Selection, Natural

In Charles Darwin's theory of natural history, selection is the principal mechanism that is responsible for the evolution of life on Earth.

Three conditions must be met if natural selection shall occur in an ecological system. First, organisms living in the system must show phenotypic variations that bear upon their survival. Second, those variations must be heritable. Third, the environment must constrain the growth of the populations in the ecosystem by placing only limited feed resources at their disposal.

Whenever all three conditions are fulfilled, natural selection will occur among the organisms. They must then compete against each other for the scarce resources. The better an organism is adapted to its environment, the higher its chances are to get more resources and, finally, to have more offspring than its rivals. The number of adult offspring of an organism is called its “fitness.” The fitter an organism is, the higher the relative frequency of its genes in the genetic pool of its population is, so that, in the long run, some genes will be present only in a few organisms or will even totally disappear. Natural selection has sorted them out—and not some mysterious drive to perfection inherent in living nature (as supposed, for example, by Jean Baptiste de Lamarck's famous pre-Darwinian evolutionary theory).

The process of natural selection applies also to the biological origin ofHomo sapiens sapiens. Evolutionary anthropology must therefore recur to natural selection as an explanation for the genetic endowment of the first human beings.

That the history of life on Earth has to be explained by natural selection was claimed simultaneously and independently by papers of Charles Darwin and Alfred Russel Wallace, which were presented at a meeting of the Linnean Society in London on July 1, 1858. Both naturalists were inspired by Thomas R. Malthus's Essay on the Principle of Population (1798). They transferred Malthus's crucial idea that population growth is geometrical but food increase only arithmetical, from human to natural history. As a meticulous naturalist, Darwin thought over evolution by natural selection following his return from the voyage on HMS Beagle, but only after Wallace had sent Darwin a paper on the same idea, could he be urged by his friends to publish a first systematic account of his thought, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1859). This work has to be honored as the birth certificate of modern evolutionary theory.

The first five chapters of The Origin of Speciesinfer the hypothesis of natural selection. Chapters 6 to 13 [Page 2070]document its explanatory power by deducing and checking its consequences for specific biological problems, so that the theory of evolution by natural selection can be affirmed as well grounded in Chapter 14.

Darwin's inference of the hypothesis of natural selection brings together four sets of empirical data. The first set contains the wealth of experience in artificially selecting parents for breeding plants and animals that shall show some desired properties. The second set includes facts about the variability of organisms of the same species. If heritable, such variations constitute the material on which natural selection can act. The third set comprises data on ecological restrictions that force organisms to struggle for survival. The fourth set consists of empirical data about reproduction and heredity, by which variations are transmitted from a generation to the next one in proportion to the fitness of the varying organisms.

...