Sources of variation in organic evolution

Variation among the organisms induces organic evolution. There are different reasons causing variation due to change in gene frequency.

1. SOURCES OF VARIATION IN ORGANIC EVOLUTION By Md Ekramuddin Assistant Professor in Zoology Shibpur Dinobundhoo Institution(college) Howrah, West Bengal. India

2. WHAT IS ORGANIC EVOLUTION 1. Advanced species originates from ancestral one and this is organic evolution . 2. Origin of new species is speciation . 3. This is very slow process. 4. Genetic composition of population gene pool is changed in response to environmental changes.

3. WHAT IS VARIATION Any kind of difference in morphology, anatomy and behaviour among the organisms is variation. Different genes are are suitable for adaptation in different environmental conditions. There is a continuous change in environment, this eventually leads to genetic variation in the population.

4. TYPES OF VARIATIONS Two types-continuous and discontinuous Continuous variation-the individuals within a population vary without no distinct boundary or category. Eg-height,weight etc. Discontinuous variation- each individual falls into only one of the two or multiple distinct categories with distinct boundary and no intermediates. Eg gender, blood group in human etc.

5. BASIS OF VARIATION Change in gene frequency in the population. Gene frequency is proportionate expression of gene in the members of population with respect to the gene pool. Frequency of a gene varies in same population occupying in different geographical area.

6. SOURCES OF VARIATION Gene frequency change is the reason of variation in the population Gene frequency change takes place in two ways- regular and inductive. Regular - by genetic recombination through crossing over during meiosis cell division Inductive - by Selection, Mutation, Gene migration and Genetic drift

7. GENETIC RECOMBINATION BY CROSSING OVER

8. SELECTION Reproductive success-adaptive value(W) When adaptive value decreases it is due to selection pressure Selection pressure is presented as ratio between number of observed offspring and absolute number expected. This ratio is called as selection co-efficient presented as ‘s’ Freqency of an allele of a locus increases and of another allele decreases due to relative reproductive success by selection. Best example is Industrial Melanism of Biston bitularia (peppered moth) which becomes dark colored when background became dark resulted from revolutionary industrialisation in Europe.

9. ARTIFICIAL SELECTION Human selects the better organisms and allow them reproduction Mostly economically important species like cattle, fishes, birds, domesticated animals etc Result is pre-determined but in natural selection result depends upon environment Both use the benefit of random mutation.

10. MUTATION Expression of gene changes by mutation so that hereditary characters are changed This happens to provide better adaptive opportunity in changing environment Rate of mutation is presented as µ. If a gene with frequency ‘p’ undergoes mutation at a rate of µ then frequency in next generation will be p.µ. Hence frequency change is (p-µp) or p(1- µ) Mutation of a threshold number of genes leads to variation in the population.

11. MUTATION INDUCES VARIATION

12. GENE MIGRATION Migration of a small group of organisms from a population of a geographical area to another population of another geographical area and interbreed freely to make conglomerate . Frequency of a gene that has different frequecy in donor and recipient population shows difference in conglomerate. The conglomerate later favours speciation through variation.

13. GENE MIGRATION cont.. MIGRATION INTERBREEDING DONOR POPULATION RECIPIENT POPULATION CONGLOMERATE

14. GENE MIGRATION cont… If frequency of gene ‘x’ in donor is pD and recepient population is pR If migration co-efficient is ‘ m ’ Then ∆Pc=m(pd-pr) Pc=pr+∆pc ∆ pC =change in gene frequency of conglomerate pC =New gene frequency of conglomerate

15. GENETIC DRIFT Changes gene frequency due to sampling error Non random force of gene frequency change Operates in small populations Most potent force as most natural populations are small in size May cause genetic death or genetic load

16. GENETIC DRIFT cont…. Gene frequency change is measured through measurement of standard deviation If frequency of dominant allele is p and recessive allele q, number of members of the population is N then, Change of frequency SD= √ pq / 2 N Most effective in founder population and in bottle neck cases

17. CONCLUSION Advanced and complicated life forms develop gradually from ancestral form in a very slow process. This is favored by change in gene frequency so that the existing gene pool is splitted into multiple pool which become reproductively isolated over time. This causes speciation. The different sources causing gene frequency as discussed here are regarded as important factors behind organic evolution.

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