Theories of Evolution

Lamarckism

It is the theory of inheritance of acquired character proposed by Jean-Baptiste Lamarck (1744-1829), a french biologist. He published a book "Philosophic Zoologique" in 1809.

Four main principles or postulation of Lamarckism

This theory includes four main principles or postulation.

Internal vital force/ Tendency to grow

According to this, organisms and their body parts tend to grow up to certain limit due to internal vital force.

Environmental effect

According to this environment is dynamic and changeable. Environmental change brings changes in organisms. To fit in the new environment, they need new organisms which are developed on the basis of desire. The principle of desire is called "Appentency".

Use and disuse of organs

According to this development of organs is directly proportional to their use and vice versa. It is more the organ is used, more it develops and less the organ is used, less it is developed and become non-functional.

Inheritance of acquired character

According to this characters acquired by organisms during lifetime are inherited from one generation to another. Accumulation of these acquired characters over a long time leads to the formation of new species.

Examples in support to Lamarckism

• Present day giraffe with a long neck is formed by continuous stretching of the neck to derive vegetation from a large tree due to the scarcity of grass.
• The long and cylindrical body of snake due to stretching of the body while moving into burrows.
• Development of the web in the hind limb of aquatic birds.
• Vestigial wings of flightless birds.

Criticism to Lamarckism

Lamarckism gets less support and high criticism regarding the first principle. There is no any internal vital force which brings the growth of body parts. Similarly, not only environmental change brings change in the organism. The third principle is acceptable to some extent. Regarding the fourth principle, not all acquired characters are inherited from one generation to another.

August Wiesman highly criticised the view of Lamarck and proposed the theory of "Continuity of Germplasm". According to this, only those characters are inherited which influence the germ cell or germplasm. To prove this he cut off the tail of rat for 22 generations and allowed them to breed but not a single rat was born without the tail. Similarly, piercing of ear by women and wrestlers powerful muscle are some example which is not inherited.

Neo-Lamarckism

It is the modified form of Lamarckism. This theory has been modified by the effort of many scientists to make more acceptable among the new generation. Some modified view of Lamarckism are;

• The environment has an influence on the organism.
• Internal vital force and appetency do not have any role in evolution.
• Not all characters are inherited from one generation to another only those character are inherited which has an influence on germplasm or germ cells.

Darwinism

It is the theory of natural selection proposed by CharlesDarwin(1809-1882). He was a British naturalist. He published a book "Origin of Species" in 1859.

Principles or postulations of Darwinism

Overproduction

It is the capacity of organisms to produce a large number of offsprings. All organisms are highly fertile as they produce more offsprings than one actually survive. Eg; Elephant, the slowest breeder matures at the age of 30 and produces 6 offsprings in the lifetime. If all survive and reproduce, at the end of 750 years their population would be 19 million.

Struggle for existence

Due to over production, there is a struggle among organisms for existence. It may be intraspecific, interspecific or with an adverse environmental condition. So, it keeps the size of the population more or less constant because in struggle few are able to get basic need whereas many fail to survive and reproduce.

Variations and Heredity

The difference in offsprings from a parent is variation and their similarity is heredity. No evolution occurs without variation. Variation brings new characters in organisms whereas heredity passes them to next generation. Variation may be continuous or discontinuous.

Natural selection (Darwin)/ Survival of the fittest

According to this nature selects those individuals which are well adapted to the environment. At the same time, nature also rejects individual which are poorly adapted to the environment. Organisms having useful variations have a greater chance of being selected. So, natural selection is a weeding out the process by which only the fittest individuals are selected. Example:

• Evolution of present day giraffe by means of natural selection.
• Industrial melanism: It is the development of black colored melanic moth along with the development of industries due to the replacement of lichens by soot.
• Development of DDT-resistant mosquitoes

Origin of species/speciation

Individuals having useful variations are selected by nature. They pass these useful characters to next generation. Accumulation of these variations in many generations leads to the formation of new species.

Criticism of Darwinism

• Darwinism lacks the knowledge of genetics.
• It failed to explain the origin of variation.
• It could not differentiate between heritable and non-heritable variation.
• It only explains the survival of the fittest but not the arrival of the fittest.
• Natural selection is not only the cause of the origin of species.
• It could not explain the inheritance of small, non-useful variation.

Neo – Darwinism

Due to the various objections, the Darwinism theory was modified and a new theory was postulated as Neo- Darwinism. According to this theory, the causes of origin of new species are the followings

Mutation;
It is defined as the sudden heritable changes which occur in the chromosomes and gene. Hugo de Vries purposed it. It causes variation in organisms, which plays very important role in evolution.

Genetic recombination;
During the crossing over of alleles in meiosis of sexual reproduction, the breakage, and recombination of the homologous chromosome is a significant phenomenon. The alleles of parental linkage groups separate and new association of alleles is formed in the gamete cells, which is known as genetic recombination. Offspring is formed from the gametes showing a new combination of characteristics recombinants. The crossing over is the main source of observable genetic variations in the population, which is the significant event in evolution.

Natural selection;
Natural selection is the principle element of Darwin’s theory. Darwin suggested that any variation which helps the individual possessing it will tend to be preserved. The principle y which this preservation of useful variation is brought about was called natural selection.

Genetic drift;
It is defined as the random changes in the gene frequencies in a population that occur either by chance or by natural selection. It was described by Sewell Wright. It may be an important mechanism in evolutionary change in small or isolated population. It is thought to have been a significant factor in the origin of new species by natural selection.

Isolation:
It is the separation of organisms from their original population resulting prevention of mating amongst interbreeding groups. It is due to physical and biotic barriers. Any factor, which prevents interbreeding, is known as isolation mechanisms. There are two types of isolation, which are as follows;

Geographical isolation; Here, original population separates due to geographical barriers like mountains, river, glacier, ocean, forests, etc. also preventing interbreeding between them. During the course of time, different mutations may occur in the separated group, this difference is such that when the separated group comes in contact again, they do not interbreed thus producing new species.

Reproductive isolation; Here, prevention of interbreeding is due to genetic differences. Here, potential mates are unable to mate due to morphological or physiological differences or the hybrid formed in unviable or sterile though being in the same area.