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PinkMonkey Online Study Guide-Biology


11.0 Introduction

Scientists have long tried to discover the origin of living organisms. Various observations about the varieties of plants and animals, the diversities in their structures and the reproductive patterns, lead to the concept of evolution. The process of evolution involves a gradual change of organisms generation after generation. Essentially it means that the present-day organisms have arisen from ancestors that were more simple in organization. Evolution involves changes in genetic composition of a population, generation after generation. Modification and development of a species takes place through hereditary transmission of slight changes (variation) from one generation to another. It results in new characteristics in a species, and the eventual formation of new species. Three main observations support the idea of evolution.

a) The development of present organisms can be traced back to the organisms living in the past. The study of fossils reveals that fossils in successive strata of rocks show a gradual change from simple forms to complex forms of life. The geological time scale explains that the oldest rocks formed show the fossils of simple animals only. The rocks formed later show more complex forms of living organisms, and thus a gradual evolution of more complex living organisms from simple forms is suggested. The Proterozoic era shows fossils of invertebrate animals. Paleozoic era shows fossils of fishes and amphibians in the lower strata and reptiles in the upper strata. The Mesozoic era shows dominance of reptiles. Coenozoic era shows appearance of birds and mammals. The recent epochs show evolution of man.

b) Another observation about the diversity in the structure of living organisms explains that there is unity in diversity of life. The animals show variations in their structures but the basic pattern of working is the same. There may be variations in the food and feeding habits, but the basic components of food are same. The proteins, carbohydrates and lipids are converted into aminoacids, monosaccharides and fatty acids, respectively. The biochemical changes during digestion are similar. The pattern of getting energy is basically the same. Similarly the nucleic acids which are involved in the inheritance of characters are similar in most of the organisms. The protein synthesis mechanism is also similar. Structures of many organs also suggest a common basic pattern, (e.g. the forelimbs of the frog, bird, horse, whale, bat and man have the same basic skeleton pattern). But necessities like running, flying, swimming, perching or grasping, variations result from various changes in shape.

c) The unit of evolution is population : An individual may show adaptation and survive. But development of one fit individual will not result in evolution. The individuals may pass on their advantageous characters to their offspring. When the individuals with new characters interbreed in a population, the characters are spread amongst the population. The varieties in the population thus have common ancestors and thus in this way a gradual evolution of more fit individuals must have taken place.

Table of Contents

11.0 - Introduction
11.1 Darwin's Theory of Natural Selection

Chapter 12


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