Natural selection acts on variation between organisms. In fact, evolution itself would be impossible without variation. If you have a population where most or all individuals are not varied, then there is no way the population is going to be able to adapt over time, so potential for change doesn’t exist. Adaptation can’t arise spontaneously just because it is needed.
For example, the case of the black and white moths during a time when tree trunks were painted white is a good illustration of this principle. You start with an even variation of about 50-50 between black moths and white moths. The tree trunks get painted white, so clearly the black moths will contrast highly with the white trunks. They are visible to predators, and so most die out, don’t get a chance to reproduce, therefore don’t pass on the gene (allele) that codes for their black colour. As a result, future generations have few black individuals, and a lot more white ones (as the white moths were well camouflaged so were not that visible to predators).
The point is, what would have happened to that moth population had there been no white moths at all? It’s not far-fetched to speculate that that specific population might well have died out. But imagine this: think about our current, mostly white moth population. What if the tree trunk dye gets washed away, and the white moths now appear obvious on the trunks? Does that mean the population is bound to die off anyway at some point in the future? It becomes obvious that, despite directional natural selection (directional means favouring one extreme attribute rather than another), some variation must be maintained. Indeed, some variation is always maintained by many very different processes, from the molecular level to the ecosystems. Find out about these in the next post.