Why have organisms that can reproduce both sexually and asexually evolved to reproduce sexually in times of stress and asexually in times of non-stress?
Organisms who have the ability to produce both sexually and asexually typically inhabit harsh environments. Many such species reside deep underwater, where pressure and temperature fluctuate widely. Access to food, mates, and habitable space are also in constant flux. In times of high stress, these species will turn to sexual reproduction in order to increase their offsprings' chances of survival. The offspring of a sexual pair have greater chances of survival than offspring produced asexually because two individuals are providing protection and resources, rather than just one. The strategy of enlisting the support of an additional parent in times of high stress has proven to be evolutionary advantageous over millions of years.