Editing Molecular evolution (section)

===Chromosome number and organization===
The ant ''Myrmecia pilosula'' has only a single pair of chromosomes<ref name=Crosland>{{cite journal | vauthors = Crosland MW, Crozier RH | title = Myrmecia pilosula, an Ant with Only One Pair of Chromosomes | journal = Science | volume = 231 | issue = 4743 | pages = 1278 | date = March 1986 | pmid = 17839565 | doi = 10.1126/science.231.4743.1278 | s2cid = 25465053 | bibcode = 1986Sci...231.1278C }}</ref> whereas the Adders-tongue fern  ''[[Ophioglossum]] reticulatum'' has up to 1260 chromosomes.<ref name="Grubben2004">{{cite book|author=Gerardus J. H. Grubben|title=Vegetables|url=https://archive.org/details/bub_gb_6jrlyOPfr24C|access-date=10 March 2013|year=2004|publisher=PROTA|isbn=978-90-5782-147-9|page=[https://archive.org/details/bub_gb_6jrlyOPfr24C/page/n404 404]}}</ref> The  [[List of organisms by chromosome count|number of chromosomes]] in an organism's genome does not necessarily correlate with the amount of DNA in its genome. The genome-wide amount of [[Genetic recombination|recombination]] is directly controlled by the number of chromosomes, with one [[Chromosomal crossover|crossover]] per chromosome or per chromosome arm, depending on the species.<ref>{{cite journal |last1=Pardo-Manuel de Villena |first1=Fernando |last2=Sapienza |first2=Carmen |title=Recombination is proportional to the number of chromosome arms in mammals |journal=Mammalian Genome |date=April 2001 |volume=12 |issue=4 |pages=318–322 |doi=10.1007/s003350020005|pmid=11309665 }}</ref>

Changes in chromosome number can play a key role in [[speciation]], as differing chromosome numbers can serve as a [[Reproductive isolation|barrier to reproduction]] in hybrids.  Human [[chromosome 2]] was created from a fusion of two chimpanzee chromosomes and still contains central [[telomeres]] as well as a vestigial second [[centromere]].  [[Polyploidy]], especially allopolyploidy, which occurs often in plants, can also result in reproductive incompatibilities with parental species.  ''Agrodiatus'' blue butterflies have diverse chromosome numbers ranging from n=10 to n=134 and additionally have one of the highest rates of speciation identified to date.<ref name="Butterflies">{{cite journal | vauthors = Kandul NP, Lukhtanov VA, Pierce NE | title = Karyotypic diversity and speciation in Agrodiaetus butterflies | journal = Evolution; International Journal of Organic Evolution | volume = 61 | issue = 3 | pages = 546–559 | date = March 2007 | pmid = 17348919 | doi = 10.1111/j.1558-5646.2007.00046.x | doi-access = free }}</ref>

[[Cilliate]] genomes house each gene in individual chromosomes.