It is the essential factor for evolution. The variation must be inheritable from parents to their offspring to contribute to evolutionary change. The genes of the organism encode the transmissible traits. The chief sources of variation include mutation and recombination.

Mutation can be defined as the change in the genomic DNA that can occur due to random replication errors or by some environmental interactions.

Gene flow is the process that introduces new mutations into a population by individuals migrating from a differentiation population. The example of gene flow is different traits of eye color such as brown, green, blue, hazel, etc. the blue eye color in humans is a result of the specific mutation in the HERC2 genes.

The mutation reduces the melanin production that causes blue eye color. The studies also reveal that the mutation first occurs in a single European ancestor around 6,000 to 10,000 years ago, which later spread in the modern population.

The consequence of sexual reproduction is usually called recombination. In fertilization, the fusion of parental gametes may bring mutations acquired in different lineages.

Later, by independent assortment and by crossing over, the mutations recombine and inherit in the following generation. The meiotic events result in the production of new allelic combinations and increase genetic variation, thus sexual reproduction escalates evolution.