A genetic bottleneck occurs when a population is reduced in size for at least one generation. A smaller number of individuals leads to less genetic variation (Berkely). Limited mate availability leads to increased inbreeding (Charruau, Fernandes, Orozco-ter Wengel, Peters... & Burger, 2011). If the population recovers and grows again, the future populations will have lower genetic diversity than before the bottleneck (Berkely).
Bottleneck Effect: Is a sharp reduction in population size due to environmental effects such as natural disasters, disease, or human activity. This decrease in population size also decreases the gene pool, which can decrease a species' ability to adapt to new situations. This reduction in genetic variability can cause a species to become extinct.
A visual example of the effect of a bottleneck on the genetic diversity of a population. Image retrieved from: http://anthro.palomar.edu/synthetic/synth_5.htm
Examples
Cheetahs:
About 12,000 years ago, the Great Ice Age led to a mass extinction during which 75% of Earth's mammal species were lost (Cheetah Conservation Fund). Before this extinction event, the cheetah and its relatives were distributed worldwide (Menotti-Raymond & O'Brien, 1993). During the extinction, the cheetah and its relatives experienced a demographic genetic bottleneck, in which all of them died off except one small group which are the ancestors of all cheetahs living today. As a result of extensive inbreeding, all of today's cheetah's live in Northern Africa or Asia and are closely related and genetically similar (Charruau et al., 2011).
Prior to 12,000 years ago, there were many different types of Cheetahs. However many of these were killed off and recently hunted making cheetah conservation important with the few animals that were left.
Northern Elephant Seals:
In the 1890's, a population of Northern elephant seals experienced a population bottleneck as a result of human hunting. The population was reduced to as few as 20 individuals. Although the population has since rebounded to more than 30,000, there is much less genetic variation in this population when compared to a more Southern population which experienced much less hunting pressure (Berkely).
References
Berkely. (n.d.) Bottlenecks and Founder Effects. Retrieved October 13, 2013 from:
http://evolution.berkeley.edu/evosite/evo101/IIID3Bottlenecks.shtml
Charruau, P., Fernandes, C., Orozco-ter Wengel, P., Peters, J.,…Burger, P.A. (2011).
Phylogeography, genetic structure and population divergence time of cheetahs in Africa and
Asia: evidence for long-term geographic isolates. Molecular Ecology, 20 (4): 706-724.Cheetah Conservation Fund. (n.d.) Education Centre Virtual Tour. Retrieved October 11, 2013
from: http://www.cheetah.org/?nd=tour_history_panel1_evolution
Menotti-Raymond, M. & O’Brien, S.J. (1993). Dating the genetic bottleneck of the African
cheetah. National Academy of Sciences of the United States of America. Proceedings 90 (8):
3172-3176.
