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, 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
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