Editing Small population size (section)

==Genetic effects==
[[File:Allele-frequency.png|thumb|The top graph shows the time to fixation for a population size of 10 and the bottom graph shows the time to fixation for a population of 100 individuals. As population decreases time to fixation for alleles increases.]]
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 [[homozygote|homozygosity]] within individuals in a population; i.e. the proportion of an individual's loci that contain homozygous rather than [[heterozygous]] [[allele]]s. Many deleterious alleles are only harmful in the homozygous form.<ref name=":3" />
* The degree of monomorphism/[[Polymorphism (biology)|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]].<ref name=":1">Purdue University. "Captive breeding: Effect of small population size". www.purdue.edu/captivebreeding/effect-of-small-population-size/. Accessed 1 June 2017.</ref> 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.<ref name=":1" /> The [[effective population size]] (Ne), or the reproducing part of a population is often lower than the actual population size in small populations.<ref>Lande, Russell, and George F. Barrowclough. "Effective population size, genetic variation, and their use in population management." ''Viable populations for conservation'' 87 (1987): 124.</ref> 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 ===
[[File:Apteryx mantelli -Rotorua, North Island, New Zealand-8a.jpg|thumb|[[Brown kiwi]]]]

* [[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]] alleles can become more frequent and 'fixed' in a population due to chance. The allele selected for by [[natural selection]] becomes fixed more quickly, resulting in the loss of the other allele at that [[locus (genetics)|locus]] (in the case of a two allele locus) and an overall loss of genetic diversity.<ref>Nei, Masatoshi. "Estimation of average heterozygosity and genetic distance from a small number of individuals." ''Genetics'' 89.3 (1978): 583-590.</ref><ref>Lande, Russell. "Natural selection and random genetic drift in phenotypic evolution." ''Evolution'' (1976): 314-334.</ref><ref>Lacy, Robert C. "Loss of genetic diversity from managed populations: interacting effects of drift, mutation, immigration, selection, and population subdivision." ''Conservation Biology'' 1.2 (1987): 143-158.</ref>  Alternatively, larger populations are affected less by genetic drift because drift is measured using the equation 1/2N, with "N" referring to population size; it takes longer for alleles to become fixed because "N" is higher. One example of large populations showing greater adaptive evolutionary ability is the red flour beetle. Selection acting on the body color of the red flour beetle was found to be more consistent in large than in small populations; although the black allele was selected against, one of the small populations observed became homozygous for the deleterious black allele (this did not occur in the large populations).<ref>TFC, Falconer, DS Mackay. "Introduction to quantitative genetics." ''4th Longman Essex, UK'' (1996).</ref> for Any allele—deleterious, beneficial, or neutral—is more likely to be lost from a small population (gene pool) than a large one. This results in a reduction in the number of forms of alleles in a small population, especially in extreme cases such as monomorphism, where there is only one form of the allele. Continued fixation of deleterious alleles in small populations is called [[Muller's ratchet]], and can lead to [[mutational meltdown]].
* [[Inbreeding]]: In a small population, closely related individuals are more likely to breed together. The offspring of related parents have a higher number of homozygous loci than the offspring of unrelated parents.<ref name=":3" /> [[Inbreeding]] causes a decrease in the [[Fitness (biology)|reproductive fitness]] of a population because of a decrease in its heterozygosity from the repeated mating of closely related individuals or selfing.<ref name=":3" /> Inbreeding may also lead to [[inbreeding depression]] when heterozygosity is minimized to the point where [[deleterious mutation]]s that reduce fitness become more prevalent.<ref>Charlesworth, D., and B. Charlesworth. "Inbreeding depression and its evolutionary consequences". ''Annual review of ecology and systematics'' 18.1 (1987): 237–268.</ref> Inbreeding depression is a trend in many plants and animals with small populations sizes and increases their risk of extinction.<ref>Newman, Dara, and Diana Pilson. "Increased probability of extinction due to decreased genetic effective population size: experimental populations of Clarkia pulchella." ''Evolution'' (1997): 354-362.
</ref><ref>Saccheri, Ilik, et al. "Inbreeding and extinction in a butterfly metapopulation." ''Nature'' 392.6675 (1998): 491.</ref><ref>Byers, D. L., and D. M. Waller. "Do plant populations purge their genetic load? Effects of population size and mating history on inbreeding depression." ''Annual Review of Ecology and Systematics'' 30.1 (1999): 479-513.</ref> Inbreeding depression is usually taken to mean any immediate harmful effect, on individuals or on the population, of a decrease in either type of genetic variation. Inbreeding depression can almost never be found in declining populations that were not very large to begin with; it is somewhat common in large populations ''becoming'' small though. This is the cause of [[purging selection]] which is most efficient in populations that are very but not dangerously inbred.
* [[Genetic adaptation]] to [[fragmented habitat]]: Over time species evolve to become adapted to their environment. This can lead to limited fitness in the face of stochastic changes. For example, birds on islands, such as the Galapagos Flightless Cormorant or the [[Kiwi (bird)|Kiwi]] of New Zealand, have been known to develop flightlessness. This trait results in a limited ability to avoid predators and disease which could perpetuate further problems in the face of [[climate change]].<ref name=":2">Frankham, R. (1997). Do island populations have less genetic variation than mainland populations?. ''Heredity'', ''78''(3).</ref><ref>Ramstad, K. M., Colbourne, R. M., Robertson, H. A., Allendorf, F. W., & Daugherty, C. H. (2013). Genetic consequences of a century of protection: serial founder events and survival of the little spotted kiwi (Apteryx owenii). ''Proceedings of the Royal Society of London B: Biological Sciences'', ''280''(1762), 20130576.</ref>  Fragmented populations also see genetic adaptation. For example, [[habitat fragmentation]] has resulted in a shift toward increased selfing in plant populations.<ref>Aguilar, R., Quesada, M., Ashworth, L., Herrerias‐Diego, Y., & Lobo, J. (2008). "Genetic consequences of habitat fragmentation in plant populations: susceptible signals in plant traits and methodological approaches". ''Molecular Ecology'', ''17''(24), 5177–5188. {{doi|10.1111/j.1365-294X.2008.03971.x}}. {{PMID|19120995}}</ref>

