5.23—Evolution by mutation and natural selection

Variation exists within a population. Random mutation, meiosis and fertilisation create heritable differences; selection pressures then make some phenotypes more likely to survive and reproduce.

Individuals do not evolve because they need to. Selection acts on existing variation: survivors pass advantageous alleles to offspring, so allele frequency changes over generations.

If foxes more often catch visible white rabbits, better-camouflaged brown rabbits leave more offspring and the brown-fur allele becomes more common in the population.

An acquired injury is not inherited, and “survival of the fittest” means reproductive success in a stated environment, not strength or intention.