In population biology, a population is a group of individuals of the same species that occupy a region and can potentially contribute genetic material to a shared pool of descendants.

Evolution is change in the heritable composition of a population across generations. At the genetic level, this is commonly tracked as a change in allele frequencies—the proportions of alternative versions of genes in the population.

This definition has several important consequences.

• Individuals do not evolve during their lifetimes. Their physiology or behavior can change, but evolutionary change is measured across generations in populations.
• Evolution does not necessarily mean increasing complexity or becoming 'better.' It means that heritable variants become more or less common.
• A visible phenotypic change is not required. Evolution can occur through genetic changes whose phenotypic effects are subtle or environment-dependent.

Example

Suppose a population contains 1,000 copies of a locus among its sampled chromosomes: 600 copies of allele $A$ and 400 copies of allele $a$.

The allele frequencies are

$$p(A)=0.60,\qquad p(a)=0.40.$$

If a later generation contains frequencies

$$p(A)=0.72,\qquad p(a)=0.28,$$

then the population has evolved at that locus, regardless of which mechanism caused the change.

Natural selection, genetic drift, migration and mutation can all change population composition. Evolution is the population-level outcome; those processes are possible causes.