GENE FLOW: MECHANISM, CONSEQUENCES AND EXAMPLES - BIOLOGY - 2025

The flow of genes or gene flow, in biology, refers to the movement of genes from one population to another. Generally, the term is used synonymously with the migratory process - in its evolutionary sense.

In its common use, migration describes the seasonal movement of individuals from one region to another, in search of better conditions, or for reproductive purposes. However, for an evolutionary biologist, migration involves the transfer of alleles from a set of genes between populations.

Source: Jessica Krueger, from Wikimedia Commons

In the light of population genetics, evolution is defined as the change in allele frequencies over time.

Following the principles of Hardy-Weinberg equilibrium, the frequencies will vary whenever there is: selection, mutation, drift and gene flow. For this reason, gene flow is considered an evolutionary force of great importance.

Mechanisms of gene flow

The mechanisms and causes that originate the movement of genes in a population are strongly linked to the inherent characteristics of the study group. It can occur due to the immigration or emigration of certain individuals in a reproductive state, or it may result from the movement of the gametes.

For example, one mechanism may be the occasional dispersal of juvenile forms of an animal species to distant populations.

In the case of plants, the mechanisms are easier to pin down. Plant gametes are transported in different ways. Some lineages use abiotic mechanisms, such as water or wind, that can carry genes to distant populations.

Similarly, there is biotic dispersal. Many frugivorous animals participate in the dispersal of the seeds. For example, in the tropics, birds and bats play a crucial role in the dispersal of plants of great importance to ecosystems.

In other words, the migration rate and gene flow depend on the dispersal capacity of the lineage studied.

Migration and the Hardy-Weinberg equilibrium

To study the effect of migration on the Hardy-Weinberg equilibrium, the island model is often used as a simplification (island-continent migration model).

As the population of the island is relatively small compared to the population of the mainland, any gene transfer from the island to the mainland has no effect on the genotype and allele frequencies of the mainland.

For this reason, gene flow would have an effect only in one direction: from the mainland to the island.

Do the allele frequencies vary?

To understand the effect of the migratory event to the island, consider the hypothetical example of a locus with two alleles A ₁ and A ₂. We must find out if the movement of genes to the island causes variation in allele frequencies.

Let's assume that the frequency of the A ₁ allele is equal to 1 - which means that it is fixed in the population, while in the continental population it is the A ₂ allele that is fixed. Prior to the maturation of the individuals on the island, 200 individuals migrate to it.

After the gene flow, the frequencies will change, and now 80% will be "native", while 20% are new or continental. With this simple example, we can demonstrate how gene movement leads to allele frequency change - a key concept in evolution.

Consequences of gene flow

When there is a marked flow of genes between two populations, one of the most intuitive consequences is that this process is responsible for diluting the possible differences between both populations.

In this way, gene flow can act in the opposite direction to other evolutionary forces that seek to maintain differences in the composition of genetic reservoirs. Like the mechanism of natural selection, for example.

A second consequence is the spread of beneficial alleles. Let us suppose that by mutation a new allele arises that gives a certain selective advantage to its carriers. When there is migration, the novel allele is transported to new populations.

Gene flow and species concept

The biological concept of species is widely known and is certainly the most widely used. This definition fits the conceptual scheme of population genetics, as it involves the gene pool - the unit where allele frequencies change.

In this way, by definition, genes do not pass from one species to another - there is no gene flow - and for this reason species exhibit certain characteristics that allow them to be differentiated. Following this line of ideas, gene flow explains why species form a “cluster” or phenetic grouping.

Furthermore, the disruption of gene flow has crucial consequences in evolutionary biology: it leads - in most cases - to speciation events or the formation of new species. The flow of genes can be interrupted by different factors, such as the existence of a geographic barrier, preferences at the courtship level, among other mechanisms.

The reverse is also true: the existence of gene flow contributes to all organisms in a region remaining as one species.

Example

The migration of the snake Nerodia sipedon is a well-documented case of gene flow from a continental population to an island.

The species is polymorphic: it may have a significant banding pattern or no band at all. In a simplification, the coloration is determined by one locus and two alleles.

In general terms, the snakes of the continent are characterized by exhibiting the pattern of bands. In contrast, those that inhabit the islands do not possess them. The researchers have concluded that the morphological difference is due to the different selective pressures to which each region is subjected.

On the islands, individuals tend to sunbathe on the surface of the rocks near the shore of the beach. The absence of bands was shown to facilitate camouflage on the rocks of the islands. This hypothesis could be tested using tagging and recapture experiments.

For this adaptive reason, we would expect the island's population to be made up exclusively of unbanded organisms. However, this is not true.

Each generation comes a new group of banded organisms from the continent. In this case, migration is acting as a force against selection.

References

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