Origin and Evolution of life Questions and Answers-3.
Q. 1. What are the main features of mutation theory?
(2 marks)
Ans. Main features of mutation theory :
(1) The new species originate as a result of large,
discontinuous variations that appear suddenly involving mutations.
(2) Mutations provide the raw material for organic
evolution by which the new species originate. They are heritable changes leading to the formation of new species.
(3) They occur frequently among the members of naturally breeding populations or species.
(4) Mutations are at random taking place in all directions. Unsuitable mutations are likely to be destroyed by Natural selection.
Q. 2. What are the main objections to mutation theory?
(2 marks)
Ans. (1) Sudden appearance of large discontinuous variations observed by Hugo de Vries were not really gene mutations, but were actually due to chromosomal aberrations that have little value in evolution.
(2) Mutation is actually a change in structure of gene which brings about a minor variation.
(3) Rate of mutation is very slow, i.e. one per million or several millions.
Q. 3. Explain modern synthetic theory theory of evolution.
(3 marks)
Ans. Modern synthetic theory of evolution :
(1) Julian Huxley proposed modern theory which was based on the work of Dobzansky.
(2) Along with these two scientists, Mayer, Simpson Stebbins, Fisher, Haldane, Sewall Wright, etc. also approved this concept.
(3) This theory explains evolution in terms of genetic change in populations leading to the origin of new species.
(4) The population is considered as unit of evolution. New species are evolved from population and not from a single individual. Therefore, Mendelian populations were considered as units of evolution.
(5) The sum total of genes of all individuals of Mendelian population are called gene pool. Every gene pool has different genes with their specific gene frequencies.
(6) Based on these assumptions, the modern synthetic theory has three main concepts, viz. genetic variation, Natural selection and isolation.
variations. The favourable among these are selected further by process of natural selection. Due to isolation interbreeding does not happen between different population. This leads to formation of new species.
Q. 4. Give any two factors responsible for genetic variation.
(2 marks) (March ’16)
Ans. Factors responsible for genetic variation are :
(1) Gene mutations (2) gene flow (3) genetic recombination (4) Genetic drift (5) Chromosomal aberrations. (6) These factors cause changes in the genes and gene frequencies and thereby bring about variations.
Q. 5. Explain the terms : Gene pool and Gene frequency.
(3 marks)
Ans. 1. Gene pool :
(1) The sum total of genes of all individuals of Mendelian population that interbreed among themselves is called gene pool.
(2) Changes in the gene pool result in the evolution. With every birth and death occurring in the Mendelian population, the gene pool changes.
(3) Migrations of reproducing individuals causing gene flow can also change the gene pool.
2. Gene frequency:
(1) The proportion of an allele in the gene pool as compared with other alleles at the same locus is called gene frequency.
(2) The useful genes which produce more adaptive phenotypes usually have more gene frequencies as compared to genes that cause harmful impacts.
(3) Nonadaptive genes have lesser gene frequency in the population.
(4) The changes in gene frequency cause evolution.
Q. 6. Write a note on "Genetic drift”.
(2 marks)
Ans. Genetic drift:
(1) Genetic drift is the alteration in allele frequency in the natural population occurring by pure chance.
(2) Genetic drift or Sewall Wright effect is one of the evolutionary forces operating by greater chance in smaller populations.
(3) Genetic drift is seen when a particular allele is eliminated from a population due to events like accidental death prior to mating of an organism.
(4) The sole possessor of particular allele if removed from a population, the genetic drift occurs.
Q. 7. What is the meaning of chromosomal
aberrations? Give the types of chromosomal aberrations.
(2 marks)
Ans. 1. Chromosomal aberrations : Chromosomal
aberrations are the structural alterations in a chromosome causing changes in the gene arrangement. They also cause variation in the Mendelian populations.
2. Types of chromosomal aberrations : Chromosomal aberrations are of the four types, viz. deletion, duplication, translocation and inversion.
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