Editing Chromosomal inversion (section)

== Evolutionary consequences ==
The suppressed recombination between inversion heterozygotes provides an opportunity for the independent evolution of the ancestral and inverted arrangements. At the beginning, the inverted arrangement lacks variation, while the ancestral one does not. If the inverted [[haplotype]] is not lost (e.g. due to [[Genetic drift|drift]]), the variation in the inverted arrangement can increase over time, and recombination rate in the inverted region is somewhat restored as more homozygotes are introduced.<ref name=":4">{{cite journal | vauthors = Faria R, Johannesson K, Butlin RK, Westram AM | title = Evolving Inversions | journal = Trends in Ecology & Evolution | volume = 34 | issue = 3 | pages = 239–248 | date = March 2019 | pmid = 30691998 | doi = 10.1016/j.tree.2018.12.005 | s2cid = 59339762 | doi-access = free }}</ref> 

Chromosomal inversions have gained a lot of attention in evolutionary research due to their potential role in local adaptation and speciation. Because non-recombining inversion haplotypes may harbor multiple co-adapted gene variants, inversions are thought to facilitate local adaptation to different environments because natural selection is more efficient in driving such linked adaptive variants to high frequency within a population.<ref>Rieseberg, L.H. (2001). Chromosomal rearrangements and speciation. Trends in Ecology & Evolution, 16, 351-358.</ref> However, empirically demonstrating the presence of linked, co-adapted gene variants within inversions is difficult because inversion haplotypes do not recombine. Moreover, this possible positive effect of chromosomal inversions for adaptation to different environments rests on the assumption that adaptive gene variants linked into distinct inversion haplotypes are indeed co-adapted. This idea is, however, likely violated in situations where populations experience spatially or temporally varying selection. Because of fluctuating selection on inversion-linked variants, the absence of recombination between inversion haplotypes harboring distinct gene variants may then constrain rather than help adaptation to distinct environments.<ref> Roesti, M., Gilbert, K. J., & Samuk, K. (2022). Chromosomal inversions can limit adaptation to new environments. Molecular Ecology, 31, 4435–4439. https://doi.org/10.1111/mec.16609 </ref> The importance of chromosomal inversions in adaptation to different environments therefore remains an open empirical problem in evolutionary genetics.      

Inversion polymorphism can be established in two ways. Genetic drift or selection can result in [[Fixation (population genetics)|fixation]] of an inversion in a local population. Inversion polymorphism can result from [[gene flow]] between this population and a population without the inversion. [[Balancing selection]] can also result in inversion polymorphism by frequency dependence or [[overdominance]].<ref name=":4" /> The [[Fitness (biology)|fitness]] differences between the inverted and the ancestral chromosome can either produce a stable polymorphism or can result in the fixation of one or the other chromosome.<ref name=":5">{{cite journal | vauthors = Kirkpatrick M | title = How and why chromosome inversions evolve | journal = PLOS Biology | volume = 8 | issue = 9 | pages = e1000501 | date = September 2010 | pmid = 20927412 | pmc = 2946949 | doi = 10.1371/journal.pbio.1000501 | doi-access = free }}</ref>

Inversions have been essential to [[sex chromosome]] evolution. In mammals, the Y chromosome is unable to recombine with the X chromosome, almost along its entire length. This non-recombining portion results from a series of inversions that overlap. Decreased recombination rate between sex determining [[Locus (genetics)|loci]] and sex-anatagonistic genes is favored by selection. This causes linkage disequilibrium between the male determining locus and an [[allele]] at another locus that is beneficial to males. This can happen through inversions resulting in a non-recombining block including both loci, as is the case in the mammalian Y chromosome.<ref name=":5" />

Inversions can also be essential in the origination of new sex chromosomes. They can cause linkage disequilibrium between a sex-determining mutation and sex-antagonistic loci and create a new sex chromosome from an autosome.<ref>{{cite journal | vauthors = van Doorn GS, Kirkpatrick M | title = Turnover of sex chromosomes induced by sexual conflict | journal = Nature | volume = 449 | issue = 7164 | pages = 909–912 | date = October 2007 | pmid = 17943130 | doi = 10.1038/nature06178 | s2cid = 4301225 | bibcode = 2007Natur.449..909V }}</ref>

Inversions can be involved in [[speciation]] in multiple ways. Since heterozygote inversions can be [[Underdominance|underdominant]], they can cause hybrid fitness loss, resulting in [[post-zygotic isolation]]. They can also accumulate selected differences between species, causing both pre- and post-zygotic isolation.<ref name=":5" />

Inversions often form geographical clines in frequency which can hint to their role in local adaptation. A prominent instance of such a cline is inversion 3RP in ''Drosophila melanogaster'' that can be observed in three different continents.<ref name=":5" /> When an inversion contains two or more locally adaptive alleles, it can be selected and spread. For example; in the butterfly ''[[Heliconius numata]],'' 18 genes controlling colors are linked together by inversions as together they confer higher fitness.<ref>{{cite journal | vauthors = Joron M, Frezal L, Jones RT, Chamberlain NL, Lee SF, Haag CR, Whibley A, Becuwe M, Baxter SW, Ferguson L, Wilkinson PA, Salazar C, Davidson C, Clark R, Quail MA, Beasley H, Glithero R, Lloyd C, Sims S, Jones MC, Rogers J, Jiggins CD, ffrench-Constant RH | display-authors = 6 | title = Chromosomal rearrangements maintain a polymorphic supergene controlling butterfly mimicry | journal = Nature | volume = 477 | issue = 7363 | pages = 203–206 | date = August 2011 | pmid = 21841803 | pmc = 3717454 | doi = 10.1038/nature10341 | bibcode = 2011Natur.477..203J }}</ref>