Natural Selection & Evolution

For natural selection to act, there has to be variation to act on. Without genetic differences between individuals, every member of a population would respond identically to every change in the environment, and the population could not evolve.

There are three main sources of genetic variation:

Mutation

A mutation is a random change in the DNA sequence (covered in detail in topic 13). Mutations are the ultimate source of all new alleles. Every new variant that has ever existed in any organism started as a mutation.

Mutations:

• Happen continuously in every cell
• Are random: they do not happen because they are useful
• Are mostly neutral (no effect on the phenotype) or harmful
• Are rarely beneficial, but even rare beneficial mutations can spread through a population by natural selection over many generations

Meiosis: independent assortment and crossing over

When gametes are made by meiosis, the existing alleles in the parent get shuffled into new combinations:

• Independent assortment: which chromosome of each pair ends up in which gamete is random, so each gamete carries a unique mix of the parent's chromosomes.
• Crossing over: matching chromosomes can swap sections of DNA before they separate, creating new allele combinations on the same chromosome.

The result is that a single person can produce a near-infinite number of genetically different sperm or eggs. Meiosis does not create new alleles, but it reshuffles existing alleles in new ways.

Random fertilisation

Any one of millions of sperm can fertilise an egg. The combination of parents' alleles in each new offspring is therefore largely random. This adds another layer of shuffling on top of the variation produced by meiosis.

Why this matters for evolution

Mutation is the only source of genuinely new alleles. Meiosis and random fertilisation reshuffle existing alleles. Both kinds of variation are needed:

• New alleles (from mutation) provide the raw material for evolution.
• Shuffling of existing alleles (from meiosis + random fertilisation) lets populations adapt quickly to changing conditions without needing to wait for new mutations.