Answer: Gene mutations and genetic recombination (also gene flow and genetic drift).
- Mutation: a sudden, heritable change in the genetic material. Gene (point) mutations alter the base sequence of DNA and produce new alleles; chromosomal mutations change the number or structure of chromosomes. Mutations are the raw material of evolution because they create entirely new variations.
- Genetic recombination: during meiosis, crossing over between non-sister chromatids of homologous chromosomes and independent assortment of chromosomes produce new combinations of genes in the gametes. Random fusion of gametes at fertilisation adds further combinations, so offspring differ from parents and from one another.
Other causes are gene flow (migration of individuals carrying different alleles between populations) and genetic drift (random change in allele frequency in small populations).
