Neutral theory of evolution

Neutral theory of evolution (molecular evolution)
By motto kimura

1. NEUTRAL THEORY AND MOLECULAR CLOCK PRESENTED BY:- NEHA DHIMAN ROLL NUMBER:- CUHP18ZOO16 SUBMITTED TO:-EKTA GULERIA COURSE NAME:- EVOLUTIONARY BIOLOGY AND BIODIVERSITY

2. NEUTRAL THEORY OF EVOLUTION Neutral theory of evolution holds that most evolutionary changes at the molecular level, and most of the variation within and between species, are due to random genetic drift of mutant alleles that are selectively neutral. The theory applies only for evolution at the molecular level, and is compatible with phenotypic evolution being shaped by natural selection as postulated by Charles Darwin.

3. NEUTRAL MUTATION A neutral mutation are changes in DNA sequences that are neither beneficial nor detrimental to the ability of an organism to survive and reproduce. The neutral theory assumes that most mutations that are not deleterious are neutral rather than beneficial. In population genetics, mutation in which natural selection does not affect the spread of the mutation in a species are termed as neutral mutations.

4. In 1968, Motoo kimura propsed :- Neutral theory of evolution/ Survival of luckiest. According to this, most genetic variation , observed in natural populations, is due to the accumulation of neutral mutation that do not affect the phenotype of the organism and are not acted on by natural selection. EXAMPLE:- A mutation within a structural gene that codes for glycine codon from GGG to GGC does not affect the amino acid sequence of encoded protein. These mutations does not affect phenotype, they spread throughout the population according to their frequency of appearance and random genetic drift.

5. Two observations underlie theory:- 1.) Most natural populations harbor high levels of genetic variation higher than would be expected if natural selection were the evolutionary force primarily responsible for influencing level of genetic variations in populations. 2.) Many mutations in sequences of genes do not alter the proteins encoded by those genes- Virtually always true for synonymous substitutions. Sometimes true for non-synonymous substitutions.

6. Kimura and his collegue Tomako ohk outlined five principles that governs the evolution of genes at the molecular level. (1.) For each protein the rate of evolution in terms of amino acid substitution is approximately constant with regard to neutral substitution that donot affect protein structure and function. EVIDENCE:- The amount of genetic variation between the coding sequence of the human alpha- globin and beta- globin genes is approximately same as the difference between the alpha and beta globin gene in the horse.

7. (2.) Proteins that are functionally less important for the survival of an organism or part of a protein that are less important for its function tend to evolve faster/ accumulate amino substitutions more rapidly than the more important ones. EVIDENCE:- Histone proteins are critical for survival which help in nucleosome formation in eukaryotes. Histone genes tolerate very few mutations and have evolved extremely slowly. Fibrinopeptides which bind fibrinogen to form a clot evolve very rapidly.

8. (3.) Amino acid substitutions that donot significantly alter the existing structure and function of a protein are found more commonly than the disruptive amino acid changes. EVIDENCE:- When comparing the nucleotide/ coding sequence within homologous genes of modern species nucleotide sequences are more likely to be occurred in the wobble base as mutation in this are silent. - Conservative substitutions (substitution with a similar amino acid, such as a nonpolar one) are fairly common.

9. - Non conservative substitutions, nonsense and frameshift mutations are very rare within coding sequence of genes. - Also, introns sequences evolve more rapidly than the exon sequences.

10. (4.) Gene duplication often proceeds the emergence of a gene having a new function. EVIDENCE:- When a single copy of gene exist in a species, it usually plays a functional role similar to that of the homologous gene in another species. Gene duplication have produced gene families in which each family member can evolve somewhat different functional rate. Example:- globin and hox gene family.

11. (5.) Selective elimination of definitely deleterious mutations and the random fixation of selectively neutral or very slightly neutral or very slightly deleterious alleles occur for more frequently in evolution than selection of advantageous mutants. EVIDENCE:- Non conservative mutations usually have a negative effect on phenotype of organisms so are effectively eliminated from population of natural selection. Example:- A nonconservative mutation in the beta- globin gene produced the Hbs allele which gives an individual resistance to malaria in the heterozygous conditions.

12. EVIDENCE SUPPORTING NEUTRAL THEORY- Psuedogenes (dead genes that have no function and no fitness effect) evolve very fast. Synonymous codon positions (3-fold, 4-fold degenerate sites) evolve faster than a non-synonymous sites, and should evolve with a constant rate (not always true). Genes that have very important functions should evolve faster.

13. MOLECULAR CLOCK HYPOTHESIS:- Concept was given by Zuckerkandle and Pauling(1965). It is used to predict the phylogeny of organism and it can also enable us to estimate the absolute time since taxa diverged.