NEO-DARWINISM (MODERN SYNTHETIC THEORY OF EVOLUTION)
(8 marks for NEB)
Due to the lack of information on cytology, genetics and molecular biology etc. during the period of Darwin, many biologists believed that Darwin was unable to describe the evolution perfectly.After the discovery of Mendel’s work and work of other biologist like Hugo de vries, Huxley, Fischer, Mayrt, Wright etc. Darwinism was re-established with the modification and formed a new integrated theory on the basis of genetic character is called Neo-Darwinism.
The modern theory explains the genetic basis of adaptation of population to new environmental condition; it included following concepts-
1. Mutation– It was put forward by botanist “Hugo de vries”(1849-1935)”. It states that” New species arise from pre existing ones in a single generation by a sudden appearance of marked, discontinuous, inheritable genetic variation called Mutation. It can be chromosomal or gene mutation. Mutation causes variation in the population in the population which plays great role in bringing evolution
2. Genetic recombination – Rearrangement of genes that occurs at the time of gamete formation is called Genetic recombination. During crossing over in prophase – I of meiosis, breaking and recombination of homologous chromosome occur, that results in recombination of genes. The genetic recombination produces produces new combination of characters in their offspring’s called recombinants. So it is also a major source of variation in the population and have significant role in evolution.
3. Natural selection – Natural selection is the principle element of Darwin’s theory. He believes that adaptation results mainly by natural selection. But according to Neo-Darwinism, Natural selection is only one of the major forces. Natural selection doesn’t produce genetic change, but once genetic change has occurred, it selects favorable change over others. Further, natural selection creates new adaptive relationship between population and environment by favoring some gene combination and rejecting others. Thus, constantly modifying the gene pool.
4. Isolation– The separation of population into 2 or more groups is called isolation. It plays a significant role in evolution. There are 2 types of isolation. i.e
- Geographical isolation – When population is divided into 2 or more groups by geographical barriers such as mountains, oceans, rivers. Etc. It is called geographical isolation. It takes place first for species formation.
- Reproductive isolation– That isolation which doesn’t allow the interbreeding among the organisms of closely related species is called reproductive isolation. Later, the reproductive isolation takes place.
(8 marks for NEB)
The random change in gene frequency, in a population occurring by chance rather than natural selection is popularized as “Genetic drift”. It may be important mechanism in evolutionary change in a small or isolated population. It may be significant factor in the origin of new species.
6. Speciation – The origin of species from the existing ones is called “speciation”. A species is a group of individuals which are closely related and interbreed with each other to form fertile offspring.
Speciation occurs in following way-
–Intra-specific speciation– It is origin of new species due to change in single species.
–Inter-specific speciation– In this type , 2 different species interbreed and give rise to fertile hybrid