Evolution: A Very Short Introduction by Brian Charlesworth & Deborah Charlesworth

Author:Brian Charlesworth & Deborah Charlesworth [Charlesworth, Brian & Charlesworth, Deborah]
Language: eng
Format: epub
ISBN: 9780192526533
Publisher: OUP Oxford
Published: 2017-05-13T00:00:00+00:00

Natural selection and fitness

A fundamental idea in the theory of evolution under natural conditions is that some heritable character differences affect survival and reproduction. For instance, just as racehorses have been selected for speed (by breeding from winners and their relatives), so antelopes have been naturally selected for speed, because the individuals that breed and contribute to the future of their species are those that did not get eaten by predators. Darwin and Wallace realized that this kind of process could explain adaptation to natural conditions. Our ability to modify animals and plants by artificial selection depends on this characteristic having a heritable basis. Provided that there are heritable differences, successful individuals in the wild will likewise pass their genes (and thus often their good characteristics) to their offspring, which will, in turn, possess the adaptive characters, such as speed.

For brevity, and to allow one to think in general terms, the word fitness is often used in biological writing to stand for overall ability to survive and reproduce, without the need to specify which characters are involved ( just as we use the term ‘intelligence’ to mean a variety of different abilities). Many different aspects of organisms contribute to fitness. For instance, speed is just one feature affecting antelope fitness. Alertness and the ability to detect predators are also important. Mere survival is not enough, however, and reproductive abilities, such as provisioning and care of the young, are also important for fitness in animals, and the ability to attract pollinators is critical for fitness in flowering plants. The word fitness can accordingly be used to describe selection acting on a wide range of different traits. As with ‘intelligence’, the generality of the term ‘fitness’ has led to misunderstandings and disputes.

To know what characters are likely to be important for the fitness of an organism, one must understand a great deal about its biology and the environment in which it lives. The same character may give high fitness in one species, but not in another. For instance, speed is not important for fitness in a lizard that evades predators by cryptic coloration. If such a lizard lives in trees and perches on twigs, it is more important for it to be good at holding on than to run fast, and so short legs, not long ones, will be associated with high fitness. Speed is adaptive for antelopes, but staying very still, so as not to be detected by predators, is an alternative means by which many animals avoid being eaten. Other animals avoid predators by frightening them away; for example, some butterflies have eye spots in their wing patterns that can be suddenly displayed in order to alarm birds. Plants obviously cannot move, and avoid being eaten by different means, including tasting bad or being prickly. All these different characteristics may increase the survival and/or reproduction of the organisms, and hence their fitness.

Given genetic variability for many characters, and environmental differences, natural selection will inevitably operate, and the genetic make-up of populations and species will change over time, as we showed in Chapter 2.

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