Answer:
Bottleneck effect.
Step-by-step explanation:
The bottleneck effect is described as an effect that happens when a certain population is drastically reduced, most likely, due to a natural event. If in this population there were 100 people and were 40 blondes, 40 black haired, and 20 red heads, the bottleneck effect would happen if, out of a natural disaster, only 20 people survived and they had 15 redheads and 5 blondes. When the numbers of that population start to grow, there will be a lot more red heads than before.
So, the bottleneck, in other words, is when a small proportion of the original population makes it through, by chance.