Mechanism of Organic Evolution

When thinking about the mechanism of evolution, it is essential to focus on the population rather than the individual organism, because it is the population as a whole that evolves over time — an individual's role is simply to pass on its own genetic variation to its offspring. The basic processes that, taken together, bring about evolution are: mutation, gene recombination, gene flow (migration), genetic drift, natural selection, isolation and speciation.

Mutations are permanent, heritable changes in an organism's genetic material (already described in detail above); gene mutations alter both the organism's own genetic make-up and, cumulatively, the population's overall gene pool.

Gene recombination refers to the variation produced when alleles are brought together in new combinations during sexual reproduction — arising from the random union of gametes, from the way chromosomes separate during anaphase, and from crossing over.

Gene flow is the transfer of genes that occurs when genetically different populations interbreed, driven ultimately by emigration and immigration of individuals; it brings about changes in allele frequency within the populations concerned.

Genetic drift is any alteration in allele frequency that occurs in a natural population purely by chance, a concept first proposed by Sewall Wright and therefore also known as the Sewall Wright effect — for example, an allele can be eliminated from a population simply because the individual(s) carrying it happened to die, by accident, before they could mate. Genetic drift is random and directionless in its effects, and it is considerably more significant in small populations than in large ones; through drift, some alleles in a population may be lost or reduced in frequency purely by chance, while others increase. Two special cases of genetic drift are worth naming individually. Sometimes a few individuals become isolated from a much larger population and go on to found a small new population in a new geographical area; because this new population's allele frequencies can differ suddenly and substantially from those of the parent population, the founding individuals are called 'founders' and the resulting change is called the founder effect. The bottleneck effect, meanwhile, is the form of genetic drift seen when a natural disaster — a tsunami, flood, tornado or disease epidemic — kills off most of a population, leaving only a small number of survivors to found the next generation; because so few individuals survive, the gene pool of the resulting population can differ sharply, and by pure chance, from that of the original populati …