Chapters
In this article, we will discuss how speciation can take place because of genetic isolation by geographical separation (allopatric speciation) and ecological and behavioural separation (sympatric speciation).
Speciation results from evolution. Evolution refers to the change in the features of a species over many generations and depends on the natural selection process.
As a result of evolution, new species are formed from pre-existing ones over time. The changes in gene pools from generation to generation resulting in the formation of new species. In speciation, two new populations are formed from a single original population. These populations should evolve in such as manner that the individuals from the new populations cannot interbreed any longer.
For speciation, the genetic isolation between the new population and the pre-existing species is critical. Speciation can occur in two different situations:
- A geographical barrier separates two groups of a species
- Two groups of species are reproductively isolated; however, they still inhabit the same area. Living in the same area implies that the two groups of species are still experiencing the same selection pressures.
Types of Speciation
There are two types of speciation:
- Allopatric speciation
- Sympatric speciation
Allopatric Speciation
Geographical isolation causes allopatric speciation, and it is the most common. In this type of speciation, a species population divides into two or more groups which are then detached from each other via geographical barriers. The barrier can be natural or man-made. An example of natural geographical barriers includes a water body or a mountain range. An example of a man-made barrier is a motorway.
Sometimes barriers, for instance, a lava flow, divide the population by altering the landscape. On the other hand, in some places, the populations separate from each other after some individuals cross a pre-existing barrier. For instance, individuals of a mainland population can become isolated on an island after floating on a piece of debris.
This separation results in two populations of the same species who are isolated from each other. Consequently, no genetic exchange can take place between them. In the presence of enough selection pressure or genetic drift that acts to change the gene pools within both populations, the populations diverge and form distinct species.
The changes in the alleles or genes of each population affect the phenotypes found in both populations. Over time, the two populations start to show physiological, behavioural, and morphological (structural) differences.
As a result of divergence, the two groups evolve characteristics that act as prezygotic and/or postzygotic barriers to reproduction. For example, if one group of population evolves a larger body size and another evolves a smaller body size, then it becomes impossible for organisms to mate physically even if they are united again.
In the presence of strong reproductive barriers that effectively prevent gene flow, the two groups continue to evolve along separate paths. It implies that they will not exchange genes with one another even when the geographical barrier is removed. Now, we can consider the two groups as distinct species.
Example of Allopatric Speciation: The Squirrels and the Grand Canyon
Colorado river gradually carved out the Grand Canyon over millions of years. Before its formation, a single species of squirrel lived in that area. With time, the canyon got deeper and deeper which made it difficult for squirrels to move between the north and south sides.
When the canyon became deep enough for the squirrels to cross, the subgroups of squirrels became isolated on each side. It means that the squirrels on the north and south sides of the canyon were reproductively isolated from one another because of the deep canyon barrier. Eventually, the squirrels on both sides diverged into distinct species.
Example of Allopatric Speciation: Trees
Consider a population of trees that are all a single species. The population splits into two after a formation of a new mountain range. This mountain range acts as a natural barrier between both groups. It also prevents the two groups from interbreeding and prevents gene flow between them. The two groups of populations now face different selection pressures and genetic drift. Thousands of years later, the divided population formed two different species that could not interbreed.
In the next section of the article, we will discuss sympatric speciation in detail.
Sympatric Speciation
Unlike allopatric speciation, sympatric speciation takes place in the absence of a geographical barrier. It implies that the group of the same species may be living in the same area, however, for the speciation to occur, there should be two populations within that group and no gene flow takes place between them.
Something happens that divides or separates the two groups:
- Ecological separation: The populations become separated because they are living in different environments within the same location. For instance, the pH of the soil can be different in different areas. The pH of soil greatly affects the growth of the plant and flowering.
- Behavioural separation: Populations become separated because they exhibit different behaviours. For instance, differences exist in communication, feeding, or social behaviour.
Example of Sympatric Speciation: Fish
Suppose a species of fish inhabits a lake. Some members of the fish population feed on the bottom, whereas others feed by remaining higher up in the open water. The difference in feeding behaviours divides the population into different environments. Behavioural separation results in ecological separation.
The separated groups face varying selection pressures. For fish feeding at the bottom, long jaws are beneficial, while shorter jaws are appropriate for fish feeding in the mid-water. Over time, natural selection diverges the populations, and they evolve distinct courtship displays. Interbreeding becomes impossible, and they are now distinct species.
Important Point
While observing the cases of sympatric speciation, do not confuse the factors that were originally responsible for causing the separation between the populations and the factors that stopped them from breeding after genetic isolation. Take an example of a fish. The separation occurred due to the differences in feeding behaviour, however, it is a difference in courtship displays (genetic isolation caused this) that stops them from interbreeding. Remember that the speciation depends on mutation, without which no new alleles or genes are present for selection to act on. The change in genetic material via mutation is critical because it generates the physiological, behavioural, and morphological differences between the species.
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