INTRODUCTION: LAMARCKISM. Paper: PAPER No. II. B.A / B.Sc 2 nd Semester. Course name: PHYSICAL ANTHROPOLOGY

Transcription

1 Title: ANTHROPOLOGY Paper: PAPER No. II B.A / B.Sc 2 nd Semester Course name: PHYSICAL ANTHROPOLOGY Module: ORGANIC EVOLUTION: Lamarckism and Darwinism (11/15) INTRODUCTION: The gradual change and development of simple and lower categories of life forms to more complex and higher categories of life forms in consequence of the geo environmental change and change of other surrounding situation are known as organic evolution. The theory of organic evolution initiated the scientific discussions on origin and development of life. We will now discuss various theories of evolution. LAMARCKISM The first persuasive theory of evolution was presented by the French naturalists, Jean Babtiste de Lamarck ( ). He was the first naturalist to put forward the fact that the animals could modify themselves to adapt to the environmental situations. According to him, the evolution of species was based

2 on the response to need and to use or disuse of organs. In his classic book entitled Philosophic Zoologique (1809), Lamarck presented a complete theory of evolution. He believed that the animals were not specially created to fit a particular pattern of life; but they have been shaped by the circumstances. Lamarck devised four laws according to which the evolution was affected through the ages. 1 st Law: Theory of elan vital or growth: The internal forces of life tend to increase the size of an organism by growth in organs and systems. The internal forces of life are capable of forming organs and systems. 2 nd Law: Theory of environmental pressure and spontaneous formation of organs: New organs and systems are formed because of need or want which has arisen due to environmental pressure and continue to be felt by the organism. 3 rd Law: Theory of use and disuse: The development of organ is directly proportional to its use. ontinuous use strengthens the organ while disuse has reverse effect and he cites examples to prove this. For example there is a. development of strong biceps muscles in blacksmith b. elongated body and loss of limbs in snakes due to continuous creeping through the holes and crevices c. migration of both the eyes towards the upper side in flat fishes living on the bottom of sea d. development of strong leg bones, muscles and tendons for fast running and thickened enamel in teeth for chewing in horses during a shift from forest life to Savannah life e. lengthening of neck in the giraffe due to its continuous use in reaching to the leaves and fruits of high rise tree f. occurrence of vestigial organs such as Pinna, Simi lunar membrane, vermiform appendix etc in man due to its constant disuse g. development of web in between the digits in water birds to facilitate swimming. 4 th Law: Theory of inheritance of acquired characters: All changes that organism acquire during the lifetime are transmitted to the next generation.

3 Lamarck s theory of evolution, though was the pioneering attempt in the formulation of biological explanation of evolutionary process, in the present period it has been severely criticized because of the fact that it has not been possible to establish that characters acquired during the lifetime of any organism are transmitted to the succeeding generation. CRITICISM OF LAMARCKISM 1. The theory put much emphasis on inner urge and forces and considers it competent enough to device ways to form organs needed for adaptation. 2. The theory considers environment as a docile force while modern theory of evolution considers environment as a dynamic force that selects the adaptive types from a range of variation. In Lamarckism, variation theory is the end product of evolution while in modern theory variation provides raw material for evolution on which natural selection and other forces act. 3. Development and atrophy of organs can be explained on the grounds of accepted modern theory of evolution. Thus lengthening of neck of giraffe occurred not because of its constant use but because of presence of individuals of variable length of neck in giraffe s population out of which those of longest necks were selected for. Thus snake with more slender form of body, aquatic birds with webs in feet, flat fishes with both eyes on one side, horses with capacity to run fast and browse, birds with perching ability would be favoured by natural selection in their respective habitat. All these arose as differing variations and perpetuated by differential reproduction because those with favourable variations had greater chances of survival and reproduction, thus leaving more offspring in the next generation i.e. contributing significantly to the gene pool of next generation. 4. Most of the experiments devised in support of Lamarckism were either biased or lacked confirmation, and a few were even forged. DARWINISM Charles Darwin, a British Naturalist, in the year 1859 published his book on the Origin of Species. Darwin s theory of Natural Selection has exercised the greatest impact on evolutionary thoughts. It was with the publication of Darwinism that Organic Evolution came to be accepted by the world. Darwin attributed several factors to the cause of evolutionary changes. These factors are as follows:

4 i) Overproduction: All living organisms tend to increase their number in a geometric ratio in nature. Potentially, each organism is capable of reproducing at a very high rate. Thus, any given population is usually able to reproduce many more young individuals that can adequately be raised in the region it occupies. ii) Variation: Darwin pointed out that all living organisms vary. There are always some variations, however minute they may be. While some variations are suited to the conditions of the environment, others are not. Further, since environment varies from region to region, the innumerable variations that living organisms show also differ from region to region. According to Darwin, these variations are preserved and transmitted to the offspring. In nature, favourable variations are more prone to be inherited than the unfavourable ones. iii) Struggle for Existence: Overproduction and consequently overcrowding of a population requires elimination of large numbers of individuals in each generation. Since the amount of space, food, residential areas and other requirements of life are limited; there is a severe competition between the individuals for these essentials of life. There is a struggle for existence. The struggle for existence can exist at intraspecific or interspecific level and also against environment. a. Intraspecific struggle: It is the struggle between the members of the same species. As the food material and area of living are limited, the individual who grows in right direction survive while the other will perished. b. Interspecific: It is the struggle between the unrelated species. c. Environmental struggle: It is the struggle against the forces of nature. Natural calamities like earthquake, flood and epidemics are all faced by the organisms in common. iv) Natural Selection and the Survival of the Fittest: In the face of the struggle for existence, only those organisms that are most well adapted to the environment for fit in life can survive. Thus, natural selection of organisms takes place in which nature eliminates those that are not fit by way of several natural calamities and preserves those that are fit. Thus natural selection maintains a more or less constant number of every species. These selected organisms serve to perpetuate their well suited variations and thus gradually a well adapted, better species evolve.

5 DRAWBACKS OF DARWINISM Although Darwin s theory paved the way for the better understanding of the process of evolution, it failed to answer certain problems like- i) It does not account for presence or origins of variations among living organisms. ii) Natural selection cannot carry a species beyond its natural range of variability. This is because variation is a progressive course in evolution which causes modification, but heredity acts as a conservative force that tends to preserve the variations as such. In spite of these drawbacks, Darwinism is widely accepted, though with few modifications.