Evolution, simply put, is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) and large-scale evolution (the descent of different species from a common ancestor over many generations). Evolution helps us to understand the history of life.
Evolution is not simply a matter of change over time. Lots of things change over time: trees lose their leaves, mountain ranges rise and erode, but they aren’t examples of biological evolution because they don’t involve descent through genetic inheritance. The central idea of biological evolution is that all life on Earth shares a common ancestor, just as you and your cousins share a common grandmother. Through the process of descent with modification, the common ancestor of life on Earth gave rise to the fantastic diversity that we see documented in the fossil record and around us today. Evolution means that we’re all distant cousins: humans and oak trees, hummingbirds and whale.
There are four main theories of evolution:
- Lamarckism or theory of inheritance of acquired characters
- Darwinism or theory of natural selection
- Mutation theory of De Vries
- Neo-Darwinism or Modern concept of evolution.
Lamarckism
This theory was proposed by a great French naturalist, Jean Baptiste de Lamarck. This theory is based on the comparison between the contemporary species of his time to fossil records. Lamarckism is based on following principles:
New needs: Every living organism is found in some kind of environment. The changes in the environmental factors like light, temperature, medium, food, air etc. or migration of animal lead to the origin of new needs in the living organisms, especially animals. To fulfill these new needs, the living organisms have to exert special efforts like the changes in habits or behavior.
Use and disuse of organs: The new habits involve the greater use of certain organs to meet new needs, and the disuse or lesser use of certain other organs which are of no use in new conditions. This use and disuse of organs greatly affect the form, structure and functioning of the organs. Continuous and extra use of organs make them more efficient while the continued disuse of some other organs lead to their degeneration and ultimate 
disappearance. So, Lamarckism is also called “Theory of use and disuse of organs.” So the organism acquires certain new characters due to direct or indirect environmental effects during its own life span and are called Acquired or adaptive characters.
Inheritance of acquired characters: Lamarck believed that acquired characters are inheritable and are transmitted to the offsprings so that these are born fit to face the changed environmental conditions and the chances of their survival are increased.
Speciation: Lamarck believed that in every generation, new characters are acquired and transmitted to next generation, so that new characters accumulate generation after generation. After a number of generations, a new species is formed. So according to Lamarck, an existing individual is the sum total of the characters acquired by a number of previous generations and the speciation is a gradual process.
In order to prove his theory Lamarck gave examples of evolution. Some of them are as follows:
Aquatic birds: Development of aquatic birds like ducks, geese etc. from their terrestrial ancestors by the acquired characters like reduction of wings due to their continued disuse, development of webs between their toes for wading purposes. These changes were induced due to deficiency of food on land and severe competition. It is an example of both extra use (skin between the toes) and disuse (wings) of organs.
Flightless birds: Development of flightless birds like ostrich from flying ancestors due to continued disuse of wings as these were found in well protected areas with plenty of food.
Snakes: Development of present day limbless snakes with long slender body from the limbed ancestors due to continued disuse of limbs and stretching of their body to suit their creeping mode of locomotion and fossorial mode of living out of fear of larger and more powerful mammals. It is an example of disuse and degeneration of certain organs.
Criticism of Lamarckism
hard blow to Lamarckism came from a German biologist, August Weismann who proposed the “Theory of continuity of germplasm” in 1892 A.D. This theory states that environmental factors do affect only somatic cells and not the germ cells.
Weismann mutilated the tails of mice for about 22 generations and allowed them to breed, but tailless mice were never born. Pavlov, a Russian physiologist, trained mice to come for food on hearing a bell. He reported that this training is not inherited and was necessary in every generation. Mendel’s laws of inheritance also object the postulate of inheritance of acquired characters of Lamarckism.
Darwinism or theory of natural selection
Charles Darwin, was the most dominant figure among the biologists of the 19th century. He made an extensive study of nature. He went on a voyage on the famous ship “H.M.S. Beagle” and explored South America, the Galapagos Islands and other islands.
He collected the observations on animal distribution and the relationship between living and extinct animals. He found that existing living forms share similarities to varying degrees not only among themselves but also with the life forms that existed millions of years ago, some of which have become extinct.
He stated that every population has built in variations in their characters. From the analysis of his data of collection and from Malthus’s Essay on Population, he got the idea of struggle for existence within all the populations due to continued reproductive pressure and limited resources and that all organisms, including humans, are modified descendents of previously existing forms of life.
Main points of Darwinism are as follows:
Geometric increase: According to Darwinism, the populations tend to multiply geometrically and the reproductive powers of living organisms (biotic potential) are much more than required to maintain their number e.g., Paramecium divides three times by binary fission in 24 hours during favourable conditions. At this rate, a Paramecium can produce a clone of about 280 million Paramecia in just one month and in five years, can produce Paramecia having mass equal to 10,000 times than the size of the earth.
