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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.
Gene flow in humans
Gene flow has been observed in humans, for example in the United States, 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
Genes can flow between species, as when bacterial DNA is transferred to animals or plants.
One source of genetic variation is gene transfer, the movement of genetic material across species boundaries, which includes horizontal gene transfer, antigenic shift, and reassortment. Viruses can transfer genes between species . Bacteria can incorporate genes from other dead bacteria, exchange genes with living bacteria, and can have plasmids "set up residence separate from the host's genome" . "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." 
"Using single genes as phylogenetic markers, it is difficult to trace organismal phylogeny in the presence of HGT [horizontal gene transfer]. Combining the simple coalescence model of cladogenesis with rare HGT [horizontal gene transfer] events suggest there was no single last common ancestor that contained all of the genes ancestral to those shared among the three domains of life. Each contemporary molecule has its own history and traces back to an individual molecule cenancestor. However, these molecular ancestors were likely to be present in different organisms at different times." 
Models of gene flow
|Basic topics in evolutionary biology||(edit)|
|Processes of evolution: evidence - macroevolution - microevolution - speciation|
|Mechanisms: selection - genetic drift - gene flow - mutation - phenotypic plasticity|
|Modes: anagenesis - catagenesis - cladogenesis|
|History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis|
|Subfields: population genetics - ecological genetics - human evolution - molecular evolution - phylogenetics - systematics - evo-devo|
|List of evolutionary biology topics | Timeline of evolution | Timeline of human evolution|
- Su, H et al. (2003). The Great Wall of China: a physical barrier to gene flow?. Heredity, Vol. 9, 212-219.
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