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# EVOLUTION ## **6.6 MECHANISM OF EVOLUTION** * Darwin was influenced by the work of Thomas Malthus on populations. * The fact that theoretically population size will grow exponentially if everybody reproduced maximally. * The question arises: What is the origin of variation and how is species for...

# EVOLUTION ## **6.6 MECHANISM OF EVOLUTION** * Darwin was influenced by the work of Thomas Malthus on populations. * The fact that theoretically population size will grow exponentially if everybody reproduced maximally. * The question arises: What is the origin of variation and how is species formed? * Mendel also had mentioned about heritable 'factors' influencing phenotype. * Darwin could not mention these observations. * Credit for this goes to Hugo de Vries who worked on Evening primrose. * Hugo de Vries (1848-1935) was a Dutch botanist, one of the independent rediscoverers of Mendelism. * He put forward his views regarding the formation of new species in 1901. * He also met some of the objections found in Darwin's theory. * According to him, new species are not formed by continuous variations but by sudden appearance of variations, which he named as mutations. * Mutations are heritable and persist in successive generations. * Hugo de Vries believed that mutation causes evolution and not the minor heritable variations which was mentioned by Darwin. * Mutations are random and directionless while Darwin's variations are small and directional. * According to Darwin evolution is gradual while Hugo de Vries believed that mutation caused species formation and hence known as saltation (single step large mutation). ## **6.7 HARDY-WEINBERG PRINCIPLE** * It was proposed by G.H. Hardy, an English mathematician and W. Weinberg, a German physician independently in 1908. * It describes a theoretical situation in which a population is undergoing no evolutionary change. * Gene frequency (also called allele frequency) is the proportion of a particular allele among all alleles at a gene locus. * The principle states that allele frequencies in a population are stable and are constant from generation to generation. * The gene pool (total genes and their alleles in a population) remains a constant. * This stability of allele frequencies is called genetic equilibrium or Hardy-Weinberg equilibrium or Hardy-Weinberg Principle. ### **Five Factors Are Known To Affect Hardy-Weinberg Equilibrium (Principle):** 1. No natural selection 2. No gene migration or gene flow 3. No mutation 4. No genetic drift 5. No genetic recombination. * Another condition is also required for this principle: there must be random mating. * According to Hardy-Weinberg Principle, gene frequencies will remain constant if all above conditions are met. ### **Illustration** * Hardy and Weinberg, through this principle have been able to explain why a population as a whole with all its variations and genotype frequencies continue to remain unchanged for many generations. * Mendelian genetic variations in Hardy-Weinberg equation are summed up as: 1. Assume a gene has two alleles A (dominant) and a (recessive). * A allele has frequency p and a allele has frequency q. In a diploid, p and q represent the frequency of allele A and allele a. 2. There would be three genotypes two homozygous and one heterozygous (e.g., AA, Aa and aa) in the parental generation. * Then, the genotype frequencies expected in the