Migration into or out of a population may be responsible for a marked change in allele frequencies (the number of individual members carrying a particular variant of a gene). Immigration may result in the addition of new genetic material to the established gene pool of a particular species or population, and conversely emigration may result in the removal of genetic material.
There are a number of factors that affect the rate of gene flow between different populations. One of the most significant factors is mobility, and animals tend to be more mobile than plants. Greater mobility of an individual tends to give it greater migratory potential.
Barriers to gene flow
Physical barriers to gene flow are usually, but not always, natural. They may include impassable mountain ranges or vast deserts, or something so simple as the Great Wall of China, which has hindered the natural flow of plant genes . Examples of the same species which grow on either side have been shown to be genetically different.
Behavioral differences in geographically isolated populations can prevent gene flow and lead to speciation. For instance, difference in seasonal timing of flowering can interrupt cross-pollination in flowering plant populations.
Gene flow in humans
Gene flow has been observed in humans, for example in the United States of America, where a white European population and a black West African population were recently brought together. The Duffy blood group gives carriers some resistance to malaria, and as a result in West Africa, where malaria is prevalent, the Fyo allele is essentially one hundred percent. In Europe, which has much lower levels of malaria, have either allele Fya or Fyb. By measuring the frequencies, the rate of gene flow between the two populations can be measured, showing that gene flow is greater in the Northern U.S. than in the South.
Gene flow between species
- See also Horizontal gene transfer
Genes can flow between species by a variety of mechanisms, including cross-hybridization (as in many land plants ), during phagocytosis of food in unicellular protists  , which often take up new genes from bacteria they engulf, or direct DNA uptake (as in bacterial DNA transformation and promiscuous conjugational mating by bacteria . Viruses can transfer genes between species.
"Sequence comparisons suggest recent horizontal transfer of many genes among diverse species including across the boundaries of phylogenetic "domains". Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes." 
Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research" therefore "biologists [should] use the metaphor of a mosaic to describe the different histories combined in individual genomes and use [the] metaphor of a net to visualize the rich exchange and cooperative effects of HGT among microbes." 
Gene flow provides serious challenges to the reconstruction of early events in the tree of life .
In pathogenic microorganisms, particularly when rates of horizontal gene transfer are high due to processes such as DNA transformation that occurs with some organisms inhabiting the upper respiratory tract of humans, evolutionary lineages can still be analysed by considering DNA sequence change that occurs in several genes. This techniques is called multi-locus sequence typing   .
Models of gene flow
- Su, H et al. (2003) "The Great Wall of China: a physical barrier to gene flow?." Heredity, Volume 9 Pages 212-219
- Rieseberg, L.H., Kim, M. J., and Seiler, G. J. (1999). Introgression between cultivated sunflowers and a sympatric wild relative, Helianthus petiolaris (Asteraceae). Int. J. Plant Sci 160, p102-108.
- Loftus B, Anderson I, Davies R, Alsmark UCM, Samuelson J, Amedeo P, Roncaglia P, Berriman M, Hirt RP, Mann BJ, Nozaki T, Suh B, Pop M, Duchene M, Ackers J, Tannich E, Leippe M, Hofer M, Bruchhaus I, Willhoeft U, Bhattacharya A, Chillingworth T, Churcher C, Hance Z, Harris B, Harris D, Jagels K, Moule S, Mungall K, Ormond D, Squares R, Whitehead S, Quail MA, Rabbinowitsch E, Norbertczak H, Price C, Wang Z, Guillen N, Gilchrist C, Stroup SE, Bhattacharya S, Lohia A, Foster PG, Sicheritz-Ponten T, Weber C, Singh U, Mukherjee C, El- Sayed NM, Petri WAJ, Clark CG, Embley TM, Barrell B, Fraser CM, Hall N: The genome of the protist parasite Entamoeba histolytica. Nature 2005, 433(7028):865-868.
- Huang J, Mullapudi N, Lancto CA, Scott M, Abrahamsen MS, Kissinger JC: Phylogenomic evidence supports past endosymbiosis, intracellular and horizontal gene transfer in Cryptosporidium parvum. Genome Biol 2004, 5(11):R88.
- Pennisi, E. (2004) Researchers Trade Insights About Gene Swapping:Genes that move between species play by rules that microbial experts are just beginning to discern SCIENCE VOL 305 16 JULY 2004 p334-335
- Urwin R, Maiden MC. (2006) Multi-locus sequence typing: a tool for global epidemiology.Trends Microbiol. 2003 Oct;11(10):479-87. Review.
- Delorme C, Poyart C, Ehrlich SD, Renault P., (2006) Extent of Horizontal Gene Transfer in Evolution of Streptococci of the Salivarius Group. J Bacteriol. 2006 Nov 3; [Epub ahead of print] PMID: 17085557
- Johnson JR, Owens KL, Clabots CR, Weissman SJ, Cannon SB. (2006) Phylogenetic relationships among clonal groups of extraintestinal pathogenic Escherichia coli as assessed by multi-locus sequence analysis. Microbes Infect. 2006 Jun;8(7):1702-13. Epub 2006 Apr 21.
- Jolley KA, Chan MS, Maiden MC.(2004) mlstdbNet - distributed multi-locus sequence typing (MLST) databases.BMC Bioinformatics. 2004 Jul 1;5:86.