Gene Flow

You have probably seen or heard about migratory animals, like some birds and mammals. Migration, or dispersal, is a common event in wild populations of animals and plants. What is the effect of an organism carrying its genetic material to another population? This receiving population is probably exposed to different environmental conditions than the migrating organism came from. We will discuss how these migrating organisms affect population evolution and genetics.

Gene Flow definition

Although we usually think of animals when we talk about migration, plants can also interchange genetic material with other populations, so sometimes, the term dispersal is preferred over migration. In plants, it is not the adult organism that moves, but the gametes carried in pollen or the seeds carried in fruits (through wind, water, or by animals like birds and mammals). Remember that gene flow occurs only when a gene or allele enters a receiving population's gene pool. Thus, an organism not only has to reach and live in the receiving population, but it must reproduce and pass on its genes to this population.

When there is a significant rate of gene flow between two populations, they can eventually share the same gene pool. Hence gene flow tends to homogenize the genetic makeup of populations.

Individuals adapt to their local environment through natural selection. Will a trait or allele that makes an individual more fitted to its environment have the same beneficial effect in the receiving population? The introduction or reintroduction of a trait to a population through gene flow is considered a random event in population genetics; therefore, it does not necessarily mean that it will benefit the population. This will depend on the selective pressures for each population.

Examples of Gene Flow and diagram

Why haven’t the alleles for the banding pattern disappeared from these island populations? About 3-10 snakes are estimated to migrate to the islands per year, reintroducing the alleles for the band pattern into the islands. Thus, gene flow from the mainland maintains these alleles in the islands despite them being disadvantageous in that environment (Figure 1).

Fig. 1 - Simplified diagram of intraspecific gene flow. Thick arrows represent natural selection over many generations

Based on the water snake example, we can infer that gene flow from the mainland tends to homogenize the genetic diversity of these populations and tends to make them more similar by the continuous reintroduction of the band pattern alleles (and, of course, all the genetic material these mainland individuals carry with them).

However, in this case, the introduced alleles are disadvantageous in the rocky environment of the receiving population. Therefore, their frequency does not increase due to natural selection. But what would happen if the introduced allele were advantageous in a receiving population?

Gene flow can occur between populations of the same species (intraspecifically) and between populations of different species (interspecifically). It is important to notice that gene flow between these two Anopheles species was possible because these are young species (they separated relatively recently). They still reproduce in areas where both species live. In these areas, they produce hybrid individuals (organisms whose parents are from different species). These hybrids mated with A. coluzzi individuals, introducing the allele to the gene pool of A. coluzzi (Figure 2).

Fig. 2 - Simplified diagram of interspecific gene flow. Thick arrows represent natural selectionover many generations, thin arrows represent gene flow, and colored dots represent alleles.

These mosquito hybrids have reduced fitness, though, signifying that strong reproductive isolation is in process. If gene flow stops between these populations, they will keep diverging genetically while each species continues to adapt to local conditions. For subpopulations of the same species, as with the water snakes, lack of gene flow would eventually lead to speciation.

Gene Flow Can Cause Evolution

Gene flow produces changes to allele frequencies in a population; therefore, it is one of the main mechanisms that drive evolution, along with genetic drift and natural selection. However, these changes will only be adaptive if the introduced genes or alleles are beneficial to the receiving population, as with the insecticide resistance allele in mosquitoes. In this sense, introducing new variants through gene flow is random, similar to a mutation. And similarly, after being introduced, it depends on the effect of natural selection to increase or decrease the frequency of an introduced allele.

How do Gene flow and Genetic drift differ?