Q.1 What is meant by the term speciation? List four factors which could lead to speciation. [Delhi]
Answer. Speciation is the evolution of reproductive isolation among once interbreeding population.
Factors which can lead to speciation are:
- Genetic drift: Over generations, genetic drift may accumulate which leads to speciation.
- Natural selection: Natural selection may work differently in different location which may give rise to speciation.
- Severe DNA change.
- A variation may occur which does not allow sexual act between two groups.
Q.2 What are homologous organs? Can the wing of a butterfly and the wing of a bat be regarded as homologous? Why? [All India]
Answer. Homologous organs are those organs which have the same basic structural design and developmental origin but have different functions and appearance.
Example: The forelimb of a frog, a lizard, a bird and a man seem to be built from the same basic design of bones, but they perform different functions.
Q.3 Distinguish between acquired and inherited traits by giving one example of each. Why are traits acquired during the lifetime of an individual not inherited? [Delhi(C)]
Answer. Acquired trait is a particular characteristic that is developed during the lifetime of an individual. Such characteristics are not genetically controlled and cannot be passed on to the next generation. Example: Loss of weight due to starvation.
Inherited trait is the transmission of particular characteristics from parents to their off springs, generation to generation. Such traits are genetically determined characteristic that distinguishes a person.
Example: Colour of skin.
The acquired traits cannot change the DNA of the germ cells. Therefore, the acquired traits cannot be inherited over generations during the lifetime of an individual.
Q.4 Define ‘evolution’. Describe Darwin’s theory of evolution. [All India]
Answer. Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of years and new species are produced. Since, the evolution is of the living organisms, so it is called ‘Organic Evolution’.
Darwin’s theory of Evolution: Charles Robert Darwin gave the theory of evolution in his famous book, ‘The Origin of Species’. The theory of evolution proposed by Darwin is known as ‘The Theory of Natural Selection’. It is also called ‘Darwinism’.
According to Darwin’s theory of evolution:
- There is natural variation within any population and some individuals have more favourable variations than others.
- Population remains fairly constant even though all species produce a large number of off springs.
- This is due to ‘competition’ or struggle for existence between same and different species.
- The struggle for survival within population eliminates the unfit individuals and those with ‘favourable variations’ survive and pass on these variations to their progeny to continue. This is called natural selection.
- The favourable variations are accumulated over a long time period leading to the origin of a new species.
Q.5 Define variation in relation to a species. Why is variation beneficial to the species? [Delhi]
Answer. Variation refers to the differences in the characters or traits among the individuals of a species.
Variations are beneficial to the species because:
- They enable the organisms to adapt themselves in changing environment.
- Variations form the basis of heredity.
- They form the raw materials for evolution and development of new species.
Q.6. Explain the terms: (i) Speciation (ii) Natural selection [Delhi]
Answer. (i) Speciation is the evolution of reproductive isolation among once-interbreeding populations, i.e. the development of one or more species from an existing species.
(ii) Natural selection is the process, according to Darwin, which brings about the evolution of new species of animals and plants.
Q7. Explain with examples how the following are evidences in favour of evolution in organisms. (i) Homologous organs (ii) Analogous organs (iii) Fossils [Delhi]
Answer. (i) Forelimb of human and bird are homologous organs. They have same structural design and developmental origin but they have different functions and appearance. Homologous organs help us to understand that the organism has evolved from a common ancestor. The more common characteristics the two species have, the more closely they are related.
(ii) Analogous organs are those organs design and developmental origin but have similar appearance and perform similar functions.
Example: The wings of birds and bats look similar but have different design in their structure. They have a common function of flying but their origins are not common. So, birds and bats are not closely related.
(iii) Fossils and their study is useful in knowing about the species which are no longer alive. They provide evidence and missing links between two classes. They are helpful in forming a sequence of organisms in the pathway of evolution. Thus, fossils have an importance in deciding evolutionary relationship. Archaeopteryx is a fossil bird. It had feathers, fused bones and beak which are exclusively bird structures. It also had some features which are found in reptiles, e.g. teeth in jaw, claws on free fingers and a long tail. This fossil provides a clue that birds have evolved from reptiles.
Q.8 State the meaning of inherited traits and acquired traits. Which of the two is not passed on to the next generation? Explain with the help of an example.
Answer. Inherited traits are the characteristics transmitted from parents to their off spring. Acquired traits are characteristics which are developed during the lifetime of an individual. Acquired traits are not passed on to the next generation. For example, if we breed a group of mice, all their progeny will have tails. Now, if the tails of these mice are removed by surgery and allowed to breed, the next generation mice will also have tails. If these tails are also removed and allowed to breed, the progeny of mice will again have tails. Removal of tail by surgery is an acquired trait and do not change the genes of germ cells and hence, are not passed on to the next generation.
Q.9 What are fossils? What do they tell us about the process of evolution?
Answer. The remains of dead plants and animals which were buried under the rocks millions of years ago are called fossils.
Fossils tell us about the process of evolution. The fossils of different organisms have some features similar to one species while some features are similar to the other species. In this way, they show the link between two species. They tell us that one species evolves from the other.
Q.10 The human beings who look so different from each other in terms of colour, size and looks are said to belong to the same species. Why? Justify your answer. [All India(C)]
Answer. Human beings are said to belong to the same species because of the following reasons:
- DNA studies.
- Number of chromosome is same,
- All have a common ancestor.
- They interbreed among themselves to produce fertile young ones of their own kind.
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