Allopatric Speciation Definition
Allopatric speciation is speciation that happens when two populations of the same species become isolated from each other due to geographic changes. Speciation is a gradual process by which populations evolve into different species. A species is itself defined as a population that can interbreed, so during speciation, members of a population form two or more distinct populations that can no longer breed with each other.
Steps of Allopatric Speciation
- A geographic change separates members of a population into more than one group. Such changes could include the formation of a new mountain range or new waterway, or the development of new canyons, for example. Also, human activities such as civil engineering, agriculture, and pollution can have an effect on habitable environments and cause some members of a population to migrate.
- Different gene mutations occur and build up in the different populations over time. The different variations of genes may lead to different characteristics between the two populations.
- The populations become so different that members of the different populations can no longer breed with each other anymore if were they to be in the same habitat in the same time. If this is the case, allopatric speciation has occurred.
The following diagram represents an experiment on fruit flies where the population was forcibly separated and the two groups were fed a different diet. After many generations the flies looked different and preferred to mate with flies from their own group. If these two populations continued to diverge for a long time, they could become two different species through allopatric speciation.
Examples of Allopatric Speciation
A major example of allopatric speciation occurred in the Galapagos finches that Charles Darwin studied. There are about 15 different species of finches on the Galapagos islands, and they each look different and have specialized beaks for eating different types of foods, such as insects, seeds, and flowers. All of these finches came from a common ancestor species that must have emigrated to the different islands. Once populations were established on the islands, they became isolated from each other and different mutations arose. The mutations that caused the birds to be most successful in their respective environments became more and more prevalent, and many different species formed over time. When many new species emerge from one common ancestor in a relatively quickly geological timeframe, this is called adaptive radiation.
Grand Canyon Squirrels
When the Grand Canyon was formed, it created a natural barrier between the squirrels living in the area. About 10,000 years ago, the squirrel population was separated from each other by this geographic change and could no longer live in the same area. Over thousands of years, the divided squirrel populations became two different species. Kaibab squirrels live on the north rim of the canyon and have a small range, while Abert squirrels live on the south rim and live in a much larger range. Members of these two species have a similar size, shape, and diet, and slight color differences, but they are no longer in contact with each other and have become so different during their separation that they are now separate species.
The Four Types of Speciation
There are three other types of speciation besides allopatric speciation: peripatric, parapatric, and sympatric. Peripatric and parapatric speciation are similar to allopatric speciation because in these types, populations also get isolated and this causes speciation. Sympatric speciation, on the other hand, occurs when the members of different populations live in the same area but speciation still occurs.
Peripatric speciation occurs when members on the periphery, or border, of a large population separate off from the main group and become a new species over time. It can be difficult to distinguish from allopatric speciation. Peripatric speciation occurs when the population that breaks off enters a different biological niche, such as eating a different food or living in a different environment. Also, these new populations that break away from the main one are usually small, so this can have an effect on the proportion of certain characteristics in the new population compared with the old one. For example, say there is a population of birds that are mostly blue, but some are red. A smaller group of birds breaks off from the main group, and most of this smaller group are red. Their offspring will probably also be mostly red, which is different than the main group. This type of change in the frequency of genes is called genetic drift. Over time, many changes may occur, and these combined with the effects of genetic drift can cause new species to arise.
Parapatric speciation occurs when subpopulations of the same species are mostly isolated from each other, but have a narrow area where their ranges overlap. This may be due to a partial geographic barrier or an unequal distribution of members of the subpopulations. It can occur between multiple subpopulations next to each other where all the populations next to each other can interbreed, but each subpopulation is so slightly different that the members on the extreme ends would not be able to interbreed with each other. This is known as a ring species.
Sympatric speciation is very different from the other forms because new species emerge from populations living in highly overlapping or even identical areas. It may be more common in bacteria than in multicellular organisms because bacteria can transfer genes to each other as well as transfer genes to offspring when they divide. It is not known how often sympatric speciation occurs, and it is much rarer than the other types of speciation, but there have been some examples seen in nature. One such example is seen in cichlid fish in Tanzania that live in a small volcanic crater lake. The population has two very different ectomorphs, or forms: a yellow-green one that lives by the shore, and a blue-black one that lives by the bottom of the lake. By looking at the fishes’ DNA, researchers could see that the two ectomorphs were very different genetically. It is believed that these two forms are currently in the gradual process of speciation.