, appearances, and reproductive traits in real time. The most famous of these, of course, is the work of Peter and Rosemary Grant on a species of Darwin’s finch. Their work showed that individuals’ beak and body characteristics could change over a single generation when an El Niño event changed the finch’s food. (See The Beak of the Finch, by Jon Weiner, for a popular account of the Grants’ work.)
Now a group of researchers from Canada and the US have reported natural selection acting in guppies, molding their reproductive behavior over a period of only 8 years: roughly 13-26 guppy generations. This work is based on the observation that in Trinidad, the common guppy (Poecilia reticulata) lives in streams both above and below waterfalls. The guppy’s downstream predators cannot migrate past the waterfall barriers, so upstream guppies experience different environments from downstream guppies. In other words, downstream populations have to deal with much stronger predation.
This leads to some evolutionary predictions. The body of evolutionary theory called “life history theory” predicts that when an animal species experiences higher predation, it should evolve a different reproductive strategy. In short, guppies harassed by predators should reproduce earlier than un-predated guppies, for the greater your chance of being chomped, the less likely you are to leave offspring if you delay reproduction. Second, fish that experience less predation should produce larger embryos, since they have the luxury of delaying reproduction and because larger embryos make the newly hatched fish more competitive. Finally, low-predation fish should have fewer offspring in each bout of reproduction, for it is to their advantage to spread their given lifetime allotment of reproductive effort over a longer time.
In 1996, some of these authors introduced guppies from downstream, high-predation populations into uninhabited upstream, above-waterfall areas. Eight years later, in 2004, they took samples from both ancestral and derived populations to see if any differences in life history had evolved. And they did — in the predicted direction. Upstream guppies had fewer but larger embryos than downstream guppies, as well as a small reproductive allotment (egg mass as a proportion of body mass). Mark-recapture experiments on adults also showed that, when tested in the upstream environment, upstream-evolved guppies had higher survival than their downstream ancestors.
The analysis is arcane, but the results are clear: guppies have changed their life histories in an adaptive way in only eight years. This certainly reflects the action of natural selection, since previous studies have found similar results for life history, and also for color. (Guppies introduced to a low-predation regime evolve brighter colors in males; brightly colored males are favored everywhere by sexual selection but become disadvantageous in high-predator environments since they are more likely to be spotted and eaten.)
In toto, the guppy work is as powerful a body of evidence for selection, if not more so, than the work on finches. This is not to denigrate the finch study, which is brilliant. That work, however, was an uncontrolled “natural experiment” that affected two characters (bill and body size), while the guppy work has involved many groups of investigators doing controlled introductions — and all finding the predicted evolutionary changes in many characters. Birds, of course, are more charismatic, but the humble guppy has a lot to show us about evolution.
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S. P. Gordon et al. 2009. Adaptive changes in life history and survival following a new guppy introduction. The American Naturalist, Volume 174, pp. 34-45.
