Survival of the fittest

Darwin had the idea that natural selection is the mechanism that drives evolution after reading An Essay on the Principal of Population by Thomas Malthus, a clergyman and political economist. Malthus argued that, in time, the growth of human populations will outstrip the food supply, and that this will lead to “famine, pestilence, and war”. Darwin applied this idea to populations of other animals and of plants. In his book on the origin of species, Darwin wrote: “There is no exception to the rule that every organic being naturally increases at so high a rate that if not destroyed, the Earth would soon be covered by the progeny of a single pair”. In spite of reproducing quickly, no single species has completely over-run the planet, although the populations of some species may be increasing at any one particular time. Darwin concluded that populations are kept in check by a “struggle for existence” as they compete for limited resources and are exposed to disease. Environmental factors that keep populations in check are called selection pressuresor environmental resistances. These include:

· disease

· competition for resources such as food and a place in which to live

· predation

· lack of light, water, or oxygen

· changes in temperature.

Those organisms best suited to the environmental conditions, with

characteristics that give them an advantage in the “struggle for existence”, will have the best chance of surviving and producing offspring. Their high natality (birth rate) gives them a selective advantage. On the other hand, those will unfavourable characteristics are more likely to die. Their high mortality (death rate) gives them a selective disadvantage. Darwin argued that this difference in natality and mortality results in natural selection. As environmental conditions change certain characteristics within a randomly varying population are favoured, and natural selection occurs. This has become known as the “survival of the fittest”.

In evolution, fitnessis defined as the ability of an organism to pass on its alleles to subsequent generations, compared with other individuals of the same species. The “fittest” individual in a population is the one that produces the largest number of offspring that survive to reproduce themselves. Natural selection by “survival of the fittest” means that the genetic characteristics of a population gradually change from generation to generation in response to changes in the environment. As we shall see in the following spreads, natural selection affects a gene pool by increasing the frequency of alleles that give an advantage, and reducing the frequency of alleles that give a disadvantage. (A gene pool is all the genes and their different alleles present in an interbreeding population.)