Gene Flow

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.

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.

As mentioned, gene flow and genetic drift are mechanisms that drive evolution, along with natural selection. Unlike natural selection, changes in alleles’ frequencies caused by gene flow or genetic drift are random (not related to an allele’s advantageous or disadvantageous effect on fitness). For example, the continuous reintroduction of the banded coloration pattern to islands’ populations is disadvantageous for water snakes.

However, gene flow and genetic drift differ in their specific causes and the direction of their outcomes.

Concerning their outcomes, we discussed how gene flow could increase the genetic diversity in a population (through the introduction of new alleles) and decrease the differences between populations (by sharing alleles). On the other hand, genetic drift (and natural selection) tend to reduce genetic diversity within a population and increase the differences between populations.