Test: Theories of Evolution (NCERT) - NEET MCQ

According to Lamarckism (proposed by Lamarck), whatever characters an individual acquires in its life time due to internal vital force, effect of environment, new needs, use and disuse of organs, they are inherited to next generations. This process continues and after several generations, the variations are accumulated upto such an extent that they give rise to new species. Melanisation in peppered moth favours Darwin's theory of natural selection.

Darwin during his famous voyage, observed that finches from various islands of Galapagos had beaks of different sizes and shapes. He observed that this was due to adaptation to different available food types. He concluded that the ancestral finches on reaching different islands occupied all empty ecological niches in absence of competition and evolved into different species. This provides evidences for origin of species by natural selection.

The main drawback of Darwinism is lack of knowledge of heredity. Darwin's theory could not explain the inheritance of variations from parents to off springs through genes.

Hugo de Vries believed that mutation causes evolution and not the minor heritable variations which were mentioned by Darwin. According to Darwin, evolution was gradual while de Vries believed mutations appear suddenly and hence called it saltation (single step large mutation)

The organisms which are provided with favourable variations would survive, because they are the fittest to face their surroundings, while the unfit are eliminated. Originally it was an idea of Herbert Spencer (1820-1903) who used the phrase 'the survival of the fittest' first time. While Darwin named it as natural selection.

According to Darwin's theory of evolution, nature selects the fittest. Fitness is based on characteristics which are inherited. Therefore, there must be a genetic basis for getting selected and to evolve. Some organisms are better adapted to survive in an otherwise hostile environment. Adaptative ability is inherited. Fitness is the end result of the ability to adapt and get selected by nature. Therefore, branching descent (inheritance) and natural selection are the two key concepts of Darwin's theory of evolution.

In 1798, Malthus, put forward a theory of humna population growth. It states that population grows geometrically, when unchecked whereas the means of its subsistence like food grwos only arithmetically.

Based on observations made during a sea voyage around the world, Charles Darwin concluded that existing living forms share similarities to varying degrees not only among themselves but also with life forms that existed millions of years ago.

Louis Pasteur by careful experimentation demonstrated that life comes only from pre-existing life. Rate of appearance of new forms is directly proportional to life span of organism. Microbes that divide fast evolve fast. For same process to occur in fish or fowl will take million of years.

According to Lamarckism, the ancestors of giraffe had small neck and forelimbs and were like horses. They were residing in places with no surface vegetation, therefore, they had to stretch their neck and forelimbs to take the leaves for food. This led to slight elongation of these parts. Whatever they acquired in one generation was transmitted to the next generation with the result that a race of long necked and long forelimbed giraffes were developed.

According to Lamarckism, all the acquired characters are inherited to the next generation. According to Darwinism, only useful variations are transferred to the next generation

Though living organisms tend to multiply geometrically, the number of individuals of a species tend to remain constant over a long period of time. Out of heterogeneous population (due to the variation) best adapted individuals are selected by nature.

Prevalence of pesticide resistant insects supports natural selection theory, e.g., DDT was thought to be an effective insecticide against household pests (like mosquitoes, houseflies, body lice, etc.) in 1945. But, within 2 to 3 years of its introduction, new DDT resistant mosquitoes appeared in the population. These mutant strains, soon became well established in the population by natural selection and thus replaced the original DDT - sensitive mosquitoes.

• August Weismann opposed the Lamarck's theory of inheritance of acquired characters. He put forward the theory of continuity of germplasm. According to this theory, the characters influencing only the germ cells are inherited. There is a continuity of germplasm (protoplasm of germ cells) but the somatoplasm (protoplasm of somatic cell) is not transmitted to the next generation, therefore, it does not carry characters to next generation.
• In 1868 in England, Charles Darwin proposed his pangenesis theory to describe the units of inheritance between parents and offspring and the processes by which those units control development in offspring.

A mutation in sperms or egg cells means mutation in offspring. This will have greatest impact on the future biological evolution of the human population.