Complex sentence: Such events can reduce the variation in the gene pool of a population; thereafter, a smaller population (of animals/people) with a correspondingly smaller genetic diversity, remains to pass on genes to future generations of offspring.
Split into simpler sentences: Such events can reduce the variation in the gene pool of a population. After an event, a smaller population (of animals/people), with a correspondingly smaller genetic diversity, remains to pass on genes to future generations of offspring.