What are the advantages and disadvantages of biological variation (on the contrary)

Biological variation may cause biological malformation, and it will lead to elimination if it does not adapt to the living environment. Moreover, mutated organisms may form new species, resulting in no natural enemies and flooding everywhere.

Many mutations, such as bacteria and pests, such as drug resistance, will bring a lot of trouble to human beings and lead to the decline of human immunity. Of course, this is also a problem of flooding.