Darwin, Charles Robert (1809–82)

3. Natural selection

The basic idea of evolution was accepted quite rapidly in the 1860s, although many people, scientists included, were unwilling to admit that the development of life on earth had no preordained goal. This meant that they were particularly worried about Darwin’s proposed mechanism of natural selection. In this theory, the species could not be regarded as having an underlying ’type’ on which all the individual organisms were modelled. The species was just the breeding population, and if the average structure of the individuals making up the population changed, then so did the species. The natural world could not be composed of a range of fixed, specific types or forms (see Species §2).

There was much debate over whether natural selection could be considered a vera causa or true cause capable of producing significant changes in species. Darwin knew that each component of the effect could be demonstrated individually, but doubted that selection could produce a new population incapable of interbreeding with the parental type within an observable timescale. For him, selection was a plausible mechanism of change, and probably the most important one – but he never ruled out the possibility of other mechanisms. Much of the initial opposition to the selection theory was based on an unwillingness to believe that the species does not have some sort of essence which prevents individuals becoming modified beyond a fixed range.

Darwin’s studies of domesticated animals convinced him that the individuals making up any population differ significantly among themselves. This is sometimes called ’random’ variation, because there seems to be no apparent purpose to the differences. Darwin knew that there was an underlying mechanism to explain the production of individual differences and their transmission through heredity, although his own explanation did not stand the test of time. Many of Darwin’s contemporaries found it difficult to accept that the variations which accumulate to give evolution could be produced by a process that imposed no fixed direction. They preferred to believe that the variations were directed along a predetermined channel by forces arising from within the organism, or were produced in response to the organism’s own purposeful activities. If characteristics acquired in response to new behaviour-patterns were inherited, this would give the rival evolutionary mechanism known as ‘Lamarckism’. Darwin himself accepted a subsidiary Lamarckian effect, but stressed the natural selection of random variation. Modern genetics has shown that acquired characteristics cannot be inherited and that mutations produce many different characteristics within a population. In the modern Darwinian theory, selection works by changing the frequencies of genes within the population (see Genetics).

Natural selection depends upon the elimination of those variant characteristics which are maladaptive, and the superior reproductive fitness of those which confer some adaptive benefit. Following Malthus, Darwin believed that the tendency for a population to breed beyond the capacity of its food supply would lead to a ’struggle for existence’ in which the least fit would be eliminated. This was what H. Spencer called the ’survival of the fittest’. Contrary to a popular anti-Darwinian argument, natural selection is not based on a tautology (the survival of those who survive), because fitness is defined in terms of the adaptation which enables an organism to live and breed more effectively. Darwin did, however, argue that there is a different mechanism, sexual selection, capable of enhancing those characteristics which, even if maladaptive, improve the organism’s chances of reproducing. Thus the peacock has developed its large tail because this characteristic has become involved with the birds’ mating behaviour – a bigger tail attracts peahens more effectively, and the reproductive advantage this confers outweighs the disadvantages of a large tail when escaping from predators.