Genetic Variation & Natural Selection

Evolution needs variation to act on. Mutation is the ultimate source of new alleles; sexual reproduction then shuffles them further through crossing over and independent assortment in meiosis, and random fertilisation.

Natural selection

In any population, individuals vary. Those with variations better suited to the environment are more likely to survive and reproduce, passing on their alleles. Over generations this changes the frequency of alleles in the population — that change is evolution.

Selection favours the best-suited variants, shifting the population's allele frequencies over time.

Selection isn't the only force: genetic drift (random change, strong in small populations) and gene flow also alter a gene pool. The environment selects — organisms don't consciously 'adapt' to it.