I predict that people will have problems with this example because it sort of vaguely implies that evolutionary change is being forced by climate change - that is, that the climate is somehow having an impact on individuals, which sounds crazy - and is. It’s a good example of somewhat sloppy science writing that’s responsible for so many misunderstandings about evolution and other matters.

In fact, the variability in drought tolerance was already there in the existing plant population. The vast majority were those well adapted to a particular climatic regime, but there was always a spectrum of drought tolerance ranging from high to low, centered on the prevailing average.

As climates shift, the raw material for change is already sitting there waiting in the wings. Those marginal cases, say those that already possess greater drought tolerance, thrive as rain decreases while their not-so-tolerant brethren do not so well. It takes very little time for a population to make substantial shifts as pre-existing, normally rare variants are favored.