The term bottleneck in general refers to the point or mouth of any bottle but in scientific term and precisely in case of genetics though the meaning is same but is used in context of population.
Bottleneck effect means the sharp reduction in population which result in loss of genetic variability and change in allele frequency. In case of bottleneck phenomenon, population is severally effected and maximum reduction in variability is caused. This means that even in a single generation, this phenomenon can reduce the population to large extent. This is either due to human activities like hunting, increasing deforestation and so on or due to natural calamities like flood, landslide, earthquake etc. The result thus is the large population of that species dies leaving behind only a small portion. This not only reduces allele frequency but also variation gets reduced in the leftover population and hence the upcoming generations have very less advantage for survival. They have to struggle more for fulfillment of every basic needs that those population that were there before the bottleneck effect. Overall the name is so designed because the large population drastically gets reduced in one generation like the base of the bottle to its neck and the left over population reproduce like the neck to the mouth of the bottle.
The result of this effect is that the chance of inbreeding will increase that will lead to increase in homozygosity. Therefore, heterozygosity will reduce thus decreasing the overall variability and the size of population.
1. When a study was conducted on the prairie chicken, it was found that they were in large number in Midwestern united state but due to uncontrolled hunting and unmanaged human activity the number reduced drastically. When the studies on the obtained work was conducted it was found that this reduction have resulted in the 29% reduction in the variability of allele in the population of modern prairie chicken than those that were present before the occurrence of this phenomenon. (Klug and Cummings)
2. The second example includes the northern elephant seal that were once abundant but due to hunting their numbers drastically reduced and now are very less in comparison to that of southern seal.
3. If we see in case of Homo sapiens, some evidences also supports that at some period of time due to certain reasons the population of homo sapines was reduced to greater extend and this is the reason that we have very less differences with each other in comparison to other living species.
4. Likewise, there are plants as well that have very less or almost negligible genetic variability and this is the result of bottleneck effect.
This is also a type of bottleneck phenomenon that gives rise to small population with very less number of individuals. In this case, new population is formed from the old population but with lots of differences. The allele and genetic diversity that is present in the original population will no more be seen in the new population. Both the original and the new population remain isolated. This can also be better explained with an example of Huntington disease. This disease is very much prevalent near the lake Maracaibo in Venezuela. It is because of the founder effect where the new parent that established in new place has the allele that is dominant for this disease. This is the reason that every upcoming generation has the dominant allele for the same disease and slowly it spread with every reproduction and hence affected the population leaving maximum number of individual.
This way bottleneck effect brings changes to Hardy-Weinberg equilibrium, where in one place Hardy-Weinberg equilibrium keeps the allele frequency same when no real forces comes in play and on the other hand bottleneck effect is the real force that bring changes in hardy-Weinberg equilibrium that in turn bring changes in allele frequency.