That means severe limitations on food, severe culling by predators, severe selection by disease, severe actions of climate -- with which some variants in the population can cope better than others.
(That; means; severe limitations on food, severe culling by predators, severe selection by disease, severe actions of climate) (some variants in the population; can cope better; )
That some variants in the population
means can cope better
severe limitations on food, severe culling by predators, severe selection by disease, severe actions of climate