A population bottleneck (or genetic bottleneck) is a sharp reduction in the size of a population due to environmental events (such as earthquakes, floods, fires, disease, or droughts) or human activities (such as genocide). Such events can reduce the variation in the gene pool of a population; thereafter, a smaller population with a correspondingly smaller genetic diversity, remains to pass on genes to future generations of offspring through sexual reproduction. Genetic diversity remains lower, only slowly increasing with time as random mutations occur. In consequence of such population size reductions and the loss of genetic variation, the robustness of the population is reduced and its ability to survive selecting environmental changes, like climate change or a shift in available resources, is reduced.