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PinkMonkey Online Study Guide-Biology

11.1 Darwin’s Theory of Natural Selection

Various hypotheses were suggested by scientists to explain the concept of evolution. As a result of many years of careful study and observations, Charles Darwin proposed his theory of evolution. He published a book The Origin of Species in the year 1859. He proposed that the new species came about by a process called ‘natural selection. While explaining his theory, Darwin gave an account of many basic facts and deductions and stated how natural selection operates and results in the evolution of a new species.

(1) Over-production. Darwin reported that all organisms tend to increase in a geometric ratio provided there are no environmental checks. Lower animals produce a large number of eggs within 10 to 14 days. A female drosophila produces 200 eggs at a time. If all flies from these eggs survive and reproduce with the same rate, they may produce 200 million organisms within a period of 40 to 50 days. Amongst the higher animals also there is a rapid increase in number. A single pair of sparrows may produce as many as 275 million individuals in a span of 10 years. Even slow breeding animals like the elephant may theoretically give rise to 19 million descendants in a period of 750 years.

(2) Population Stability. In spite of the high reproductive potential, the number of individuals in a species remains relatively constant, suggesting struggle for existence.

(3) Struggle for Existence. There is an inevitable competition between the organisms for space, food, shelter, nesting sites, water, etc. This struggle takes place in three ways-

a) Intraspecific struggle

b) Interspecific struggle and

c) Environment struggle

a) Intraspecific struggle takes place between the individuals of the same species. This is often most severe and fatal, as the needs of the individuals of the same species are similar.

b) Interspecific struggle takes place between the individuals of different species. In a community, some organisms feed on others. Both animals and plants are affected by this kind of struggle.

c) Environmental struggle is the struggle of organisms against the physical environment. Organisms struggle against excess of moisture, drought, extreme heat or cold, lightning, volcanic eruptions, etc.

(4) Genetic variations within a species. Animals and plants show variations in physical structure. Some of these variations are simply caused by external conditions (environmental), such as accidents, temperature, food abundance, etc. (e.g. animals look different when they are starving than when they are well fed). Such somatic variations die with the organism and not inherited. Thus, they have no effect on evolution.

Heritable variations are called genetic variations. Such variations arising from changes in DNA are passed on within families and to the offspring from the parents, (e.g. parents with a larger stature body generally have larger children than parents with a small statured body).

(5) Natural selection, or "survival of the fittest". Organisms struggle for existence. Those individuals who have favorable variations have better chances of living long enough to reproduce. They pass on their advantageous characters to the next generation. Thus, organisms with advantageous characters survive, while those which lack such variations perish. The advantageous characters are passed on to the offsprings generation after generation and the organisms become better suited for survival. Darwin described that nature selects such organisms, i.e. there is natural selection. Herbert Spencer referred to this as ‘survival of the fittest.’

(6) Environmental change. The physical environment changes continuously. There are seasonal changes in availability of moisture, light, severe heat or cold, etc. Mountain ranges are formed where there were no mountains before. Volcanoes erupt and cause changes in the environment. Similarly floods and fires convert plains and forests into temporary deserts.

(7) Inheritance of adaptive characters. Because organisms are continually tested by their environment, their forms change to suit new conditions. New advantageous characters are inherited along newer generations and their cumulative effect results in an organism with new characters.

(8) Origin of species. Darwin explained that favorable variations are passed on to the offsprings down the generations. After a certain period of time the organisms appear so different from the original species that ultimately a new species is evolved.

The basic facts and deductions of Darwin’s theory are summarized as follows:

Table of Contents

11.0 - Introduction
11.1 Darwin's Theory of Natural Selection

Chapter 12


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