Genetic Bottleneck.
Step-by-step explanation:
Genetic bottle neck is a phenomenon in which a large part of population is lost within a very short period of time. This evolutionary event may occur as a result of certain natural calamity, habitat destruction etc. This phenomenon radically changes in the allele frequencies and sometimes the recessive traits establish themselves in the population.
In this case also certain traits are lost and diversity is reduced so it is an instance of genetic bottleneck.