To begin with, let's examine a simple model of a population of fictional organisms called driftworms . In the following examples, the driftworms have only one gene, which controls skin color.
Worms reproduce asexually and are connected to their parents by lines.
In the population of five worms below, each worm gives rise to exactly one worm in the next generation. With no diversity in generation 0 and no forces of evolution acting on the population, the model above begins and ends with all worms in the population having the same allele. For evolution to occur, there must be mutation, selection, or random genetic drift. These are the three major forces of evolution. The cause changes in genotypes and phenotypes over time.