That's called selection pressure. When a species develops a feature or trait that allows it to survive over another. So the Samurai face was a selection pressure for the other crabs. It is part of Darwin's theory of evolution "survival of the fittest."

There is a similar story of two moth species in pre-industrial England.
There were white moths and black moths before industrialization in England. As industry developed and grew so did the amount of soot deposits on trees and buildings. The white moths stood out against the sooty background but the black moths blended in and were harder to see. Predators naturally ate more white than black moths and so white moths where less likely to survive to procreate (pass on the white moth genes) so black moths soon predominated. How long it takes depends on how fast a species procreates and the level of selection pressure applied, along with the initial population.