How does a bottleneck affect a population?
A bottleneck typically affects a population by lowering genetic diversity.
A bottleneck occurs when there is a serious decline in the population. Floods, disease, and even hunting can all cause population bottlenecks.
After the disturbance, whatever it may be, the size of the population is significantly smaller than the original population's, and genetic diversity is reduced (see the image above). Lowering genetic diversity can be very dangerous for a population. The more genetic diversity a population has, the more resilient to change it is.
Let's pretend all of the dots in the image above are phenotype s. If a disease kills all of the cheetahs with the green phenotype before the bottleneck, the cheetah population still has multiple individuals with different genetic makeup. If this same disease strikes after the bottleneck, the population will be left with three different versions (colors) of this phenotype, whereas before the population had four versions of this phenotype. The disease has a much larger impact.
On the other hand, a bottleneck could strengthen a population, if all individuals surviving the bottleneck have superior fitness . This is unlikely though, as eventually new pressures will be introduced to the population and the variation that these pressures would have acted on no longer exists in the population, causing further decline.
Here are some links to learn more: how the idea of a population bottleneck has directly affected humans, cheetahs and population bottlenecks, Florida panthers and their bottleneck .