Additional levels of mortality experienced by a population that can result in population extinction absent successful adaptation.

To the extent that the additional mortality is biased such that it is experienced to a greater extent by some genotypes versus others then the result of hard selection can be adaptation and specifically adaptations that, in the course of natural selection, result increases in the absolute fitness of the population. Contrast .

A defining feature of hard selection, and one that contrasts it with soft selection, is that hard selection can result in population extinction. It is also important to keep in mind that while adaptation can be necessary for a population to avoid this extinction, the adaptations explicitly may be beneficial to the carrying organisms only under conditions where the agent mediating hard selection is present.

Adaptations that allow an organism to resist predators, antibiotics, , etc., thus may be advantageous — thereby increasing absolute fitness — only so long as selection by these agents are present or operating.