Answer: They become extinct.
Adaptation can be defined as the physiological, structural and behavioral changes that occur in an organism either plant or animals which supports it's survival in the unfavorable environment. These changes are the result of natural selection which suggests that nature selects those individuals which are fit for survival in drastic environmental conditions.
According to the given situation, after a major environmental change the species which are not adapted to the changing conditions they are likely to extinct because adaptation increases the chances of survival of the organisms.
