Monday, September 28, 2009

Evolution and Theories of Evolution

Evolution means development of life with time. Evolution means that living things change. A species may slowly change into new species or even into two or more new species. It means that the plants and animals that are present on earth now are not the first plants and animals, it also means that many plants and animals that once flourished are no longer alive, as one species evolves into other species, the original species no longer exists. The knowledge about these extinct species is provided by their remains preserved in the rocks called fossils Theory of evolution has special place in the study of history of life. It helps us to understand the unit as well as the diversity of plants and animals. But evolution has not made the organisms completely different DNA always carries the instructions for inheritance. Evolution provides an unifying picture of life. Life has common pattern because all life is interrelated through evolutionary descent. This is evident from classification which has its scientific basis in evolutionary interrelationship. By evolution scientists try to discover how this process of change occurs.

(1) THEORY OF EVOLUTION BY NATURAL SELECTION

Charles Darwin during voyage accumulated geological and fossil evidence that supported the idea that life changes with time. He studied flora and fauna of main land South America. He found evidence development of a variety of forms among finches (a kind of birds) from single ancestral group due to adoption to feeding on different kinds of food. He also studied the effect of light on plant growth that later led to discovery of plant hormones, plants of various habitats. He said there is contest among the members of a population for food to which called ‘Survival of the fittest’. He found that same struggle exists among all living things. He suggested that all species show variations with time. Some variations are of advantage in struggle for existence. Organisms with favourable variations are most likely to breed and to pass on their favourable characteristics. In this way new species arise from existing ones because the natural selects the fittest, evolution by ‘natural selection’.

LAMARCK’S THEORY OF EVOLUTION - LAMARCKISM

French naturalist Lamarck in 1809 gave hypothesis based on two conditions.

(1) The use and disuse of parts

(2) The inheritance of acquired characters.

According to Lamarck changes in the environment may lead to changed patterns of behaviour which can necessitate new or increased or disuse of certain organs or structures. Extensive use would lead to increased size, disuse would lead to degeneracy and atrophy. These traits acquired during life time of the individual were believed to be heritable and thus transmitted to offspring.

Weisman theory of continuity of Germplasm

Weisman proposed theory of continuity of germplasm. He postulated that somatic or body acquired characters resulting in variation did not affect the germ or gamete cells (in other words DNA, the genetic material) which are responsible for transmission of genetic characters. The germ cells are protected and nourished by somatic cells and are passed on intact and unmodied from generation to generation. Weisman on the basis of his theory argued that bodily modifications brought out by the environmental changes and by the use and disuse of organs cannot affect the germ cells and therefore cannot be transmitted to the next generation.

According to Weisman only germ cells of an individual contain all the determinants for all hereditary characters but the somatic cells contain only those determinants which determine the development of particular characters of the tissues or organs which they form. For example the nucleus of nerve cell contains those determinants which control the development of characters of nerve cell. Same nucleus behaves differently in different cells because of differences in their cytoplasm.

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