Evolution throws spaghetti at the wall and anything that sticks it keeps. Usually stuff sticks because it’s useful in some way, but some stuff sticks just because of random chance.
For example if a few individuals colonize a new location, then whatever genes those founders have will be prevalent in the new population. The classic example is the deaf people on Martha’s Vineyard. Some of the original settlers of the island were deaf and passed that down to their descendants.
The smaller a population is the more it’s affected by random genetic drift. (It’s easier for a gene to randomly spread to an entire population if the population is small.) The larger it is the more it’s affected by natural selection.