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| population genetics | |
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Migration into or out of a population may be responsible for a marked change in gene pool frequencies (the number of individual members with a particular trait). Immigration may result in the addition of new genetic material to the established Gene Pool of a particular species or population, and conversely emigration results 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 {Link without Title} . 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. Gene flow is greater in the North 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 s "set up residence separate from the host's genome" {Link without Title} . "Sequence comparisons suggest recent horizontal transfer of many Gene s 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." {Link without Title} Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research" therefore "biologists 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." [http://www.esalenctr.org/display/confpage.cfm?confid=10&pageid=105&pgtype=1] "Using single Gene s as Phylogenetic Marker s, it is difficult to trace organismal Phylogeny in the presence of HGT gene transfer . Combining the simple Coalescence model of Cladogenesis with rare HGT 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." [http://web.uconn.edu/gogarten/articles/TIG2004_cladogenesis_paper.pdf] MODELS OF GENE FLOW Models of gene flow can be derived from Population Genetics , e.g. Sewall Wright 's Neighborhood Model , Wright's Island Model and the Stepping Stone Model . REFERENCES
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