Sewell Wright, Fisher, Mayer, Huxley, Dobzhansky, Simpson and Haeckel explained Natural Selection in the light of Post-Darwinian discoveries. According to this theory gene mutations, chromosomal mutations, genetic recombinations, natural selection, and reproductive isolation are the five basic factors involved in the process of organic evolution.
i. Gene mutation refers to the changes in the structure of the gene. It is also called gene/ point mutation. It alters the phenotype of an organism and produces variations in their offsprings.
ii. A chromosomal mutation refers to the changes in the structure of chromosomes due to deletion, addition, duplication, inversion or translocation. This too alters the phenotype . of an organism and produces variations in their offspring.
iii. Genetic recombination is due to crossing over of genes during meiosis. This brings about genetic variations in the individuals of the same species and leads to heritable variations.
iv. Natural selection does not produce any genetic variations but once such variations occur it favors some genetic changes while rejecting others (driving force of evolution).
v. Reproductive isolation helps in preventing interbreeding between related organisms.

(i) According to the theory of chemical evolution primitive organisms in the primordial environment of the earth evolved spontaneously from inorganic substances and physical forces such, as lightning, UV radiations, volcanic activities, etc.
(ii) Oparin (1924) suggested that the organic compounds could have undergone a series of reactions leading to more complex molecules. He proposed that the molecules formed colloidal aggregates or 'coacervates' in an aqueous environment. The coacervates were able to absorb and assimilate organic compounds from the environment.
(iii) Haldane (1929) proposed that the primordial sea served as a vast chemical laboratory powered by solar energy. The atmosphere was oxygen free and the combination of CO₂, NH₃ and UV radiations gave rise to organic compounds.
(iv) The sea became a 'hot' dilute soup containing large populations of organic monomers and polymers. They proposed that groups of monomers and polymers acquired lipid membranes and further developed into the first living cell. Haldane coined the term prebiotic soup and this became the powerful symbol of the Oparin-Haldane view on the origin of life (1924-1929).
(v) Oparin and Haldane independently suggested that if the primitive atmosphere was reducing and if there was appropriate supply of energy such as lightning or UV light then a wide range of organic compounds can be synthesized.

Adaptive radiation is the process of evolution of different species in a given geographical area starting from a point and literally radiating to other geographical area habitats.
1. Darwins finches were the small black birds which Darwin observed in Galapagos islands.
2. There were many varities of these birds in the same island.
3. Darwin reasoned that after originating from a common ancestral seed eating stock the finches must have radiated to different geographical areas and undergone adaptive changes especially in their breaks enabling some of them to become insectivorous.
4. Living isolation for long new kinds of finches must have evolved which could survive in the new habitats.
5. Australian marsupials (Kangaroos) also show adaptive radiation
6. Marsupials in Australia and placental mammals in north America are two subclasses of mammals they adapted in similar way to a particular food resources, locomotary skill or climate.
7 They were separated from the ori-on ancestor more than 100 million year ago and each lineage continued to evolve independently despite temporal and geographical separation marsupials in Australia and placenta mammals in North America have reduced varieties of species living in similar habitats with similar way of life their overall resemblance in shape, locomotary mode, feeding and foraging are superimposed upon different modes of reproduction.

1. Natural selection can be explained clearly through Industrial melanism.
2. Industrial melanism is classical case of natural selection exhibited by the peppered moth.
Biston Betularia
1. These were available in two colours, white and black. Before industrialization peppered moth both white and black coloured were common in England (Manchester).
2. Pre-industrialization witnessed white coloured background of the wall of buildings hence the white coloured moths escaped from their predators.
3. Post Industrialization the tree trunks become dark due to smoke and soot let out from the lndustries.
4. The black moths camouflaged on the dark bark of the trees and the white moths were easily identified by their predators (birds).
5. Hence the dark coloured moth population was selected and their number increased when compared to the white moths nature offered positive selection pressure to the black coloured moths.
6. The above proof shows that in a population organisms that can adapt will service and produce more progenies resulting in increase in population through natural selection.

