Editing Conservation genetics (section)

==Genetic diversity==
[[Genetic diversity]] is the total amount of genetic variability within a species. It can be measured in several ways, including: observed [[Zygosity#Heterozygous|heterozygosity]], expected heterozygosity, the mean number of [[alleles]] per [[Locus (genetics)|locus]], the percentage of loci that are [[polymorphism (biology)|polymorphic]], and estimated [[effective population size]]. Genetic diversity on the population level is a crucial focus for conservation genetics as it influences both the health of individuals and the long-term survival of populations: decreased genetic diversity has been associated with reduced average [[fitness (biology)|fitness]] of individuals, such as high juvenile mortality, reduced immunity,<ref name=":1">{{Cite journal |last1=Ferguson |first1=Moira M |last2=Drahushchak |first2=Lenore R |date=1990-06-01 |title=Heredity - Abstract of article: Disease resistance and enzyme heterozygosity in rainbow trout |journal=Heredity |volume=64 |issue=3 |pages=413–417 |doi=10.1038/hdy.1990.52 |issn=0018-067X |pmid=2358369 |doi-access=free}}</ref> diminished population growth,<ref name=":0">{{Cite journal |last=Leberg |first=P. L. |date=1990-12-01 |title=Influence of genetic variability on population growth: implications for conservation |journal=Journal of Fish Biology |language=en |volume=37 |pages=193–195 |doi=10.1111/j.1095-8649.1990.tb05036.x |bibcode=1990JFBio..37S.193L |issn=1095-8649}}</ref>  and ultimately, higher extinction risk.<ref>{{Cite journal |last=Frankham |first=Richard |date=2005-11-01 |title=Genetics and extinction |journal=Biological Conservation |volume=126 |issue=2 |pages=131–140 |doi=10.1016/j.biocon.2005.05.002|bibcode=2005BCons.126..131F }}</ref><ref>{{Cite journal |last1=Saccheri |first1=Ilik |last2=Kuussaari |first2=Mikko |last3=Kankare |first3=Maaria |last4=Vikman |first4=Pia |last5=Fortelius |first5=Wilhelm |last6=Hanski |first6=Ilkka |date=1998-04-02 |title=Inbreeding and extinction in a butterfly metapopulation |journal=Nature |language=en |volume=392 |issue=6675 |pages=491–494 |bibcode=1998Natur.392..491S |doi=10.1038/33136 |issn=0028-0836 |s2cid=4311360}}</ref>

[[Heterozygosity]], a fundamental measurement of genetic diversity in [[population genetics]], plays an important role in determining the chance of a population surviving environmental change, novel pathogens not previously encountered, as well as the average fitness within a population over successive generations. Heterozygosity is also deeply connected, in population genetics theory, to [[population size]] (which itself clearly has a fundamental importance to conservation). All things being equal, small populations will be less heterozygous{{Nbsp}}– across their whole genomes{{Nbsp}}– than comparable, but larger, populations. This lower heterozygosity (i.e. low genetic diversity) renders small populations more susceptible to the challenges mentioned above.<ref>{{Cite web |title=Effective Population Size - an overview {{!}} ScienceDirect Topics |url=https://www.sciencedirect.com/topics/earth-and-planetary-sciences/effective-population-size |access-date=2023-02-11 |website=www.sciencedirect.com}}</ref>

In a small population, over successive generations and without [[gene flow]], the probability of mating with close relatives becomes very high, leading to [[inbreeding depression]]{{Nbsp}}– a reduction in average fitness of individuals within a population. The reduced fitness of the offspring of closely related individuals is fundamentally tied to the concept of heterozygosity, as the offspring of these kinds of pairings are, by necessity, less heterozygous (more homozygous) across their whole genomes than outbred individuals. A diploid individual with the same maternal and paternal grandfather, for example, will have a much higher chance of being homozygous at any loci inherited from the paternal copies of each of their parents' genomes than would an individual with unrelated maternal and paternal grandfathers (each diploid individual inherits one copy of their genome from their mother and one from their father).

High homozygosity (low heterozygosity) reduces fitness because it exposes the phenotypic effects of recessive alleles at homozygous sites. Selection can favour the maintenance of alleles which reduce the fitness of homozygotes, the textbook example being the sickle-cell beta-globin allele, which is maintained at high frequencies in populations where malaria is endemic due to the highly adaptive heterozygous phenotype (resistance to the malarial parasite ''[[Plasmodium falciparum]]'').

Low genetic diversity also reduces the opportunities for [[chromosomal crossover]] during [[meiosis]] to create new combinations of alleles on chromosomes, effectively increasing the average length of unrecombined tracts of chromosomes inherited from parents. This in turn reduces the efficacy of selection, across successive generations, to remove fitness-reducing alleles and promote fitness-enhancing alleles from a population. A simple hypothetical example would be two adjacent genes{{Nbsp}}– A and B{{Nbsp}}– on the same chromosome in an individual. If the allele at A promotes fitness "one point", while the allele at B reduces fitness "one point", but the two genes are inherited together, then selection cannot favour the allele at A while penalising the allele at B{{Nbsp}}– the fitness balance is "zero points". Recombination can swap out alternative alleles at A and B, allowing selection to promote the optimal alleles to the optimal frequencies in the population{{Nbsp}}– but only if there are alternative alleles to choose between.

The fundamental connection between genetic diversity and population size in population genetics theory can be clearly seen in the classic population genetics measure of genetic diversity, the [[Watterson estimator]], in which genetic diversity is measured as a function of [[effective population size]] and [[mutation rate]]. Given the relationship between population size, mutation rate, and genetic diversity, it is clearly important to recognise populations at risk of losing genetic diversity before problems arise as a result of the loss of that genetic diversity. Once lost, genetic diversity can only be restored by [[mutation]] and gene flow. If a species is already on the brink of extinction there will likely be no populations to use to restore diversity by gene flow, and any given population will be small and therefore diversity will accumulate in that population by mutation much more slowly than it would in a comparable, but bigger, population (since there are fewer individuals whose genomes are mutating in a smaller population than a bigger population).