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.
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
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.
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 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.
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.