Evolution through natural selection
Evolution through natural selection

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Evolution through natural selection

3.4 Variation

Guppies vary in a number of characters; in particular, male guppies vary in the number, size and brightness of the coloured spots that decorate their bodies (Figure 2). This variation can be detected within a single population in a given stretch of stream, but is much more obvious when different populations, from different streams, are compared. Biologists working in Trinidad have shown that this variation is related to the presence of predatory fish. Male guppies from streams where predators are absent are much more brightly coloured than those from streams that contain predators.

Figure 2
Figure 2 A variety of guppies produced by selective breeding by aquarists. The two females in the foreground are relatively plain. The four brightly coloured males show variation in the number, size and colour of the spots on their bodies.

Question 2

Suggest an explanation, in terms of adaptation, for the relationship between the presence or absence of predatory fish in streams and the brightness of male guppies.?

Discussion

Bright colouration makes male guppies more conspicuous to predators. Thus, where predators are present, it will be the less colourful males that tend to survive and reproduce. Putting it another way, in streams where predators are present, males have evolved less bright coloration, an adaptation that reduces their risk of being eaten.

As we shall see shortly, the explanation given in the answer to Question 2 is supported by other observations. But it does beg an important question: 'why are male guppies brightly coloured at all?' It is quite common among animals that males are more brightly coloured than females (an example of sexual dimorphism;). The explanation for this is quite complex, but can be summarized briefly. In the majority of animal species, males are the more active partner in initiating mating behaviour and they perform a variety of behaviour patterns to attract the attention of, and stimulate, females. Commonly, females are more effectively attracted and stimulated by the most brightly coloured males, giving such males an advantage in terms of enjoying a higher mating success. For example, peacocks with the greatest number of 'eyespots' in their tails mate with more females than those with fewer eyespots. Likewise, male guppies with more brightly coloured spots are more attractive to females than are those with fewer spots.

The possibility that bright coloration makes male guppies more conspicuous to predators, and the observation that such males are more attractive to females, suggests that the evolution of coloration in male guppies must be seen as an example of a trade-off. In other words, there is a balance between the advantage that the more brightly coloured males experience in terms of enhanced mating success and the disadvantage they suffer in terms of increased predation risk. Moreover, the point of balance in this trade-off is likely to differ between streams or to shift over time in any one stream, depending on the presence or absence of predators. This example illustrates an important point about trying to explain specific characters of organisms in terms of adaptation. It is not sufficient to explain adaptations just in terms of their apparent advantages. Characters typically also involve costs of some kind and so the actual form of a particular character is the result of a trade-off between costs and benefits.

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