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. There are five alleles (skin colors) at generation 0 and the same five alleles at generation 4.
Note that the model above starts with a diverse population (5 worms, 5 alleles). What would the model look like if there were no diversity to begin with?
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.