Essay on Speciation of Organism!
Individuals of a species are similar and they can breed among themselves. At the same time, there are some small, but significant, differences (variations) between the individuals of a species.
Heritable variations are transmitted to the offspring. These variations are important as they produce changes in the characters of that particular species.
This leads to microevolution or evolution on a small scale with the emergence of new varieties or new subspecies.
To understand how such small variations lead to the formation of a new speeds, let us take the beetle’s example again. Suppose there are beetles spread over a large area. If the population of beetles gets divided into two subpopulations by a barrier (say, a river or a mountain) then it will be difficult for the members of one subpopulation to go to the other side for mating.
Therefore, exchange of genetic material, or gene flow, between them will decrease. They will be restricted to mate within their own subpopulations. In other words, they will be forced to inbreed, or mate with closely related individuals in their own isolated subpopulations.
In this process, the recessive mutant genes of each parent have a much greater chance of coming together. The genes will now be expressed giving benefit or harm to the offspring. These new characters, or variations, may be selected by nature and may lead to the formation of a new species.
The new generations differ so much from the original population that they can no longer interbreed to produce fertile offspring. This leads to speciation, that is, the formation of one or more species from an existing species.
After a few years, if a male beetle from one isolated area and a female from another area are brought together, they may or may not mate with each other. If they mate but are unable to reproduce, then they have become two different species. If they are able to reproduce, then they are still the same species.
Over many generations, different variations are accumulated in each subpopulation. Suppose, for example, in one area with a beetle subpopulation, crows are scarce due to the presence of eagles. And in another area, crows are present in large numbers.
Natural selection will not select the green variety of beetles in the first area as there are no crows to eat the beetles. But the green variety will be selected in the second area as the crows will eat the other beetles there.
Thus, natural selection may operate differently on the same variations in subpopulations of different areas. Nature will select those variations that help to adapt better in a particular environment.
Over a period of time, the processes of genetic drift and natural selection will cause the two isolated subpopulations to become more and more different from each other.
Microevolution is generally a consequence of gene mutation. But larger changes in the genetic make-up, like change in the number of chromosomes, may not allow the germ cells of two subpopulations to fuse together. This prevents interbreeding and causes the emergence of new species.
Speciation due to inbreeding, genetic drift and natural selection will be applicable to all sexually reproducing organisms. Geographical isolation does not play any role in the speciation of asexually reproducing organisms. It also does not play any major role in the speciation of self-pollinating plants.