(i) Mutations. These have been recognised as the ultimate source of biological changes and hence the raw material of evolution. The mutation in chromosomes may be due to changes in structure, number or gene.

(ii) Gene Recombination takes place during crossing over in meiosis. New combinations of genes produce new phenotypes.

(iii) Hybridisation is the intermingling of the genes of the members of closely related species.

(iv) Genetic drift is the elimination of the genes of some original characteristics of a species by extreme reduction due to different reasons.

In monoparental reproduction only chromosomal and gene mutation are sources of genetic variation.

(2) Natural Selection. If differential reproduction (i.e. some individuals produce abundant offspring, some only a few and some organisms none) continues for many generations, genes of the individuals which produce more offspring will become predominant in the gene pool of the population. Thus natural selection occurs through differential reproduction in successive generations.

(3)Migration of individuals from one to other population is an accessory factor for speciation (origin of new species).

(4) Isolation.  The isolation plays a significant role in evolution. Isolation of a species population into different geographical zones may lead to formation of a new species.