Population bottlenecks

What are they? They occur when a wild population is reduced to a small number of individuals, and then increases again. They are typically caused by an environmental change such as unusual climatic conditions, or, increasingly, the actions of humans, as in over-hunting and habitat destruction. But if the population size is restored, everything ends up fine, surely? No. The small population size causes all the individuals in the population to end up closely related to each other, and they lose genetic variability. They become 'inbred'. But there won't be much of an effect if the bottleneck only lasts for a few generations. Not true. Genetic diversity depends on the population size: larger populations will be more diverse. But if population size changes, diversity is greatly affected by periods of small size, even if these are short. We talk about the 'effective' size of a varying population, which is how small a constant-sized population would have to be in order to have the same genetic impact. Suppose we look at five generations of a population going through a bottleneck, and the numbers of individuals in these generations are 1,000, 1000, 10, 100 and 1,000. The mean size is 622 but the 'effective' size is only 44. But what is so bad about losing variability and becoming inbred? Unfortunately, all populations contain individuals with harmful recessive alleles. With inbreeding, these recessive alleles start to appear in their homozygous form, and individuals' abilities to survive and reproduce are diminished. Is a population which passes through a bottleneck permanently harmed by it? Good question. There are two schools of thought. One idea is that the bad alleles are homozygous in a few individuals, these individuals die, and the bad alleles are purged from the population so there's no permanent damage. The alternative scenario is that damaging alleles spread by chance through the tiny population. All individuals get bad alleles only. Now the population is harmed. Which of these happens depends on the severity of the bottleneck and how harmful the alleles are. How do we find out about bottlenecks? Two methods: we can look at the genetic variation now and see if it's unexpectedly low, or, better, we can observe the changes in numbers directly, as when some of our favourite species came close to extinction and then recovered. Sometimes, for recent bottlenecks, we can compare the diversity after the bottleneck with that …