Gene Flow as a Mechanism of Evolution
Movement of Genes Between Populations Results in Evolutionary Change
Jun 15, 2009
Gene flow (also known as gene migration) results when genes are transferred from one population to another. If genes are carried to a population where those genes did not previously exist, gene flow can be an important source of genetic variation.
Mobility Determines the Rate of Gene Flow
The most significant factor affecting the rate of gene flow between different populations is the mobility of the organisms making up those populations. Animal populations are more mobile than plant populations and thus show a greater tendency to experience gene flow.
Migration (new individuals coming in) can add new genes to an established gene pool while emigration (individuals leaving a population) may result in the removal of genes. If two populations maintain a gene flow between them, this can lead to a combination of both their gene pools, reducing the genetic variation between the two groups.
“The very idea of a gene pool has no meaning if there is no sex. 'Gene Pool' is a persuasive metaphor because the genes of a sexual population are being continually mixed and diffused, as if in a liquid. Bring in the time dimension, and the pool becomes a river, flowing through geological time..."
(Richard Dawkins, The Ancestor's Tale, page 432).
Human Development is a Major Barrier to Gene Flow
Impassable mountains, open oceans, or vast tracts of deserts present natural barriers to the migration of genes between populations. Also, human development of the landscape has thrown up many artificial barriers. A multi-lane highway stretching for miles can be a major barrier for the movement of small animals.
Human development and occupation can also fragment ecosystems into isolated islands and in the process seriously reduce the genetic variation of the creatures imprisoned on those islands. A great challenge facing conservation biologists is the maintenance or establishment of corridors or connections between these isolated fragments in order to maintain a healthy genetic variation through the facilitation of gene flow.
Human Mobility Generates High Gene Flow
While human constructions, occupation, and activity may pose barriers to other species, humans themselves are the most mobile and wide-ranging animal on the planet. Genetic analyses of humans of different geographical regions and races reveal tremendous rates of gene flow and a subsequent mixing of many human gene pools or the “dilution of the races” as it has been called.
Gene Flow in Action
Imagine a population of small fish in a lake (Lake A). Suppose this population of small fish is mixed with both dark and bright color variants. The dark-colored fish are the ancestral phenotype and represent the usual coloration of the species. The bright-colored fish, however, arose through germ line mutation. Both populations are being maintained by natural selection. Also suppose that a land barrier of a short distance separates Lake A from another small lake with a population of only the normal dark colored fish (Lake B). After torrential rains lasting for days, the lakes overflow and a few of the bright-colored fish only from Lake A are washed into Lake B. This unintentional migration of bright-colored fish has resulted in gene flow from Lake A to Lake B.
Mechanisms of Evolution
Evolution encompasses changes on two vastly different scales – from an increase in the frequency of a gene for colored spots on the feathers of a bird (microevolution) to something as grand in scale as the evolution of the entire bird lineage (macroevolution). Despite the scale on which it happens, evolution at both levels is driven by the primary mechanisms of natural selection, mutation, genetic drift, and gene flow.
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