Micro-evolution consists of change in the population either to fit it to a new place or time in a changing environment, or to improve its success in a constant environment. The modifications which take place are genetic. Sewall Wright pointed out in an article written in 1949 that ‘the elementary evolutionary process in a reasonably large homogeneous population maybe considered to be change in gene frequency’. He has provided the best general classification of the processes changing gene frequency under the simplified conditions outlined in Chapter 1, and it is used in the following account.