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The genetic drift caused by a population bottleneck can change the proportional random distribution of alleles and even lead to loss of alleles. The chances of inbreeding and genetic homogeneity can increase, possibly leading to inbreeding depression. Smaller population size can also cause deleterious mutations to accumulate. [3]
The estimated remaining effective population sizes are around 10,000 individuals, larger than the estimated non-African size during their bottleneck. [ 76 ] [ 77 ] [ 78 ] Unlike the non-African populations, there is no consensus as to the cause of the African bottleneck.
Another possible cause of genetic divergence is the bottleneck effect. The bottleneck effect is when an event, such as a natural disaster, causes a large portion of the population to die. By chance, certain genetic patterns will be overrepresented in the remaining population, which is similar to what happens with the founder effect. [4]
The impact of a population bottleneck can be sustained, even when the bottleneck is caused by a one-time event such as a natural catastrophe. An interesting example of a bottleneck causing unusual genetic distribution is the relatively high proportion of individuals with total rod cell color blindness ( achromatopsia ) on Pingelap atoll in ...
A population bottleneck may also cause a founder effect, though it is not strictly a new population. The founder effect occurs when a small group of migrants—not genetically representative of the population from which they came—establish in a new area.
China's population is projected to crash 55% by the turn of the next century. Italy's will sink 41%, and Brazil's will drop 23%. But helped by immigration, the U.S. should see an increase of 23%.
A genetic bottleneck can occur when a population goes through a period of low number of individuals, resulting in a rapid decrease in genetic diversity. Even with an increase in population size, the genetic diversity often continues to be low if the entire species began with a small population, since beneficial mutations (see below) are rare ...
The environment can directly affect the survival of a small population. Some detrimental effects include stochastic variation in the environment (year to year variation in rainfall, temperature), which can produce temporally correlated birth and death rates (i.e. 'good' years when birth rates are high and death rates are low and 'bad' years when birth rates are low and death rates are high ...