Certainly you agree that the phrase "too quickly" is a relative matter. After the the Dinosaur-Killing asteroid hit, and all terrestrial animals larger than 50 kilos disappeared, that is evidence of a "quick" change.
But I didn't want to rely on that particular event. I intended my meaning to refer to extinctions that occurred all over the planet for all sorts of reasons and circumstances with which the now extinct populations had insufficient allele diversity to cope.
Naturally, we have to be careful with the broad brush about "extinctions". It would be completely normal and expected that as a "continuous population" rides the crest of change
- - using the allegory of a surging tidal wave, carrying the most adaptive populations into new times, conditions and regions - -
it is definitional that even the successful populations leave fossils of "the older models" ... that may have looked and behaved differently from the more recent genetic snapshot of a surviving population.
Alligators and crocodiles appear to fit that class easily. And for a long time, mammoths held out ... evolving into much smaller models when they were confined to small northern islands as the world continued to change around them.
Ultimately, mammoths became one of the clear cases of a population that just ended - - they were terminal. We can tell by their pattern of fossils and the fact they no longer compete with the Eskimo, that they were not carried on to the next era by the allegorical crests of change the world brought them. The Terror Birds that eventually became confined to South America rose up well after the disappearance of the dinosaurs ... but their population too became terminal.
Terror Birds did not become parrots, nor falcons. The population came to an end without any survivors for the next wave of environmental change that would sweep into South America -
- the tidal crest represented by humankind!
W.K., I suppose it would be too much to think that you could take spiritual solace from one such as me - - a Unitarian Universalist - - but I'm convinced that God has it in his plan to save all humanity one way or another, just as a World-Ranked Chess Master, playing a 100 hopefuls (sequentially, each at a different board), has no trouble serially engaging these many dozens without constraining their Free Will. But all of these eager players have no way to resist the inexorable mastery of the top Chess expert in their region.
God has an additional advantage over the Chess Master. Not only does God know every move and counter-move necessary to make a struggling soul find a peaceful end to his vain resistance - - but unlike the Chess Master, God invented the whole system. His Mastery is beyond question, and thus his effect will be complete and uncontested!