This is an evolutionary event in which population size is Radically reduced usually because of of a catastrophic event

bottleneck effect

Explanation:

The bottleneck effect is an evolutionary event in which population size is radically reduced, usually due to a catastrophic event.

This effect occurs when sudden changes in the environment, such as natural disasters and habitat fragmentation, reduce the size of a population and hence genetic diversity. An example of this effect is what happened to the prairie grouse, a bird that lived on the grasslands of the state of Illinois, in the United States. The conversion of grasslands to crops and pastures throughout the nineteenth century drastically reduced their populations to less than 50 individuals in 1993. This also reduced genetic diversity and led to increased frequency of deleterious genes. Thus, compared to other populations of the species, egg hatching rates in Illinois capercaillie populations were much lower, below 50%.