What is the bottleneck effect?

A bottleneck is a phenomenon where the performance or capacity of an entire system is limited by a single or limited number of components or resources. The term bottleneck is taken from the 'assets are water' metaphor. As water is poured out of a bottle, the rate of outflow is limited by the width of the conduit of exit?that is, bottleneck. By increasing the width of the bottleneck one can increase the rate at which the water flows out of the neck at different frequencies. Such limiting components of a system are sometimes referred to as bottleneck points.

The bottleneck effect occurs due to catastrophic events. These events cause rapid dramatic changes in natural selection since most individuals die without passing on their genes. The few survivors of these evolutionary "bottlenecks" then reproduce successful, resulting in large populations in subsequent generations. The consequence of this bottleneck effect is the extraordinary reduction in genetic diversity of a species since most variability is lost at the time of the bottleneck.

So basically, a catastrophe causes much of the population to die and those that are left reproduce, but only pass on the genes that they small minority of the survivors had so the genetic variation is small.

Hope this helps!!