Gene Flow

Gene Flow Definition

Gene flow is the exchange of alleles between two or more populations. For this reason it is sometimes referred to as allele flow or gene migration. While migrating animals often carry new alleles from one population to another, they must interbreed with the new population for gene flow to occur. In the image below, a beetle from a population of brown beetles migrates into a population of green beetles.

Gene flow

Gene flow

If the brown beetle finds a mate, the alleles which cause a brown exoskeleton may be passed onto his offspring. However, these two populations of beetles have evolved over time to become different colors. The reason might have been genetic drift or the founder effect, from when one population was established from the other. The gene flow may be a good thing for the new population, as genetic diversity tends to help species survive. The gene flow may also be negative, in that it may carry harmful alleles into the new population.

If the two populations constantly interbreed (have a high gene flow), then the two population can be considered one. While they may be separated by barriers which appear to make them separate populations, they share the same allele frequencies and are essentially the same population.

Examples of Gene Flow

Dogs

There are dogs of every shape and size in the world. The largest domestic dogs can dwarf a wild wolf. The smallest domestic dog, even as an adult, could easily be mistaken for a newborn wolf. From wolves, dogs have changed almost every aspect of their appearance in one population or another. Dogs are one of the best known examples of artificial selection, a process through which traits are established through selective breeding.

Around 15,000 years ago, all dogs were essentially wolves. However, some of these pre-dogs were much more likely to scavenge from the new human settlements springing up everywhere. The wolves moved further away from civilization, while the pre-dogs moved closer to the humans. Eventually a “social contract” of sorts was worked out between the humans and the dogs. In this contract, dogs provided a service such as waste removal, vermin control, or a hunting guide. Humans would then provide shelter and food. However, the many different human populations had different uses for their dogs.

Some needed dogs to protect their sheep. So, they bred the dogs with the biggest build and a protectionist mentality. These dogs became the large sheep-dog breeds. Other dogs were needed to hunt mice and rabbits in tiny holes. Thus, the Dachshund was born. Need a dog with fluffy hair that likes to fetch? Golden retriever. As these breeders zeroed in on their desired traits, the populations of dogs became more distinct. Yet, they are still all the same species.

Gene flow, in this case, can be imagined as the Labradoodle. Or the half-Beagle, half-Pug mix: the Puggle. Gene flow is the Chiweenie (Chihuahua/Dachshund), shown below. As one dog from a specific population is allowed to breed within a pure-breeding group, new alleles are brought into the mix. The gene pool is expanded, and new varieties are seen. Thus, the labradoodle has a Labrador mentality, but has Poodle hair. Artificial selection allows scientists and breeders to manipulate the timing and specifics of gene flow, to produce desirable traits.

Zoey the Chiweenie

Birds on an Island

Unlike the case of dogs, most cases of gene flow involve natural selection. Imagine a large population of birds on a mainland. When a big storm brews up, it forces some of the birds high into the air to avoid the storm. When the small flock comes down, they find themselves over the ocean. The wind carries them to a small island, where they set up a new home. The two populations are now sufficiently separated that they cannot regularly interbreed.

Over time, the environmental factors affecting the two different populations will differ. The island birds may have to learn to eat a new food, and may be subject to completely different weather patterns. Over time, this may even change the alleles present in the populations. However, there are always more storms. In another storm, some birds may get transferred back to the mainland. Here, they can once again interbreed with the main population, and gene flow occurs as the new alleles from the island are introduced into the population.

Likewise, if any birds go from the main population to the island population, they will bring with them the alleles selected for on the mainland. This gene flow will help add diversity to the island population. Because of the founder effect, the birds on the island may not have all the alleles on the mainland, and may benefit from gene flow from the mainland. The mainland birds can also benefit from the novel alleles developed on the island.

Bacteria

Bacteria are very interesting when it comes to gene flow. Unlike the rest of the organisms discussed in this article, bacteria are asexual. Without sexual reproduction, how do bacteria exchange genetic variation?

Bacteria, and other asexual organisms, sometimes transfer genetic variation through alternative processes. These processes, like horizontal gene transfer, allow DNA to pass between organisms without the need for sexual reproduction. In fact, much of the diversity present in life today was caused by these gene transfers millions of years ago. The chart below shows gene flow between the different domains of life.

Tree Of Life (with horizontal gene transfer)

Tree Of Life

A horizontal line shows any place which gene flow allowed genetic variation to pass between the various populations of organisms. It is through this horizontal gene flow that eukaryotes gained the pathways for both mitochondria and plastids such as chloroplasts.

Quiz

1. Which of the following is NOT gene flow?
A. A bird flies to an island, and breeds with the birds there. He introduces new alleles.
B. Several hippos escape from the zoo and start a new population in New York City.
C. A tiger raised in captivity is released to the wild, where he reproduces with a wild tiger.

Answer to Question #1
B is correct. The tiger raised in captivity is technically from a separate population than the wild tigers. In breeding with the wild tiger, he will introduce captive alleles into the population. These could be detrimental or beneficial, only evolution can decide. The bird example was covered in the article. The hippos, while they are founding a new population, are not experiencing gene flow from another population.

2. What is the difference between gene flow and migration?
A. Migration and gene flow describe the same process
B. Migration can occur without gene flow
C. Gene flow can occur without migration

Answer to Question #2
B is correct. Migration occurs whenever an organism physically moves into a new area or joins a new population. However, gene flow only occurs when the populations interbreed. Even then, it is only considered gene flow if the populations are exchanging alleles and changing the allele frequency of one or both populations.

3. Which of the following represents a BENEFIT of gene flow to a population?
A. Increased genetic diversity
B. Increased genetic load
C. Decreased adaptability

Answer to Question #3
A is correct. When animals migrate to a new population, they often bring with them beneficial alleles which can be introduced to the new population. Sometimes, though, the migrant organisms bring unwanted alleles. Alleles which decrease adaptability and bring disease are considered a genetic load, which is unfavorable to a species.

References

  • Darwin, C., & Wallace, A. (1980). On the Tendency of Species to Form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection. In P. H. Barrett (Ed.), The Collected Papers of Charles Darwin (Vol. 2, pp. 3-18). Chicago: The University of Chicago Press.
  • Feldhamer, G. A., Drickamer, L. C., Vessey, S. H., Merritt, J. F., & Krajewski, C. (2007). Mammology: Adaptation, Diversity, Ecology (3rd ed.). Baltimore: The Johns Hopkins University Press.
  • Hartwell, L. H., Hood, L., Goldberg, M. L., Reynolds, A. E., & Silver, L. M. (2011). Genetics: From Genes to Genomes. Boston: McGraw Hill.
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