Guppies Evolve Their Spots in Experimentation

Guppies are popular freshwater aquarium fish. As with the pheasants we met in Chapter 3, the males are more brightly coloured than the females, and aquarists have bred them to become even brighter. Endler studied wild guppies (Poecilia reticulata) living in mountain streams in Trinidad, Tobago and Venezuela. He noticed that local populations were strikingly different from each other. In some populations the adult males were rainbow-coloured, almost as bright as those bred in aquarium tanks. He surmised that their ancestors had been selected for their bright colours by female guppies, in the same manner as cock pheasants are selected by hens. In other areas the males were much drabber, although they were still brighter than the females. Like the females, though less so, they were well camouflaged against the gravelly bottoms of the streams in which they live. Endler showed, by elegant quantitative comparisons between many locations in Venezuela and Trinidad, that the streams where the males were less bright were also the streams where predation was heavy. In streams with only weak predation, males were more brightly coloured, with larger, gaudier spots, and more of them: here the males were free to evolve bright colours to appeal to females. The pressure from females on males to evolve bright colours was there all the time, in all the various separate populations, whether the local predators were pushing in the other direction strongly or weakly. As ever, evolution finds a compromise between selection pressures. What was interesting about the guppies is that Endler could actually see how the compromise varied in different streams. But he did much better than that. He went on to do experiments.

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After the experiment had been running for five months, Endler took a census of all the ponds, and counted and measured the spots on all the guppies in all the ponds. Nine months later, that is, after fourteen months in all, he took another census, counting and measuring in the same way. And what of the results? They were spectacular, even after so short a time. Endler used various measures of the fishes' colour patterns, one of which was 'spots per fish'. When the guppies were first put into their ponds, before the predators were introduced, there was a very large range of spot numbers, because the fish had been gathered from a wide variety of streams, of widely varying predator content. During the six months before any predators were introduced, the mean number of spots per fish shot up. Presumably this was in response to selection by females. Then, at the point when the predators were introduced, there was a dramatic change. In the four ponds that had the dangerous predator, the mean number of spots plummeted. The difference was fully apparent at the five-month census, and the number of spots had declined even further by the fourteen-month census. But in the two ponds with no predators, and the four ponds with weak predation, the number of spots continued to increase. It reached a plateau as early as the five-month census, and stayed high for the fourteen-month census. With respect to spot number, weak predation seems to be pretty much the same as no predation, over-ruled by sexual selection by females who prefer lots of spots.

Spots are attractive to females, but also attractive to predators.