Many species can reproduce either asexually or sexually. What might be the evolutionary significance of the switch from asexual to sexual reproduction that occurs in some organisms when the environment becomes unfavorable?

When an organisms reproduces asexually its offspring is identical, this means it has the same DNA code.

For example viruses look for specific protein types at cell membranes to "anchor" or "stick" and then inject their genetic material. If all the individuals of a population have the same membrane proteins the whole population is vulnerable to be infected by the virus.

Sexual reproduction creates genetic variability due to mechanisms like crossing over and the assortment of chromosomes during meiosis. On top of it, different parents contain different information which leads to offspring with similar but different characteristics which can end up having different membrane proteins that can save them from the virus.