1. There are five basic processes that affect the Hardy-Weinberg equilibrium and cause variations at genetic level.
2. These include (i) Mutation (ii) recombination (iii) Gene migration (iv) Genetic drift and (v) natural selection.
3. Among the above five processes, lets discuss how mutation, natural selection and genetic drift affect Hardy Weinberg equilibrium.
(a) Mutation:
1. Mutation are sudden heritable changes in the characteristics of the organisms.
2. These mutations are random (indiscriminate) with respect to the adoptive needs of organisms.
3. Most mutations are harmful or with no effects (neutral) on their bearers.
4. However under changed environmental conditions, the alleles, which were previously harmful or neutral may become advantageous.
5. The rate of mutation is very slow usually one mutation occurs in a million still these mutation rates are sufficient to create considerable genetic variation.
6. Mutations help in maintaining variations within population.
7. Mutation also introduce new genes and alleles into the gene pool (sum total of all genotypes or alleles found in a population).
8. It is the gene pool in which new genes are added or removed. This variability of genes (or) alleles in a gene pool, then serve as a raw material for evolutionary changes.
9. The accumulation of many mutations over hundreds of generation cause large scale changes.
(b) Natural selection
1. Natural selection is the most important evolutionary process that brings about changes in allele frequencies.
Definition
1. The process of differential reproduction that lead to differential contribution of genotypes to the gene pool of the next generation.
2. Natural selection results a change in allele frequencies of a population.
(c) Genetic drift
1. It refers to the elimination of certain genes in a small isolated population either due to intensive inbreeding causing permanent fixation of some genes or death of a small section of population by natural calamity.
Definition
1. The random changes in the allele frequency occurring by chance alone are called genetic drift.
2. This means that the alleles that form the gene pool of the next generation are a sample of the allele from current generation.
3. Sampling errors (chances) often lead to the elimination of certain alleles and fixation by others thus reducing the genetic variability of the population.
4. The effect of genetic drift is very small in large but can be large in a small population.
5. Thus these forces that change the allele and genotypic frequencies in a population and so called evolutionary agents.
6. As long as Hardy-Weinberg Equilibrium remains constant the evolution cannot occur since the genetic equilibrium is a sort of inertia whose tendency is rather conservative. Evolution become possible only if the inertia is overcome or the equilibrium is broken.

Structures which are similar in origin but perform different functions are called homologous structures that bring about divergent evolution.
Eg: Thorn of Bougainvillea is used for defense and tendrils of Cucurbita are used for climbing but both are homologous structures.

Mould	Cast
Even after disintegration, the body of an animal might leave an indelible impression on the soft mud which later becomes hardened into stones. Such impressions are called moulds.	The cavities of the moulds may get filled up by hard minerals and get fossilized, which are called casts.

Lamarcks 'theory of acquired character's was disproved by August Weismann who conducted experiments on mice for twenty generations by cutting their tails and Weismann proved that change in the somatoplasm will not be transferred to the next generation but changes in the germplasm will be inherited.

1. Fitness according to Darwin refers ultimately and only to reproductive fitness
2. Those individuals who are better fit in an environment leave more progeny than the others the populations come to posses more fit individuals as nature selects for fitness.
3. The so called fitness is based on characteristics which are inherited hence there must be. genetic basis for getting selected and to evolve.

Ernst Von Haeckel propounded the "biogenetic law or theory of recapitulation" which states that the life history of an individual (ontogeny) briefly repeats or recapitulates the evolutionary history of the race (phylogeny). In other words "Ontogeny recapitulates Phylogeny".

Sudden appearance of vestigial organs in highly evolved organisms is called atavistic organs. Eg: presence of tail in a human baby is an atavistic organ.

(i) Neanderthal human was found in Neander Valley, Germany with a brain size of 1400 cc and lived between 34,000 - 1,00,000 years ago.
(ii) They differ from the modern. human in having semierect posture, flat cranium, sloping forehead, thin large orbits, heavy brow ridges, protruding jaws and no chin. 

(i) Darwin failed to explain the mechanism of variation.
(ii) Darwinism explains the survival of the fittest but not the arrival of the fittest.
(iii) He focused on small fluctuating variations that are mostly non-heritable.
(iv) He did not distinguish between somatic and germinal variations.
(v) He could not explain the occurrence of vestigial organs, over specialization of some organs like large tusks in extinct mammoths, oversized antlers in the extinct Irish deer, etc.,