Island genetics

Small populations can behave differently from larger populations. They are often the result of population bottlenecks from larger populations, leading to loss of heterozygosity and reduced genetic diversity and loss or fixation of alleles and shifts in allele frequencies. A small population is then more susceptible to demographic and genetic stochastic events, which can impact the long-term survival of the population. Therefore, small populations are often considered at risk of endangerment or extinction, and are often of conservation concern.

Demographic effects

The influence of stochastic variation in demographic (reproductive and mortality) rates is much higher for small populations than large ones. Stochastic variation in demographic rates causes small populations to fluctuate randomly in size. This variation could be a result of unequal sex ratios, high variance in family size, inbreeding or fluctuating population size.Frankham, R., Briscoe, D. A., & Ballou, J. D. (2002). ''Introduction to conservation genetics''. Cambridge university press. The smaller the population the greater the probability that fluctuations will lead to extinction.

One demographic consequence of a small population size is the probability that all offspring in a generation are of the same sex, and where males and females are equally likely to be produced (see sex ratio), is easy to calculate: it is given by $1/2^$ (the chance of all animals being females is $1/2^n$; the same holds for all males, thus this result). This can be a problem in very small populations. In 1977, the last 18 kakapo on a Fiordland island in New Zealand were all male, though the probability of this would only be 0.0000076 if determined by chance (however, females are generally preyed upon more often than males and kakapo may be subject to sex allocation). With a population of just three individuals the probability of them all being the same sex is 0.25. Put another way, for every four species reduced to three individuals (or more precisely three individuals in the effective population), one will become extinct within one generation just because they are all the same sex. If the population remains at this size for several generations, such an event becomes almost inevitable.

Environmental effects

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) that lead to fluctuations in the population size. Again, smaller populations are more likely to become extinct due to these environmentally generated population fluctuations than the large populations.

The environment can also introduce beneficial traits to a small population that promote its persistence. In the small, fragmented populations of the acorn woodpecker, minimal immigration is sufficient for population persistence. Despite the potential genetic consequences of having a small population size, the acorn woodpecker is able to avoid extinction and the classification as an endangered species because of this environmental intervention causing neighboring populations to immigrate. Immigration promotes survival by increasing genetic diversity, which will be discussed in the next section as a harmful factor in small populations.

Genetic effects

Conservationists are often worried about a loss of genetic variation in small populations. There are two types of genetic variation that are important when dealing with small populations:

* The degree of homozygosity within individuals in a population; i.e. the proportion of an individual's loci that contain homozygous rather than heterozygous alleles. Many deleterious alleles are only harmful in the homozygous form.
* The degree of monomorphism/ polymorphism within a population; i.e. how many different alleles of the same gene exist in the gene pool of a population. Genetic drift and the likelihood of inbreeding tend to have greater impacts on small populations, which can lead to speciation.Purdue University. "Captive breeding: Effect of small population size". www.purdue.edu/captivebreeding/effect-of-small-population-size/. Accessed 1 June 2017. Both drift and inbreeding cause a reduction in genetic diversity, which is associated with a reduced population growth rate, reduced adaptive potential to environmental changes, and increased risk of extinction. The effective population size (Ne), or the reproducing part of a population is often lower than the actual population size in small populations. The Ne of a population is closest in size to the generation that had the smallest Ne. This is because alleles lost in generations of low populations are not regained when the population size increases. For example, the Northern Elephant Seal was reduced to 20-30 individuals, but now there are 100,000 due to conservation efforts. However the effective population size is only 60.

Contributing genetic factors

* Genetic drift: Genetic variation is determined by the joint action of natural selection and genetic drift (chance). In small populations, selection is less effective, and the relative importance of genetic drift is higher because deleterious