Increasing diversity under domestication

Abstract

In any group of plants and animals evolution is the history of changes in genetic diversity. These changes are the result of subtle interactions between internal factors such as mutations, and external factors, mostly natural selection. The basic biological unit which is the subject of evolutionary process is the breeding population, i.e. a group of individuals which are bound by breeding relationships. At any given point in space and time genetic diversity is an outcome of three different, and sometimes conflicting, forces: factors and mechanisms acting toward greater diversity; factors decreasing diversity; and mechanisms to maintain existing diversity. The effect of these forces in shaping plant and animal evolution has been studied in detail in natural populations and they are also effective under domestication. The difference between the two is that in the latter, man has become an important element affecting trends and rates. To learn more about the nature of evolution under domesti-cation it would be useful to examine the relative importance of each of the various forces and their effect. Increasing diversity is achieved through mutations, recombination, immigration and introgression. Mutation is the only means by which novel alleles can be formed and is an essential element in evolution. Recombination is the process by which gene combinations are formed, and immigration and introgression are means by which diversity of a local population can be enriched by contributions from other populations of the same species or from other species. 2.1 MUTATIONS AND THEIR EFFECT Mutations occur as a result of changes in the DNA sequence of the gene and are referred to as gene mutations or point mutations. The change may involve substitution of a nucleotide in the coding region of the gene or inversion of the order of nucleotides, which may result in encoding of a different amino acid in the polypeptide chain, compared with the G. Ladizinsky, Plant Evolution under Domestication