Editing Microevolution (section)

===Gene flow===
{{Main|Gene flow}}
Gene flow is the exchange of genes between populations, which are usually of the same species.<ref>{{cite journal |author=Morjan C, Rieseberg L |title=How species evolve collectively: implications of gene flow and selection for the spread of advantageous alleles |journal=Mol. Ecol. |volume=13 |issue=6 |pages=1341–56 |year=2004 |pmid=15140081 |doi=10.1111/j.1365-294X.2004.02164.x |pmc=2600545|last2=Rieseberg |bibcode=2004MolEc..13.1341M }}</ref> Examples of gene flow within a species include the migration and then breeding of organisms, or the exchange of [[pollen]]. Gene transfer between species includes the formation of [[Hybrid (biology)|hybrid]] organisms and [[horizontal gene transfer]].

Migration into or out of a population can change allele frequencies, as well as introducing genetic variation into a population. Immigration may add new genetic material to the established [[gene pool]] of a population. Conversely, emigration may remove genetic material. As [[reproductive isolation|barriers to reproduction]] between two diverging populations are required for the populations to [[speciation|become new species]], gene flow may slow this process by spreading genetic differences between the populations. Gene flow is hindered by mountain ranges, oceans and deserts or even man-made structures such as the [[Great Wall of China]], which has hindered the flow of plant genes.<ref>{{cite journal |author=Su H, Qu L, He K, Zhang Z, Wang J, Chen Z, Gu H |title=The Great Wall of China: a physical barrier to gene flow? |journal=Heredity |volume=90 |issue=3 |pages=212–9 |year=2003 |pmid=12634804 |doi=10.1038/sj.hdy.6800237|last2=Qu |last3=He |last4=Zhang |last5=Wang |last6=Chen |last7=Gu |s2cid=13367320 }}</ref>

Depending on how far two species have diverged since their [[most recent common ancestor]], it may still be possible for them to produce offspring, as with [[horse]]s and [[donkey]]s mating to produce [[mule]]s.<ref>{{cite journal |author=Short RV |title=The contribution of the mule to scientific thought |journal=J. Reprod. Fertil. Suppl. |issue=23 |pages=359–64 |year=1975 |pmid=1107543}}</ref> Such [[Hybrid (biology)|hybrid]]s are generally [[infertility|infertile]], due to the two different sets of chromosomes being unable to pair up during [[meiosis]]. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype.<ref>{{cite journal |author=Gross B, Rieseberg L |title=The ecological genetics of homoploid hybrid speciation |doi= 10.1093/jhered/esi026 |journal=J. Hered. |volume=96 |issue=3 |pages=241–52 |year=2005 |pmid=15618301 |pmc=2517139|last2=Rieseberg }}</ref> The importance of hybridization in developing [[hybrid speciation|new species]] of animals is unclear, although cases have been seen in many types of animals,<ref>{{cite journal |author=Burke JM, Arnold ML |title=Genetics and the fitness of hybrids |journal=Annu. Rev. Genet. |volume=35 |pages=31–52 |year=2001 |pmid=11700276 |doi=10.1146/annurev.genet.35.102401.085719 |last2=Arnold |issue=1 |s2cid=26683922 }}</ref> with the [[gray tree frog]] being a particularly well-studied example.<ref>{{cite journal |author=Vrijenhoek RC |title=Polyploid hybrids: multiple origins of a treefrog species |journal=Curr. Biol. |volume=16 |issue=7 |year=2006 |pmid=16581499 |doi=10.1016/j.cub.2006.03.005 |pages=R245–7 |s2cid=11657663 |doi-access=free |bibcode=2006CBio...16.R245V }}</ref>

Hybridization is, however, an important means of speciation in plants, since [[polyploidy]] (having more than two copies of each chromosome) is tolerated in plants more readily than in animals.<ref name=Wendel>{{cite journal |author=Wendel J |title=Genome evolution in polyploids |journal=Plant Mol. Biol. |volume=42 |issue=1 |pages=225–49 |year=2000 |pmid=10688139 |doi=10.1023/A:1006392424384 |s2cid=14856314 }}</ref><ref name=Semon>{{cite journal |author=Sémon M, Wolfe KH |title=Consequences of genome duplication |journal=Current Opinion in Genetics & Development |volume=17 |issue=6 |pages=505–12 |year=2007 |pmid=18006297 |doi=10.1016/j.gde.2007.09.007 |last2=Wolfe }}</ref> Polyploidy is important in hybrids as it allows reproduction, with the two different sets of chromosomes each being able to pair with an identical partner during meiosis.<ref>{{cite journal |author=Comai L |title=The advantages and disadvantages of being polyploid |journal=Nature Reviews Genetics |volume=6 |issue=11 |pages=836–46 |year=2005 |pmid=16304599 |doi=10.1038/nrg1711 |s2cid=3329282 }}</ref> Polyploid hybrids also have more genetic diversity, which allows them to avoid [[inbreeding depression]] in small populations.<ref>{{cite journal |author=Soltis P, Soltis D |title=The role of genetic and genomic attributes in the success of polyploids |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=97 |issue=13 |pages=7051–7 |date=June 2000 |pmid=10860970 |pmc=34383 |doi=10.1073/pnas.97.13.7051 |last2=Soltis |bibcode=2000PNAS...97.7051S |doi-access=free }}</ref>

