Introduction to Evolution:
It is believed, that the present state of mankind on the earth is the outcome of three kinds of evolution – chemical, organic and social or cultural. Therefore, the concept of evolution has been applied not only to the living world but also to the non-biological systems.
Appropriate evolution of all the systems in a concerted manner has created todays earth. So defining evolution from any one aspect of the complex system may be misleading, as a definition requires numerous parameters.
However, Dobzhansky, Ayala, Stebbins and Valentine (1976), in their book ‘Evolutions’ attempted to define evolution in the following words, which satisfy most of the requirements.
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“Organic evolution is a series of partial or complete and irreversible transformations of the genetic compositions of populations, based principally upon altered interactions with their environment. It consists chiefly of adaptive radiations into new environments, adjustment to environmental changes that take place in a particular habitat, and the origin of new ways for exploiting existing habitats. These adaptive changes occasionally give rise to greater complexity of developmental pattern, of physiological reactions, and of interactions between populations and their environment.”
Organic and Other Kinds of Evolution:
Obviously there are relationships between organic and other kinds of evolution. It is now experimentally proved that the origin of life on earth is the outcome of pre- cellular chemical evolution, which took place over millions of years. Social or cultural evolution of organisms is an important factor to perpetuate a life successfully on earth.
To understand evolution one must discuss all kinds of evolutions, so as to place organic evolution in its proper theoretical perspective. Strickberger (1994) considered evolution from a historical point of view, both biologically and conceptually.
Historical information’s are passed on by transmitted genetic material that connects the biology of organisms to events of the past, while evolutionary concepts are derived from previous concepts transmitted culturally. Most of the aspects of evolution indicate that knowledge of the past has become essential for fully understanding the present.
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Ultimately it is becoming very much true, as the great evolutionary geneticist Theodosius Dobzhansky stated, “Nothing in biology makes sense except in the light of evolution.”
Elementary Forces of Evolution:
Evolution is a gradual phenomenon and evolutionary changes occur at several levels on a species. The magnitude of the forces which work on the evolutionary levels determines the effect on the species population.
On the basis of magnitude of the forces, evolution can be classified as micro-, macro- and mega-evolution:
(a) Microevolution:
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Evolution resulting from interaction of variations, natural selection and genetic drift to produce relatively small population changes, is called micro- evolution. Sequential evolution is always the product of micro-evolutionary processes.
Example:
In the scarlet tiger moth (Panaxia dominula), one pair of gene is responsible for white spots on the anterior wing. British geneticists studied the change of the spot pattern through 23 years and found remarkable changes. The changed pattern survived for a long period. Therefore, it can be said that the new pattern of wing is selected by nature and ultimately gave rise to new species.
(b) Macroevolutions:
When evolutionary changes involve a number of species populations and usually occurs above the species level, it is called macroevolution or ‘adaptive radiation’.
Macroevolution is characterised by:
(i) Subdivision of the group into many new subgroups;
(ii) An invasion of numerous new environmental situations; and
(iii) Diversification of structure and biology.
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While microevolution and speciation tend to produce special adaptation, macroevolution usually develops from a general adaptation with a number of special adaptations following from the general one.
Example:
Marsupials and placental mammals, e.g., native cat (Dasyurus) and ocelot (Felis) among marsupials; flying phalanger (Petaurus) and flying squirrel (‘Glaucomys) among placentals, show parallel adaptational features although they belong to divergent groups.
(c) Mega-evolution:
When, on rare occasions, new combinations of characteristics cause the appearance of new biological organisations of general adaptation, then these are called mega-evolution.
Example:
Evolution of vertebrate classes, such as from fish to amphibia, from amphibia to reptilia, from reptilia to birds etc. All the above mentioned levels differ considerably from one another in fundamental features, but all are based upon microevolution and contribute to the adaptation.
Microevolution causes sequential adaptive changes, while macro and mega-evolution produce divergent adaptation. At all level, the result of evolution is same — i.e., the development of organisms adapted to a changing environment and having a more efficient relation with the present environment than their predecessors.
Results of Evolution:
The result of evolution is to produce better adapted organisms in the present environment. Evolution can be classified on the basis of environment as convergent and divergent.
(a) Convergent Evolution:
Organisms from the different habitats when migrating to a new habitat and acquiring adaptive features in a similar fashion, then the resultant evolution is called convergent evolution.
Example:
The wings of insects, birds and bats. The predecessors of these organisms were inhabitants of different places. But these organisms developed wings to explore the same volant atmosphere.
(b) Divergent evolution:
Ancestral organisms when migrating to different habitats from its original one and becoming suitably adapted to give rise to new species, then the resultant evolution is called divergent evolution.
Example:
Forelimb of whales, birds, frogs, lizards etc. Ancestors of these animals were terrestrial, but during evolution the structure of fore-limbs changed according to the habitat they intend to inhabit.