Evolution and Classification 

Classification

Classification is the system of arrangement of organisms in certain groups or subgroups on the basis of hierarchies of certain characteristics.

Flow chart of basic characters shared By most of the organisms

The more characteristics two species will have in common, the more closely they are related and the more closely they are related, the more recently they will have had a common ancestor. For example, a brother and a sister are closely related. They have common ancestors in the first generation before them, namely, their parents. A girl and her first cousin are also related, but less than the girl and her brother. This is because cousins have common ancestors, their grandparents, in the second generation before them, not in the first one. We can now appreciate that classification of species is in fact a reflection of their evolutionary relationship.

1.0Tracing Evolutionary Relationships

When we try to follow evolutionary relationships, we identify characteristics in common. These characteristics in different organisms would be similar because they are inherited from a common ancestor.

1. Homologous organs: Homologous organs are those organs having similar basic structure but have been modified to perform different functions. e.g. forelimbs of reptiles, frog, lizard, bird and human are homologous organs. Such homologous characteristic helps to identify an evolutionary relationship between apparently different species. This relationship is called divergent evolution or homology.

Forelimb in Vertebrates

Leaves are an example of homology in plants. In many plant species, leaves have been modified for functions other than photosynthesis. A cactus spine and a pea tendril, although quite different in appearance, are homologous because both are modified leaves.

Analogous Organs  

Those organs which have different origin and structural plan but appear similar and perform similar functions are called analogous organs, while this relationship is called convergent evolution or analogy. e.g. Wings of an insect, bird and bat.

Analogy in these organs is due to similar adaptations to perform similar functions rather than their common ancestory.

Both sweet potato and potato possess tubers for storage of food. Tubers of sweet potato are root structure while those of potato are stem structure.

Note: Convergent evolution is the emergence of biological structure or species that exhibit similar function and appearance but that evolved through widely divergent evolutionary pathways.

2.0Species and Speciation

Biological species concept

A species is a sexually interbreeding group of individuals separated from other species by the absence of genetic exchange. Members of species are capable of breeding with one another and produce living, fertile offspring but are unable to breed with members of other species normally.

Speciation

The process of formation of one or more new species from an existing species is called speciation.

Reproductive Isolation

It refers to the mechanism which prevents populations of two different species from interbreeding.

Speciation occurs when the gene pool of a population is somehow reproductively isolated from other sister populations of the parent species and gene flow no longer occurs between them. Then a population splits into independent species which become reproductively isolated from each other.

Geographical Isolation

It is the separation of groups of related organisms by geographical barriers like mountains, deserts, glaciers, etc. 

It can be explained by following illustration. Suppose large population of beetles occur on a mountain range and few beetles started feeding in neighbourhood. Gene flow continued in two places. They may get isolated at larger distance because of existence of river. Gene flow decreases and finally stops.

Over generations, genetic drift will accumulate different changes in each sub-population. Also, natural selection may also operate differently in these different geographic locations. Thus, for example, in the territory of one sub-population, crows are eliminated by eagles. But this does not happen for the other sub-population, where crow numbers are very high. As a result, the green variation will not be selected at the first site, while it will be strongly selected at the second. 

Together, the processes of genetic drift and natural selection will result in these two isolated sub-populations of beetles becoming more and more different from each other. Eventually, members of these two groups will be incapable of reproducing with each other even if they happen to meet.

There can be a number of ways by which this can happen. If the DNA changes are severe enough, such as a change in the number of chromosomes, eventually the germ cells of the two groups cannot fuse with each other or a new variation emerges in which green females will not mate with red males, but only with green males. This allows very strong natural selection for greenness. Now, if such a green female beetle meets a red male from the other group, her behavior will ensure that there is no reproduction between them. Effectively, new species of beetles are being generated.

(i) Speciation due to genetic drift and natural selection would be applicable to all sexually reproducing animals.

(ii) Geographical isolation does not play any role in the speciation of asexually reproducing animals and self-pollinating plants.

Note: Sum of all the genes of all the members of a species is called gene pool.

3.0Also Read