[[Horizontal gene transfer]] is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among [[bacteria]].<ref>{{cite journal |author=Boucher Y, Douady CJ, Papke RT, Walsh DA, Boudreau ME, Nesbo CL, Case RJ, Doolittle WF |title=Lateral gene transfer and the origins of prokaryotic groups |doi=10.1146/annurev.genet.37.050503.084247 |journal=Annu Rev Genet |volume=37 |pages=283–328 |year=2003 |pmid=14616063|last2=Douady |last3=Papke |last4=Walsh |last5=Boudreau |last6=Nesbø |last7=Case |last8=Doolittle |issue=1 }}</ref> In medicine, this contributes to the spread of [[antibiotic resistance]], as when one bacteria acquires resistance genes it can rapidly transfer them to other species.<ref>{{cite journal |author=Walsh T |title=Combinatorial genetic evolution of multiresistance |journal=Current Opinion in Microbiology |volume=9 |issue=5 |pages=476–82 |year=2006 |pmid=16942901 |doi=10.1016/j.mib.2006.08.009 }}</ref> Horizontal transfer of genes from bacteria to eukaryotes such as the yeast ''[[Saccharomyces cerevisiae]]'' and the adzuki bean beetle ''Callosobruchus chinensis'' may also have occurred.<ref>{{cite journal |author=Kondo N, Nikoh N, Ijichi N, Shimada M, Fukatsu T |title=Genome fragment of Wolbachia endosymbiont transferred to X chromosome of host insect |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=99 |issue=22 |pages=14280–5 |year=2002 |pmid=12386340 |doi=10.1073/pnas.222228199 |pmc=137875 |last2=Nikoh |last3=Ijichi |last4=Shimada |last5=Fukatsu |bibcode=2002PNAS...9914280K |doi-access=free }}</ref><ref>{{cite journal |author=Sprague G |title=Genetic exchange between kingdoms |journal=Current Opinion in Genetics & Development |volume=1 |issue=4 |pages=530–3 |year=1991 |pmid=1822285 |doi=10.1016/S0959-437X(05)80203-5}}</ref> An example of larger-scale transfers are the eukaryotic [[Bdelloidea|bdelloid rotifers]], which appear to have received a range of genes from bacteria, fungi, and plants.<ref>{{cite journal |author=Gladyshev EA, Meselson M, Arkhipova IR |title=Massive horizontal gene transfer in bdelloid rotifers |journal=Science |volume=320 |issue=5880 |pages=1210–3 |date=May 2008 |pmid=18511688 |doi=10.1126/science.1156407|last2=Meselson |last3=Arkhipova |bibcode=2008Sci...320.1210G |s2cid=11862013 |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:3120157 |type=Submitted manuscript |url-access=subscription }}</ref> [[Virus]]es can also carry DNA between organisms, allowing transfer of genes even across [[domain (biology)|biological domains]].<ref>{{cite journal |author=Baldo A, McClure M |title=Evolution and horizontal transfer of dUTPase-encoding genes in viruses and their hosts |journal=J. Virol. |volume=73 |issue=9 |pages=7710–21 |date=1 September 1999|pmid=10438861 |pmc=104298 |last2=McClure |doi=10.1128/JVI.73.9.7710-7721.1999 }}</ref> Large-scale gene transfer has also occurred between the ancestors of [[eukaryote|eukaryotic cells]] and prokaryotes, during the acquisition of [[chloroplast]]s and [[Mitochondrion|mitochondria]].<ref name = "rgruqh">{{cite journal |author=Poole A, Penny D |title=Evaluating hypotheses for the origin of eukaryotes |journal=BioEssays |volume=29 |issue=1 |pages=74–84 |year=2007 |pmid=17187354 |doi=10.1002/bies.20516 |last2=Penny }}</ref>

''Gene flow'' is the transfer of [[alleles]] from one population to another.

Migration into or out of a population may be responsible for a marked change in allele frequencies. Immigration may also result in the addition of new genetic variants to the established [[gene pool]] of a particular species or population.

There are a number of factors that affect the rate of gene flow between different populations. One of the most significant factors is mobility, as greater mobility of an individual tends to give it greater migratory potential. Animals tend to be more mobile than plants, although pollen and seeds may be carried great distances by animals or wind.

Maintained gene flow between two populations can also lead to a combination of the two gene pools, reducing the genetic variation between the two groups. It is for this reason that gene flow strongly acts against [[speciation]], by recombining the gene pools of the groups, and thus, repairing the developing differences in genetic variation that would have led to full speciation and creation of daughter species.

For example, if a species of grass grows on both sides of a highway, pollen is likely to be transported from one side to the other and vice versa. If this pollen is able to fertilise the plant where it ends up and produce viable offspring, then the alleles in the pollen have effectively been able to move from the population on one side of the highway to the other.