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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 of Homo 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 Species infer the hypothesis of natural selection. Chapters 6 to 13 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.

Knowing the laws of variation, heredity, and ecological dynamics, and given sufficient data about present-day biodiversity, the Darwinian biologist should be able to deduce that natural selection is the principal mechanism by which life on Earth has evolved. Darwin himself did not know those laws— especially he did not know the laws of genetics. But in principle the hypothesis of natural selection is a deductive conclusion from premises discovered by the analysis of the above-mentioned data sets. It is therefore unnecessary to posit a principle of natural selection as a kind of not provable axiom for biology. It is even false to do so, since such a principle would limit natural selection to a particular class of entities, which are called “organisms.”

Nowadays it is possible to explain the mechanism of natural selection as an effect of general natural laws that act upon systems consisting of self-reproducing varying elements under restrictions on growth (Manfred Eigen). Darwin’s natural selection is then just one instance of this more general selective process—with organisms as self-reproducing elements, genetic differences as variations, and the environment as a set of restrictions on the system’s growth.

Selection is generated by a natural dynamics that, on the one hand, can be described formally without referring to the history of life on Earth, and that, on the other hand, states what conditions real evolution must meet if natural selection shall be a confirmed explanatory mechanism in biology. This twofold way of thinking about selection originated in the work of the founding fathers of theoretical population genetics—Ronald A. Fisher, John B. S. Haldane, and Sewall Wright—in the 1920s. They began to study mathematically the genetic base of natural selection and thereby made possible the modern synthesis of Darwinian evolutionary theory and Mendelian genetics.

Whether natural selection is really the principal mechanism responsible for the evolution of life on Earth is a highly debated question. Current criticism often aims at some tendencies in orthodox Darwinism to turn natural selection into an almighty power. Research on molecular evolution has shown that most genetically transmitted mutations are selectively neutral, so that random genetic drift of such neutral mutations constitutes another important mechanism effective in real evolution (Motoo Kimura). Other critics underline the importance of nonadaptive phenotypic properties of organisms, which have to be seen either as features necessary for the stability of their anatomical structure or as vestiges of formerly functional adaptations (Stephen Jay Gould).

Bibliography:

Darwin, C. (1982). The origin of species. Harmondsworth, UK: Penguin.
Eigen, M., & Winkler, R. (1981). Laws of the game: How the principles of nature govern chance. New York: A. A. Knopf.
Fisher, R. A. (1999). The genetical theory of natural selection (variorum ed.). Oxford, UK: Oxford University Press.

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