Gene flow

Gene flow is the transfer of genetic material between different populations of a species. This process is also known as migration or allele flow. It occurs when individuals move from one group to another and reproduce. By doing so, they introduce new alleles into the new population. Alleles are the different versions of a gene that carry specific traits.

Gene flow is a vital mechanism for moving genetic diversity across a species. It changes the distribution of this diversity by modifying allele frequencies. Allele frequency is the proportion of members in a group who carry a specific genetic variant.

How gene flow affects populations depends on its rate. If the rate of gene flow is high, the two populations will develop equivalent allele frequencies. In this case, they can be considered a single effective population. High rates of gene flow increase homogeneity, which means the groups become more similar. This process can actually constrain speciation. Speciation is the process where new species form. By combining gene pools, gene flow prevents populations from developing the distinct differences needed to adapt to different environments. However, if selection pressure is strong enough, populations can still diverge even while exchanging alleles.

Several factors determine how quickly genes move between groups. Mobility is a primary factor in the dispersal rate. Highly mobile individuals generally have much greater prospects for movement. While animals are typically more mobile than plants, plants still achieve significant dispersal. For example, wind, water, or animals can carry pollen and seeds over great distances. Conversely, gene flow is often lower in species with low mobility. It is also reduced when habitats are fragmented or when population sizes are small. When gene flow is blocked, populations may suffer from increased inbreeding. Inbreeding is measured by the inbreeding coefficient, represented by the symbol F.

Scientists use different methods to measure the level of gene flow. One direct method is to observe the dispersal of individuals and record their reproductive success. However, this is only suitable for certain types of organisms. Most researchers use indirect methods instead. They infer gene flow by comparing allele frequencies among different population samples. The more genetically differentiated two populations are, the lower the estimate of gene flow will be. This is because gene flow has a homogenizing effect that reduces differences. Researchers can also use the effective population size (Ne) and the net migration rate per generation (m) to calculate these effects.

Physical barriers can stop gene flow through a process called allopatric speciation. This happens when geographic isolation prevents populations from exchanging genetic material. Natural barriers include oceans, vast deserts, or impassable mountain ranges. Even man-made structures can act as barriers. The Great Wall of China is an example that has hindered the gene flow of native plants. For instance, the plant Ulmus pumila uses wind-pollination to spread, so it can cross the wall. However, plants like Vitex negundo rely on insects for pollination. These insect-dependent plants show more genetic differentiation because the wall blocks their movement.

Barriers do not always have to be physical. Sympatric speciation occurs when new species arise within the same geographic range. This is often caused by a reproductive barrier. On Lord Howe Island, two species of palm called Howea show this. They have different flowering times that correlate with their soil preferences. This difference in timing creates a barrier that inhibits gene flow. Other causes of limited gene flow in the same area include habitat fragmentation, specialist pollinators, or the production of unfit hybrids. In some cases, humans cause genetic differentiation through endogamy, which is mating within a specific social group like a caste or religion.

Humans can also intervene in gene flow through management strategies. Genetic rescue is one method used to help species threatened by extinction. When small populations face high risks of inbreeding, scientists can introduce unrelated individuals. This increases genetic diversity and can reduce the negative effects of inbreeding. In laboratory studies, crossing two bottlenecked strains of Drosophila melanogaster reversed inbreeding effects. Another strategy is assisted gene flow. This involves moving individuals or gametes to help species adapt to climate change. By introducing genotypes that are pre-adapted to new conditions, managers can increase the overall genetic fitness of a population.

However, human activity can also cause genetic pollution. This happens when the movement of species or landscape changes leads to hybridization or genetic swamping. This is especially dangerous for rare species. For example, the abundant Mallard duck interbreeds readily with many other duck species. This can threaten the genetic integrity of those rarer species. Urbanization also changes gene flow in two ways. Habitat fragmentation can decrease diversity by breaking up environments. However, the urban facilitation model suggests that cities can actually connect populations. In urban environments, the Western black widow spider shows increased gene flow and higher diversity.