The Bottleneck Effect

Overview

The Bottleneck Effect can make a profound impact on genetic variation (particularly within a small population). This phenomenon occurs when a population’s size is reduced for at least one generation. Since genetic drift often acts more rapidly to reduce genetic variation in a small population, the bottleneck effect can reduce a population’s genetic variation by a significant amount. On the other hand, the genetic variation of a small population can even be tremendously impacted if the bottleneck does not occur for several generations. Evolutionary biologist Masatoshi Nei emphasized this biological phenomenon in his mathematical study. According to Nei, “When a population goes through a small bottleneck, the genetic variability of the population is expected to decline rapidly but, as soon as the population size becomes large, it starts to increase owing to new mutations” (Nei, Maruyam, & Chakroborty, 1975). This, therefore, ties in another evolutionary subject (mutations) into the topic of the bottleneck effect, showing how various evolutionary mechanisms are intertwined.

One common example of the bottleneck effect in nature is seen in Northern Elephant Seals. In the 1980s, hunting seriously reduced the population of these seals in the wild. This caused a massive reduction in genetic variation. While this seal population has rebounded since then, the genetic variation which exists within the Northern Elephant Seal population is significantly smaller than a population of Southern Elephant Seals (which were not so intensely hunted by humans).