A group of organisms potentially capable of successful reproduction. Individuals of a population tend to choose mates from within the group. Gene pool: the sum of all alleles carried by the members of a population. Evolution: a change in allele frequencies from one generation to the next. Genetic equilibrium: no changes in allele frequencies i. e. no evolution is occurring in the population. An individual"s genotype and phenotype will never change. The genetic make-up of the population is what changes over time. This is how potential new advantageous traits arise. Mutations in gametes are where you get variation added to the gene pool. Without strong selective pressure, populations will naturally change at random. Genetic drift is also random, purely by chance. Big population, small groups band out & form a new colony & the new colonies have a completely new genetic makeup than the original population.