Limited food and space: Darwinism states that though a population tends to increase geometrically, the food increases only arithmetically. So two main limiting factors on the tremendous increase of a population are: limited food and space which together form the major part of carrying capacity of environment. These do not allow a population to grow indefinitely which are nearly stable in size except for seasonal fluctuation
Struggle for existence: Due to rapid multiplication of populations but limited food and space, there starts an everlasting competition between individuals having similar requirements. In this competition, every living organism desires to have an upper hand over others.
Variations: According to this law of nature, no two individuals except identical (monozygotic) twins are identical. This everlasting competition among the organisms has compelled them to change according to the conditions to utilize the natural resources and can survive successfully. Darwin stated that the variations are generally of two types—continuous variations or fluctuations and discontinuous variations. On the basis of their effect on the survival chances of living organisms, the variations may be neutral, harmful and useful. Darwin proposed that living organisms tend to adapt to changing environment due to useful continuous variations {e.g., increased speed in the prey; increased water conservation in plants; etc.), as these will have a competitive advantage.
Natural selection or Survival of the fittest: Darwin stated that as many selects the individuals with desired characters in artificial selection; nature selects only those individuals out of the population which are with useful continuous variations and are best adapted to the environment while the less fit or unfit individuals are rejected by it. Darwin stated that if the man can produce such a large number of new species/varieties with limited resources and in short period of time by artificial selection, then natural selection could account for this large biodiversity by considerable modifications of species with the help of unlimited resources available over long span of time. Darwin stated that discontinuous variations appear suddenly and will mostly be harmful, so are not selected by nature. He called them “sports”. So the natural selection is an automatic and self going process and keeps a check on the animal population. This sorting out of the individuals with useful variations from a heterogeneous population by the nature was called Natural selection by Darwin and Survival of the fittest by Wallace. So natural selection acts as a restrictive force and not a creative force
Inheritance of useful variations: Darwin believed that the selected individuals pass their useful continuous variations to their offsprings so that they are born fit to the changed environment.
Speciation: according to Darwinism, useful variations appear in every generation and are inherited from one generation to another. So the useful variations go on accumulating and after a number of generations, the variations become so prominent that the individual turns into a new species. So according to Darwinism, evolution is a gradual process and speciation occurs by gradual changes in the existing species.
Criticism of Darwinism
- Darwinism is not able to explain the inheritance of vestigial organs.
- The inheritance of small variations in those organs which can be of use only when fully formed e.g. wing of a bird. Such organs will be of no use in incipient or underdeveloped stage.
- It was also refuted by Mendel’s laws of inheritance which state that inheritance is particulate.
- It did not explain the Presence of neuter flowers and sterility of hybrids.
Mutation Theory of Evolution
The mutation theory of evolution was proposed by a Dutch botanist, Hugo de Vries. Some of the Important conclusions of experiments of Hugo de Vries were as follows:
- The evolution is a discontinuous process and occurs by mutations (L. mutate = to change; sudden and inheritable large differences from the normal and are not connected to normal by intermediate forms). Individuals with mutations are called mutants.
- Mutability is fundamentally different from fluctuations
- Mutations are recurring so that the same mutants appear again and again. This increases the chances of their selection by nature.
- Mutations occur in all directions so may cause gain or loss of any character.
- Mutations are recurring so that the same mutants appear again and again. This increases the chances of their selection by nature.
So according to mutation theory, evolution is a discontinuous and jerky process in which there is a jump from one species to another so that new species arises from pre-existing species in a single generation (macrogenesis or saltation) and not a gradual process as proposed by Lamarck and Darwin.
Modern Concept Theory of Evolution(Neo-Darwinism)
Theories of Lamarckism, Darwinism and Mutation theory of evolution showed that no single theory is fully satisfactory. Neo-Darwinism is a modified version of theory of Natural Selection and is a sort of reconciliation between Darwin’s and de Vries theories. Modern or synthetic theory of evolution was designated by Huxley (1942). It emphasises the importance of populations as the units of evolution and the central role of natural selection as the most important mechanism of evolution. The scientists who contributed to the outcome of Neo-Darwinism were: J.S. Huxley, R.A. Fischer and J.B.S. Haldane of England; and S. Wright, Ford, H.J. Muller and T. Dobzhansky of America.
Some important observations and conclusions of modern concept of evolution are as follows:
- Variability is an opposing force to heredity and is essential for evolution as the variations form the raw material for evolution. The studies showed that the units of both heredity and mutations are genes which are located in a linear manner on the chromosomes.
- Natural selection of Neo- Darwinism differs from that of Darwinism that it does not operate through “survival of the fittest” but operates through differential reproduction and comparative reproductive success. Differential reproduction states that those members, which are best adapted to the environment, reproduce at a higher rate and produce more offsprings than those which are less adapted. So these contribute proportionately greater percentage of genes to the gene pool of next generation while less adapted individuals produce fewer offsprings. If the differential reproduction continues for a number of generations, then the genes of those individuals which produce more offsprings will become predominant in the gene pool of the population.
- Any factor which reduces the chances of interbreeding between the related groups of living organisms is called an isolating mechanism. Reproductive isolation is must so as to allow the accumulation of variations leading to speciation by preventing hybridization. In the absence of reproductive isolation, these variants freely interbreed