Gene flow is responsible for the reduction of genetic variation in a population. This BiologyWise post explains this phenomenon in detail and provides some examples of gene flow.
Gene Affects Genetic Variation
The red wolf, endemic to southeastern United States, has become extinct in the wild due to habitat destruction, predator control programs, and hybridization of these wolves with coyotes. This hybridization of the two has led to the mixing of their gene pools and caused the allele frequency of the red wolf to change drastically.
All life on Earth is said to have originated from a single ancestor many billion years ago. It is through evolution of that single ancestor that we have a number of species of organisms today. Evolution can be described as the phenotypical changes that occur in heritable traits over successive generations. This process may give rise to diversity at every level of biological organization. These changes are brought about by four main evolutionary processes: mutation, natural selection, genetic drift, and gene flow.
Mutation is the change in the nucleotide sequence in the DNA, i.e., the hereditary unit of life to bring about a phenotypic change in the organism. Natural selection is the gradual process through which these phenotypic changes become more or less common in the population over time. Genetic drift can be simply described as the change in the number of individuals possessing a particular allele in a population over time. We will now talk about gene flow in detail in the following section.
Gene flow can be defined as the transfer of alleles or gametes from one population to another. It is also known as gene migration. When individuals of one population migrate to another population the allele frequency (the proportion of individuals carrying the same allele) of the population changes.
In simple words, if individuals of population A are introduced into population B, there is a change brought about in the composition of the gene pool of population B (through interbreeding). It may also result in the addition of new variants of that allele in the population.
The rate of gene flow depends on a lot of factors, the most important being the migratory potential of the individuals of a population. In plants, the rate of gene flow depends on the effectiveness of the mechanism of dispersal of pollen and seeds used. Mate choice is also a contributing factor to gene flow as the individuals that have immigrated to the population might not find suitable mates, or the off springs (hybrids) born might not be viable, and therefore, have no effect on the allele frequency. Other factors that may affect the rate of gene flow may include the distance between the two population, or certain physical barriers like mountains, rivers, or certain man-made structures.
How Does Gene Flow Prevent Speciation?
Speciation is an evolutionary process that may give rise to the formation of a new species. It is basically the splitting of lineages. Genetic variation due to accumulation of mutations and natural selection of the individuals with these variation are required for speciation to take place.
A continuous gene flow that is maintained between two populations may lead to the aggregation of the two gene pools. This homogenizes the gene frequency by reducing the genetic variation of the two populations. This results in nullifying the genetic differences that are needed for speciation to take place. It can therefore be said that gene flow has a negative effect on speciation.
Example of Gene Flow in Humans
❖ In recent years, gene flow has been observed between the Caucasian population and the African-American population. African-Americans are descendants of the natives of West Africa, whereas the Caucasians are descendants of the natives of Europe. The African-American population is inherently resistant to malaria whereas, the European population isn’t. The offspring produced by the mating of the individuals of these populations were seen to be resistant to the disease.
❖ Another example of gene flow was during the Vietnam War, when the American soldiers mated successfully with Vietnamese women, in the 1960s and 1970s, and altered the allele frequency of the Vietnamese population.
Other Examples of Gene Flow
❖ A pollen grain of a wind pollinated plant manages to fertilize some other plant to produce seed that give rise to viable offspring, then a change in the allele frequency may be brought about.
❖ A population of moths that are white in color migrate to a population of brown-colored moths and successfully mate to give rise to viable offspring. Here, we can say that there is a change in the allele frequency. Over time, the number of these white moths will increase.
❖ When the gray wolf mates with a coyote, this may give rise to viable red wolves, and therefore, a change in the number of individuals carrying a particular allele has been observed.
❖ In the Atlantic cod populations, high gene flow has been observed over a large geographical area. Thus, the genetic variations between these cod populations was low.
Horizontal Gene Transfer
Gene flow does not occur only by vertical gene transfer (from parents to offspring). Horizontal or lateral gene transfer is vital to bring about gene flow in lower eukaryotes, prokaryotes, and viruses. Horizontal gene transfer is basically the transfer of genes between organisms via methods other than asexual or sexual reproduction. The gene transfer, in this case, is not from parents to offspring here. This phenomenon may occur by a number of processes like transformation, transduction, and conjugation, and is responsible for the development of antibiotic resistance in bacteria.
❖ The evolution of Shiga toxin in E.coli has been through the transduction of this gene from Shigella species.
❖ Plasmodium vivax, the malarial pathogen, has acquired some human genes to prolong its infection in the human host.
Gene flow can have a profound effect on the organisms living in an ecosystem and can therefore determine the fate of an ecosystem. The study of gene flow is thus, essential to the conservation of biodiversity.