Examples of genetic consequences that have happened in inbred populations are high levels of hatching failure,<ref name="Briskie">{{cite journal |last1=Briskie |first1=James |title=Hatching failure increases with severity of population bottlenecks in birds |journal=PNAS |date=2004 |volume=110 |issue=2 |pages=558–561 |doi=10.1073/pnas.0305103101 |pmid=14699045 |pmc=327186 |ref=Briskie|doi-access=free }}</ref><ref>{{cite journal |last1=Brekke |first1=Patricia |title=Sensitive males: inbreeding depression in an endangered bird |journal=Proc. R. Soc. B |date=2010 |volume=277 |issue=1700 |pages=3677–3684 |doi=10.1098/rspb.2010.1144|pmid=20591862 |pmc=2982255 |doi-access=free }}</ref> bone abnormalities, low infant survivability, and decrease in birth rates. Some populations that have these consequences are cheetahs, who suffer with low infant survivability and a decrease in birth rate due to having gone through a population bottleneck. Northern elephant seals, who also went through a population bottleneck, have had cranial bone structure changes to the lower mandibular tooth row. The wolves on Isle Royale, a population restricted to the island in Lake Superior, have bone malformations in the vertebral column in the lumbosacral region. These wolves also have syndactyly, which is the fusion of soft tissue between the toes of the front feet. These types of malformations are caused by inbreeding depression or [[genetic load]].<ref>Raikkonen, J. et al. 2009. Congenital Bone Deformities and the Inbred Wolves (Canis lupus) of Isle Royale. Biological Conservation. 142: 1025–1031.</ref>