Genetic equilibrium
Genetic equilibrium is the condition of an allele or genotype in a gene pool (such as a population) where the frequency does not change from generation to generation.{{cite web | url=http://medical-dictionary.thefreedictionary.com/genetic+equilibrium | title=Genetic equilibrium}} Genetic equilibrium describes a theoretical state that is the basis for determining whether and in what ways populations may deviate from it. Hardy–Weinberg equilibrium is one theoretical framework for studying genetic equilibrium. It is commonly studied using models that take as their assumptions those of Hardy-Weinberg, meaning:
- No gene mutations occurring at that locus or the loci associated with the trait
- A large population size
- Limited-to-no immigration, emigration, or migration (genetic flow)
- No natural selection on that locus or trait
- Random mating (panmixis)
It can describe other types of equilibrium as well, especially in modeling contexts. In particular, many models use a variation of the Hardy–Weinberg principle as their basis. Instead of all of the Hardy–Weinberg characters being present, these instead assume a balance between the diversifying effects of genetic drift and the homogenizing effects of migration between populations.{{Cite journal |doi=10.1111/j.1365-294X.2007.03648.x |pmid=18302693|year=2008|last1=Duvernell|first1=D. D.|title=Relative influences of historical and contemporary forces shaping the distribution of genetic variation in the Atlantic killifish, Fundulus heteroclitus|journal=Molecular Ecology|volume=17|issue=5|pages=1344–60|last2=Lindmeier|first2=J. B.|last3=Faust|first3=K. E.|last4=Whitehead|first4=A.|bibcode=2008MolEc..17.1344D |s2cid=27306569 }} A population not at equilibrium suggests that one of the assumptions of the model in question has been violated.
Theoretical models of genetic equilibrium
The Hardy–Weinberg principle provides the mathematical framework for genetic equilibrium. Genetic equilibrium itself, whether Hardy-Weinberg or otherwise, provides the groundwork for a number of applications, in including population genetics, conservation and evolutionary biology. With the rapid increase in whole genome sequences available as well as the proliferation of anonymous markers, models have been used to extend the initial theory to all manner of biological contexts.{{Cite journal |doi=10.1002/gepi.20612 |pmc=4141651 |pmid=21922537|year=2011 |last1=Shriner |first1=D. |title=Approximate and exact tests of Hardy-Weinberg equilibrium using uncertain genotypes |journal=Genetic Epidemiology |volume=35 |issue=7 |pages=632–7 }} Using data from genetic markers such as ISSRs and RAPDs as well as the predictive potential of statistics, studies have developed models to infer what processes drove the lack of equilibrium. This includes local adaptation, range contraction and expansion and lack of gene flow due to geographic or behavioral barriers, although equilibrium modeling has been applied to a wide range of topics and questions.
Equilibrium modeling have led to developments in the field. Because allelic dominance can disrupt predictions of equilibrium,Kramer, Koen; van der Werf, D. C. (2010). [http://revistas.inia.es/index.php/fs/article/download/1931/1494 "Equilibrium and non-equilibrium concepts in forest genetic modeling: population- and individually-based approaches,"] Forest Systems, 19(SI): 100–112. some models have moved away from using genetic equilibrium as an assumption. Instead of the traditional F-statistics, they make use of Bayesian estimates.{{Cite journal |url=http://www.genetics.org/content/163/3/1177.full.pdf |pmid=12663554|pmc=1462502|year=2003|last1=Wilson|first1=G. A.|title=Bayesian inference of recent migration rates using multilocus genotypes|journal=Genetics|volume=163|issue=3|pages=1177–91|last2=Rannala|first2=B.|doi=10.1093/genetics/163.3.1177 }} Holsinger et al. developed an analog to FST, called theta.{{Cite journal |doi=10.1046/j.1365-294X.2002.01512.x |pmid=12074723|year=2002|last1=Holsinger|first1=K. E.|title=A Bayesian approach to inferring population structure from dominant markers|journal=Molecular Ecology|volume=11|issue=7|pages=1157–64|last2=Lewis|first2=P. O.|last3=Dey|first3=D. K.|bibcode=2002MolEc..11.1157H |s2cid=11461448 |url=http://opencommons.uconn.edu/eeb_articles/1|type=Submitted manuscript}} Studies have found Bayesian estimates to be better predictors of the patterns observed.{{Cite journal |doi=10.1111/j.1442-1984.2008.00211.x|title=Conservation of rare species with island-like distributions: A case study of Lasthenia conjugens(Asteraceae) using population genetic structure and the distribution of rare markers|journal=Plant Species Biology|volume=23|issue=2|pages=97–110|year=2008|last1=Ramp Neale|first1=Jennifer M.|last2=Ranker|first2=TOM A.|last3=Collinge|first3=Sharon K.|doi-access=free}} However, genetic equilibrium-based modeling remains a tool in population and conservation genetics-it can provide invaluable information about previous historical processes.
Biological study systems
Genetic equilibrium has been studied in a number of taxa. Some marine species in particular have been used as study systems. The life history of marine organisms like sea urchins appear to fulfill the requirements of genetic equilibrium modeling better than terrestrial species.{{Cite journal |last1=Palumbi |first1=Stephen R. |author-link=Stephen Palumbi |last2=Grabowsky |first2=Gail |last3=Duda |first3=Thomas |last4=Geyer |first4=Laura |last5=Tachino |first5=Nicholas |year=1997 |title=Speciation and Population Genetic Structure in Tropical Pacific Sea Urchins |journal=Evolution |volume=51 |issue=5 |pages=1506–1517 |doi=10.1111/j.1558-5646.1997.tb01474.x |pmid=28568622 |doi-access=free }}. They exist in large, panmictic populations that don’t appear to be strongly affected by geographic barriers. In spite of this, some studies have found considerable differentiation across the range of a species. Instead, when looking for genetic equilibrium, studies found large, widespread species complexes.{{Cite journal |doi=10.1146/annurev.es.24.110193.001201|title=Sibling Species in the Sea|journal=Annual Review of Ecology and Systematics|volume=24|pages=189–216|year=1993|last1=Knowlton|first1=Nancy|issue=1 |bibcode=1993AnRES..24..189K }}. This indicates that genetic equilibrium may be rare or difficult to identify in the wild, due to considerable local demographic changes on shorter time scales.{{Cite journal |doi=10.1111/j.1558-5646.1992.tb02069.x |pmid=28568658|year=1992|last1=Whitlock|first1=M. C.|title=Temporal Fluctuations in Demographic Parameters and the Genetic Variance Among Populations|journal=Evolution; International Journal of Organic Evolution|volume=46|issue=3|pages=608–615|s2cid=20983027 }}
In fact, although a large population size is a required condition for genetic equilibrium according to Hardy–Weinberg, some have argued that a large population size can actually slow the approach to genetic equilibrium.{{Cite journal |url=http://www.genetics.org/content/103/3/513.full.pdf |pmc=1202037 |pmid=6840539|year=1983 |last1=Birky Jr |first1=C. W. |title=An approach to population and evolutionary genetic theory for genes in mitochondria and chloroplasts, and some results |journal=Genetics |volume=103 |issue=3 |pages=513–27 |last2=Maruyama |first2=T. |last3=Fuerst |first3=P. |doi=10.1093/genetics/103.3.513 }} This can have implications for conservation, where genetic equilibrium can be used as a marker of a healthy and sustainable population.