NCERT Class 12 Biology Exemplar Chapter –7 Evolution Part 7

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Question 7:

State and explain any three factors affecting allele frequency in populations.

Answer:

Five factors are known to affect allele frequency in populations i.e., Hardy-Weinberg equilibrium. These are gene migration or gene flow, genetic drift, mutation, genetic recombination and natural selection.

Gene migration or gene flow – it is movement of alleles into a gene pool or out of a gene pool. When this movement of alleles takes place gene frequencies change in the original as well as in the new population. New genes/alleles are added to the new population and these are lost from the old population. This will lead to a gene flow if this gene migration, happens multiple times.

Genetic drift – If the movement of alleles into a gene pool or out of a gene pool takes place by chance it is called Genetic Drift. This always influences frequencies of alleles and s inversely proportional to the size of the population. It sometimes result into founder effect i.e., when a small group of individuals get isolated from a larger population to form a new population. Genetic drift may result into reduced genetic variation.

Mutation – is the large difference arising suddenly in a population, they are random and occur in all directions.

Natural Selection – is a process in which heritable variations enabling better survival are enabled to reproduce and leave greater number of progeny. That is nature selects only those traits which help an organism to survive in the changed conditions. Those that are not fit for survival perish in the long run leading to evolution of new species.

Genetic recombination – Production of new offspring with traits which differ from parents is called Genetic recombination.

Question 8:

Gene flow occurs through generations. Gene flow can occur across language barriers in humans. If we have a technique of measuring specific allele frequencies in different population of the world, can we not predict human migratory patterns in pre-history and history? Do you agree or disagree? Provide explanation to your answer.

Answer:

Gene flow occurs through generations.

As gene flow occur thorough various barriers like language over generations, by studying specific allele frequencies in different population of the world we can predict human migratory patterns in pre-history and history.

Nowadays there are many projects like human genographics project which uses previous studies data on specific genes to trace evolutionary history and migratory patterns of humans.

Question 9:

How do you express the meaning of words like race, breed, cultivars or variety?

Answer:

Race – it is a classification system used to categories human into many groups or populations. Race is an informal taxon and is kept below the species. This means that members from different races of the same species can be interbreed.

Breed – It is a specific group of domestic animals or plants having homogenous appearance, characteristics or behaviours that distinguish it from other animals or plants of the same species. This homogeneity is through selective breeding.

Cultivators – A selected group of plants selected for desirable characteristics which can be raised through vegetative propagation.

Variety – Variety is similar to race but is used for the plants. Thus variety is informal taxon which is kept below the species in the plant kingdom. A variety arises naturally in the plant kingdom and plant grown from its seeds will typically come out true to type.

Question 10:

When we say “survival of the fittest”, does it mean that?

a. those which are fit only survive, or

b. those that survive are called fit? Comment.

Answer:

Survival of the fittest means those who survive are fit.

In the struggle for existence, the individuals who have more favourable variations will enjoy an advantage over others. They are fit and survive and reproduce.

Natural Selection – is a process in which heritable variations enabling better survival are enabled to reproduce and leave greater number of progeny. That is nature selects only those traits which help an organism to survive in the changed conditions. Those that are not fit for survival perish in the long run leading to evolution of new species.

Question 11:

Enumerate three most characteristic criteria for designating a Mendelian population.

Answer:

The characteristic criteria for designating a Mendelian population are as follows:

i. Population must be sufficiently large. The population to be selected as Mendelian’s population should be very large so that experiments can be done easily.

ii. Population should have potential for free flow of genetic materials is ensured through sexual reproduction. Here genetic material should be present among the individuals of the population to be considered as Mendelian’s population.

iii. There should be negligible or nil migration in the population.

Question 12:

“Migration may enhance or blur the effects of selection”. Comment.

Answer:

Migration is movement of individuals from one place to another.

When movement of alleles takes place, gene frequencies change in the original as well as in the new population. New genes/alleles are added to the new population and these are lost from the old population. This will lead to a gene flow if this migration happens multiple times.

Immigration brings in those alleles which confer the traits that are not selected by nature. And emigration may lead to removal of alleles that confer better adaptations.

Hence “Migration may enhance or blur the effects of selection”.

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