Blackwell Publishing

Adaptive explanation - Why is the study of adaptation difficult?


Explaining the shell color of snails

Cepaea nemoralis (opposite) is a land snail, and the background color of its shell may be either yellow or some darker shade of brown or pink. On top of the background color are a number, usually between zero and five, of dark bands. The snails are therefore highly variable in their external appearance; an individual snail may have any combination of background color and banding pattern. Why do Cepaea vary in this way? One possibility is that it does not matter, and Mayr remarked in 1942 that

"There is no reason to believe that the presence or absence of a band on a snail shell would be a noticeable selective advantage or disadvantage."

The analysis of adaptation may be difficult, but not impossible.

So many selective factors have been shown to influence the pattern and coloration of Cepaea shells that 35 years after Mayrís remark, the polymorphism was called "a problem with too many solutions". A series of ecological geneticists, particularly Cain and Sheppard, have studied the question over many years. The first suggestive observation was that the proportions of the different shell types vary from place to place.

• Camouflage

Initially, the geographic distribution of the shell types had seemed to be random, but it was soon found that the banded forms tend to be found in diversified habitats, such as mixed hedgerows, and the unbanded snails against more uniform backgrounds, such as those of dense woodlands. Perhaps the banding pattern is an adaptation for camouflage. Cain and Sheppard duly measured the rate at which the different snail types were eaten by birds. Birds, such as the thrush (Turdus philomelos ), use stones as anvils to break open the snails, and it is possible to count the proportions of different snail types taken by the birds in the shell debris around an anvil; these proportions can then be compared with those in the local habitat. At one site near Oxford (UK), for example, where the habitat is relatively uniform, Cain and Sheppard found that 47.1% of snails were banded; but 56.3% of the snails taken by thrushes were banded.

The difference is statistically significant. In this habitat, the thrushes were taking the banded snails disproportionately. Similar results for other habitats also support the idea that differences in the color pattern and banding number serve as camouflage to protect the snails from bird predators.

• Thermoregulation

The appearance of Cepaea is also influenced by the need for thermoregulation. How dark a shell is affects how quickly the snail warms up in sunshine, and it can be a matter of life and death for a snail if it has too many bands and lives on a sunny hill-slope. It can no longer be doubted that the color and banding patters of Cepaea are adaptive, and adaptive in relation to more than one property of their local environments.

In 1942, as Mayr's remark shows, it was not obvious that shell pattern in Cepaea was an adaptation. It was certainly not obvious what it was an adaptation for. In order to find out why natural selection favored the particular characters possessed by Cepaea, Cain and Sheppard simply looked more closely at the character and noted what consequences it has in the animal's life. There are other methods too, and we go on to discuss them now.

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