UNIT 7

    SURVIVAL OF THE FITTEST

    1 On each tree there is a moth. They are both quiet clearly visible. Any predator

    would see his prey very clearly. But suppose the light moth was sitting on the light tree and vice versa. A dark moth on a dark tree would be less visible and

    have a better chance of survival from the attacks of predators. This is knows as camouflage. Some animals, like the chameleon, for instance, are able to change

    colour according to their background. This kind of change is not evolutionary

    change (though of course the chameleon has evolved this ability to make the change). However, a change in colour is evolutionary if the new creature is able to reproduce itself so that its young also have the new colour.

    2 The example of the moth is a real one and was investigated in England in the 1950’s by a scientist called Dr Kettlewell. It is a very well- known example of evolutionary change.

    3 The species of moth is the Peppered Moth. It was typically light brown in colour

    and settled on the trunks of trees which were a similar colour and camouflaged it. Then came a change in the environment. Industry began to grow up in parts of England with the result that smoke and other form of pollution began to fill the atmosphere.

    4 The pollution from the factories covered the bark on the tree trunks with soot

    and grime so the light brown Peppered Moths became very visible to their predators and were eaten. Then gradually they began to change colour. The darker ones were more likely to survive, so their colour gradually became darker. How did they become darker? This is one of the mysteries of science, but it has been called ‘natural selection’ since Charles Darwin published his famous book

    The Origin of Species in 1859. Natural selection does not make anything happen. You cannot force a moth to change colour, for instance, nor can a moth decide to change. The point is that every creature has a genetic structure consisting of

    genes and chromosomes. This structure can change naturally, by accident.

    Perhaps this change does not matter. Perhaps, on the other hand, it produces a ‘deformed’ individual which the others reject or even kill because it is different. These things happen all the time. But if the change (or ‘mutation’) happens to fit

    the new environment, then the new creature, instead of being rejected or killed by the others, will survive.

    5 This is natural selection. A mutation (which is always possible) happens to suit a new environment, and the ‘odd’ creature survives because it is better fitted. Then it reproduces and a new type of creature evolves. Meanwhile the others have become unsuited to the changed environment. They must either change their behaviour or become extinct.

    6 Dr kettlewell wanted to discover whether the dark Peppered Moths were in fact a new type of Peppered Moth which had adapted to its environment. In the first experiment, he released light and dark moths into the woods near Birmingham (a large industrial city in England). In the second he released his moths into the

    woods in a country district called Dorset in the south of England. Finally, he

    placed examples of each kind of moth on trees of the opposite colour and

    watched what happened.

    7 Here are the results from his experiment in Birmingham.

    light dark

    Number of moths released 201 601

    Number of moths recaptured 34 205

    Percentage of moths recaptured 16% 34.1%

    Dr kettlewell’s technique was to release moths which were specially marked, then recapture as many as possible after a time.

    8 Later, to find additional support for his hypothesis, Dr Kettlewell repeated the

    same experiment in Dorset. The result proved he was right:

     light dark

    Number of moths released 496 473

    Number of moths recaptured 63 30

    Percentage of moths recaptured 12.5% 6.3%

    9 Finally, he placed an equal number of each colour on trees and watched what

    happened. He quickly discovered that several species of birds searched the tree

    trunk for moths and other insects and that these birds more readily found the one

    that contrasted with its background than the one that blended with the bark:

     Moths eaten by birds

     light dark

    Unpolluted woods 26 164

    Polluted woods 43 15