NEET Biology Notes Evolution Modern Synthetic Theory of Evolution
Modern Synthetic Theory of Evolution
The modern theory of origin of species or evolution is known as Modern Synthetic Theory of Evolution. It is the combination of Darwinian selection and genetic theory. This concept evolved after a book by Julian Huxley (1942) entitled Evolution, the Modern Synthesis. Theodosius Dobzhansky reviewed the Darwinian concept of evolution by Natural Selection in Mendelian populations.
Postulates of Modem Synthetic Theory
This theory recognizes four basic types of processes
- Gene mutations
- Changes in chromosome structure and number
- Genetic recombination mod
- Natural selection D’hois,
- Besides these, three accessory processes affect the working of above four basic processes
- Gene mutations, change in chromosome structure and number and genetic recombination provide the genetic variability.
- Migration of individuals frofti one population to another, hybridisation between races or closely related species both increase the genetic variability.
- The effect of chance, acting on small population may alter the way.
- All sexually reproducing organisms contain a large gene pool of genetic variability, which maintains a dynamic equilibrium between in flow and out flow of genes.
- Genes may be added to gene pool by immigration from
other gene pool and mutation. .
- Natural selection which results from interaction between populate and their environment, may either stabilise gene composition by eliminating most immigrants and mutants or change it in various ways.
- Reproductive isolation, which includes all the barriers to gene exchange between populations has a canalising effect.
Hardy-Weinberg law was proposed by GH Hardy, an English mathematician and G Weinberg, a German physician in 1908 states that the gene and genotypic frequencies in a Mendelian population remain constant generation after generation, if there is no selection, mutation, migration or random drift.
Hardy-Weinberg used the binomial expression p2 +2pq +q2 to calculate the genotypic and allele frequencies of a population. The original proportions of the genotype in a population will remain constant from generation to generation as long as
- The population size is very large.
- Random mating is occurring.
- No mutation takes place.
- No genes are input from other sources (i.e. no immigration).
- No selection occurs.
When some individuals of a population migrate to other populations or when certain individuals come into a population, the gene frequencies of the given population, change, i.e. some genes are lost. If this migration occurs a number of times, gene flow occurs.
Genetic drift or Sewall-Wright effect or Random drift refers to the random changes in the allele frequency that occur in all populations but are much more pronounced in smaller populations. Bottleneck effect and founder effect are two special cases of genetic drift.
A species is a group of similar organisms, which can breed among themselves, producing fertile offsprings. The members of a species not only share a common gene pool, but also share a common ancestry. Origin of a new species from the existing one is called spéciation.
It is the prevention of mating amongst inter-breeding groups as a result of physical barriers (i.e. ecological, geographical, etc.) and biotic barriers (i.e. physiological, behavioural, mechanical, genetical, etc).
It is an evolutionary process in which an ancestral stock gives rise to new species adapated to new habitats and new ways of life.
These were small black birds, which Darwin observed in Galapagos islands. There were many varieties in the same kind. Darwin reasoned that after originating from a common ancestral seed-eating stock, the finches must have radiated to different geographical areas and undergone adapative changes, especially in the type of beak. Living in isolation for long, the new kinds of finches emerged that could function and survive in the new habitats.
A number of marsupials, each different from the other, evolved from an ancestral stock within Australia. When one more than one adaptive radiation appeared to have occurred in an isolated geographical area with different habitats. It can be called as convergent evolution.
The common ancestor of apes and humans is a primate Dryopithecus, that lived 15 million years ago. The next stage in the hominid evolution is Ramapithecus. After this,
human evolution is as follows:
- Australopithecus They had a brain capacity of 450-600 cc. They were about four feet tall and walked nearly upright. They hunted with stone weapons but ate fruits. They were found in Tanzania and Ethiopia.
- Homo habilis They lived in East African grasslands. They had a brain capacity of 650-800 cc. They probability did not eat meat.
- Homo erectus Their fossils were found in Java. They had a brain capacity of about 900 cc. They probably ate meat.
- Homo sapiens The fossils were found in East and Central Asia. The brain capacity was about 1400 cc. They must lived between 100000-40000 years before. They used hides to protect the body and also buried the body after death. They became extinct about 25000 years before.
- Homo sapiens sapiens He arose during the ice age between 75000-10000 years ago. He learned to cultivate plants and domesticate animals. Agriculture started around 10000 years back. Human settlement and civilisation started.