CHAPTER 7.3
EVIDENCES OF EVOLUTION
Theories of Organic Evolution:-
The process of organic evolution has been explained by different scientists by formulating different theories.
1. Lamarckism (Theory of inheritance of Acquired characters)
2. Darwinism (Theory of Natural Selection)
3. Modern Synthetic Theory (Neo-Darwinism)
1. Lamarckism (Theory of inheritance of Acquired characters):
One of the first theories of evolution that was published by Jean Baptiste Lamarckin 1809 in the book called Zoological Philosophy. His idea of evolution was based on the observation that the changes in the environment lead to the changed patterns in the species. His conclusions about the evolution are summarized as follows:-
I. Tendency to grow: The internal forces of life tend to increase the size of an organism i.e. the whole body and also the different parts up to a limit which it brings about itself.
II. Formation of new organs: The production of new organ in an animal body is the result of the need or want of which has arisen and continuous to be felt by the organisms.
III. Use and disuse of organs and structures: The continuous use of an organ makes them functional and strong and thus leads to its better development , while continuous disuse makes the organs undeveloped, weakened and finally disappeared. E.g. long neck of Giraffe.
IV. Inheritance of acquired characters: The characteristics that are acquired by an organism during its life time due to direct or indirect environmental effect is preserved and transmitted to its offsprings. After certain generations the offspring produced become totally different from the original parents and establish new species.
Examples of Lamarckism
A. Giraffe:
The ancestors of giraffes were short-necked and short-limbed deer-like animals. They lived in places with no surface vegetation. They had to stretch their neck and fore limbs to take the leaves of long trees, which resulted in the slight elongation of neck and limbs. Whatever they acquired in one generation was transmitted to the next generation. As a result of that, a race of long necked and long limbed giraffes were developed.
B. Snake:
The absence of feet in snake is an example of disuse of organs. The snakes were originally four limbed animals. But they adopted creeping habits and lived in the burrows. Due to this, they lost their limbs.
C. Flightless birds: The ancestors of these birds (e.g., Ostrich) were capable of flying. Due to some environmental factors they had plenty of food and were well protected on land. So, they did not use their wings and that is why wings become vestigial.
Objections to Lamarckism:
Mutations (removal of part of the body)are not inherited.
Eyes that are being continuously and constantly develop defects instead of being improved as is suggested by the theory of use and disuse of organs.
Piercing of ears and nose in woman has been continued as a tradition from centuries but the progeny do not show any trace of holes in their ears and nose.
The wrestler’s power muscles are not transmitted to their offsprings.
2. Darwinism (Theory of Natural Selection):
The theory of natural selection that was published by Charles Darwin (1809-1882) in 1839 is one of the most important facts of evolution. In 1859, Darwin published his findings in book, “The Origin of Species”. The publication of this book was an important event in biological history and it was regarded as the most important book of the 19th century. The elements of this theory may be enumerated as follows:-
I. Over production:-
All organisms produce much more offsprings than can actually survive. A fern plant may produce 50 million spores and insects like a housefly, may lay thousands of eggs. Paramecium divides three times in 48 hrs.
II. Struggle for existence:-
Due to over production, there is struggle for existence. The individuals face intraspecific as well as interspecies competition for food, water, air and space. Organisms should struggle with the natural calamities and environmental disasters such as excess of moisture or drought, excessive heat and cold, flood, earthquake, volcanoes, etc. so a few of them only are capable to get these basic requirements and the rest, a large number fails to fails to survive or reproduce. As a result of it, all population sizes are limited or checked and the numbers of individuals in a population remain nearly constant.
III. Natural selection (Survival of the fittest):-
In nature, organisms face everlasting competition or struggle among them. In that competition the stronger win and survive, while the weaker are rooted out. Similarly, organisms show some variations, which are proved to be beneficial in facing the environmental calamities. The individual with favorable variations have a better chance of survival in the nature and others die. Thus, the nature is the super power that selects best fitted individuals and rejects those that are poorly developed, and thus they
usually die. This process is known as natural selection.
IV. Variation and heredity:-
The offsprings of the same parents also differ from one another after numerous generations and show variations. In addition to visible phenotypic variation, the hidden genetic variations also exist. The variations are very important for evolution as some of these may help the individuals to survive in a certain type of environment while others may not.
V. Origin of species:-
As the favorable variations of the fittest animals are inherited to the offsprings, more and more these accumulate in their progenies. After several generations over a long period of time, the organisms become so different from the individuals of theoriginal species that they constitute a new species.
Objections/Criticisms to Darwin’s Theory of Natural Selection:-
It does not explain the causes of variations.
He did not explain the effect of use and disuse of organs.
The theory of natural selection doesn’t give an adequate explanation of the occurrence of vestigial organs generation after generation.
Darwinism explained the survival of the fittest but not the arrival of the fittest.
Natural selection did not explain the over specialization of particular structure, which in some cases has led to the extinction of the race.
3. Neo-Darwinism (Modern Synthetic Theory of Evolution):-
Neo-Darwinism is a modified form of Darwinism. The Neo-Darwinians like Wallace, Haeckel, Weismann, Mendel etc believed that natural selection has accounted everything that is involved in evolution. In their dogmatic belief they went further to secure enough proof towards natural selection. Darwin believed that the adaptations result mainly by single source, i.e. natural selection but Neo-Darwinians thought that adaptation result from multiple forces and natural selections is only one of these many forces. Later on this theory was modified by different other scientists such as Fisher, Huxley, Haldane, Hugo-de varies etc in the form of modern synthetic theory of evolution. This modern theory explains the genetic basis of the adaptation of a population to new environmental conditions.
It includes following concepts:-
I. Mutation: – It can be define as the sudden, random, and heritable changes in the organisms. It was first proposed by Hugo de Vries. According to him, new species are formed due to discontinuous variations caused due to mutation.
II. Genetic recombination: – The genes or alleles of parental linkage groups separate and new combinations of alleles are formed in the gamete cells, a process is known as genetic recombination. As a result new recombinants with new genotypes are produced from already existing genes.
III. Natural selection: – In nature, organisms face everlasting competition or struggle among them. In that competition the stronger win and survive, while the weaker are rooted out. Similarly, organisms show some variations, which are proved to be beneficial in facing the environmental calamities. The individual with favorable variations have a better chance of survival in the nature and others die. Thus, the nature is the super power that selects best fitted individuals and rejects those that are poorly developed, and thus they usually die. This process is known as natural selection.
IV. Isolation: – Isolation is the process of separation of organisms from their original population under physiological or geographical factors. Since these isolations do not allow the interbreeding among the organisms of different species, which is called reproductive isolation. It is supposed to be one of the most significant factors responsible for evolution.
V. Genetic drift: – The modern changes in gene frequencies in a population occurring by chance alone rather than natural selection are called genetic drift.
