How does an adaptation first happen?

b. heterotroph

explanation:

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.

the right option is; decreased genetic variation  

decreased genetic variation describes the effect of this environmental change.

genetic variation is the naturally occurring genetic diversities or differences between organisms of the same species in a population. genetic variation allows the survival and adaptability of a species when they encounter changing environmental conditions. when an endangered species reduces, it loses genetic variation which may not be restored even if the species rebound and will only be recovered through the gradual collection of mutations over many years.