Species formation is believed to be the product of four principal factors: mutation, recombination, selection, and isolation.' A mutation is a heritable, spontaneous, and within certain limits random change in the chemical composition of a molecular segment of a chromosome known as a gene cr gene locus.' These changes take place normally in all organisms at individual frequency rates that can be predicted. As most mutations produce unfavorable effects, relatively few are passed on or participate in species formation. The same mutation, favorable or otherwise, can appear time after time, at its own rate, in individuals of different races. Yet mutation is the primary element in evolution. The other three are secondary.
Recombination, known as Mendel's second law, is the process by which rows of gene-molecules strung together on chromosomes break up and form new associations.' At meiosis, that critical moment in fertilization when a single array of paternal chromosomes lines up with and joins a single set of maternal chromosomes, the pairs do not always merge with each other in a regular fashion. Some chromosomes cross over each other at various loci and trade strings of genes. Others break up and the fragments attach themselves to other chromosomes or get lost. These new arrangements can also cause changes in the resultant organism.
Selection is the well-known pruning process by which the environment determines which novelty produced by mutation or recombination shall gradually spread through the group because of its superiority to the old trait it replaces, and which novelty shall be eliminated because it is unfavorable. As most mutations are unfavorable, when a species is not perceptibly changing, selection serves almost entirely to preserve the status quo. However, the process of replacement is characteristically slow. Old genes have a habit of hanging on as minorities, and if the environment changes back once more, they may re-emerge as majorities, in new combinations.
Isolation, the fourth factor, is necessary for the rise of new species because, unless a breeding population is self-contained, natural selection may be unable to eliminate old, unfavorable genes from its pool. A constant gene flow from neighboring populations may renew the old genes as fast as they are being lost. In a monotypic species such gene flow is impossible by definition. But in a polytypic species only those genes can be eliminated which are unfavorable to all its component units. When this happens, the species evolves as a whole, whereas its component populations may retain their local differences.