Science [3 Mark Questions]

Some amount of variations are produced even during asexual reproduction but it is very small. The number of variations produced during sexual reproduction is, however, very large. It is due to these variations that no two human beings look alike. The number of successful variations is maximized by the process of sexual reproduction, so we can conclude that the variation is a necessity for organic evolution.
Example: Animal reproduced by sexual reproduction show large number of variations.

The evidence was given by Stanley L. Miller and Harold C. Urey in 1953. They assembled an apparatus to create an early earth atmosphere which was supposed to consist of gases like ammonia, methane and hydrogen sulphide, but no oxygen) over water. This was maintained by them at a temperature just below 100 C and electric sparks were passed through the mixture of gases to stimulate lightning. At the end of a week, they found that 15% of the carbon (from methane) had been converted to simple compounds of carbon including amino acids which make up protein molecules.

According to Mendel’s second law of inheritance: In the inheritance of more than one pair of traits in a cross simultaneously, the factors responsible for each pair of traits are distributed independently to the gametes.

The important factors which could lead to the rise (or formation) of a new species are the following:

1. Geographical isolation of a population caused by various types of barriers (such as mountain ranges, rivers and sea). The geographical isolation leads to reproductive isolation due to which there is no flow of genes between separated groups of population.
2. Genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.
3. Variations caused in individuals due to natural selection.

The gene which decides the appearance of an organism even in the presence of an alternative gene is known as dominant gene. The gene which can decide the appearance of an organism only in the presence of another identical gene is called a recessive gene. The dominant gene is represented by a capital letter and the corresponding recessive gene is represented by the corresponding small letter. For example, in pea plants, the dominant gene for tallness is T and the recessive gene for dwarfness is t.

A trait of an organism which is ‘not inherited’ but develops in response to the environment is called an acquired trait.
Example: If a beetle does not get sufficient food for a considerable time, its weight will be reduced due to starvation. The low weight of the beetle is an example of acquired trait. A trait of an organism which is caused by a change in its genes (DNA) is called an inherited trait.
Example: The change of colour from red beetle to green beetle is an example of inherited trait.

The remains of dead animals or plants that lived in the remote past are known as fossils. The fossils provide evidence of evolution.
For example: a fossil bird called Archaeopteryx looks like a bird but it has many features which are found in reptiles. This is because Archaeopteryx has feathered wings like those of birds but teeth and tail like those of reptiles. Therefore, Archaeopteryx is a connecting link between the reptiles and birds and suggests that birds have evolved from reptiles. Thus, fossils provide evidence that the present plants and animals have originated from the previously existing ones through the process of continuous evolution.

In evolutionary terms, we can say that bacteria has a ‘better’ body design than spiders, fish, and, chimpanzees. This is because though bacteria is one of the simplest and primitive life forms but it still inhabits and survives in some of the most inhospitable (most unfavourable) habitats such as hot springs, deep-sea thermal vents and ice in Antarctica. Most other organisms (including spider, fish and chimpanzees) cannot survive in such harsh environments.

X are trilobites, Y are ammonites and Z are dinosaurs.

There is a pair of genes for each characteristic of an organism, one is dominant gene and the other is recessive gene. Each parent passes only one of the two genes of the pair for each characteristic to its progeny through gametes. Thus, the male gamete and the female gamete carry one gene for each characteristic from the gene pairs of the parents. When a male gamete fuses with a female gamete during fertilisation they make a new cell called zygote which grows and develops to form a new organism having characteristics from both the parents which it has inherited through genes.