Irish Elk,They have big antlers, but there is variation in population. Some antlers are smaller
When drought comes, it divided the habitat into 2, and that also separated the population. The original could not survive due to using too much effort to carried those big antlers. But the variation survived.
Over time, the smaller antlers survive and reproduce. The population then have only small antlers. They all totally different compared to its ancestor, that's how "Sausage elk" evolved