CBSE NCERT Class X (10th) | Science
Heredity And Evolution
Heredity:- The passing of traits from the parents to offspring is called heredity. This is heredity which is responsible for many commonly observable facts; like siblings looking similar in overall appearance.
Genotype:- The complete set of genes in an organism's genome is called genotype.
Phenotype:- The observable characters in an organism make the phenotype. Phenotype is a result of genotype's interaction with the environment. Due to this reason, many phenotypes are not inheritable.
Acquired Traits:- Traits; which are acquired due to interaction with the environment; are called acquired traits. Acquired traits are not inheritable. For example; if a boxer develops bulging biceps, it does not mean that his son would be borne with bulging biceps.
Inheritable Traits:- Traits; which can be expressed in subsequent generations; are called inheritable traits. Such traits bring a change in the genotype of the organism and hence become inheritable.
Accumulation of Variations During Reproduction:-
Asexual reproduction involves a single parent and is hence not ideal ideal for facilitating variations. Some minor variations do occur due to inaccuracies in DNA replication. But the quantum of variations would be too little and would take too many years to show effect.
Sexual reproduction, on the other hand, is ideal for facilitating variations because two parents are involved in it. The offspring's genotype is contributed by two parents and hence chances of variations are very high.
Rules of Inheritance
Gregor Johann Mendel conducted experiments on pea plants and proposed the rules of inheritance; based on his observations. Mendel observed that characters are often present in pairs. A pair of contrasting characters is called allele.
Possible Reasons of Pea Plants Used by Mendel:
- Pea can be termed as biennial plant, i.e. two generations of a pea plant can grow in a given year. This means that Mendel could get enough time to observe a larger number of generations.
- Many easily identifiable and contrasting characters are present in pea plants.
- Cross pollination can be easily induced in pea plants.
Monohybrid Cross: The cross in which just two contrasting characters are studied is called monohybrid cross. Mendel did monohybrid cross for his first experiment. He selected a pair of contrasting characters for that experiment.
Let us take the example of cross between tall plants and short plants. The figure; given here shows the results of this experiment.
TT represents the genotype of tall plant and tt represents the genotype of short plants. In the F2 generation, all plants were tall but their genotype was Tt; which means they were not pure tall plants. This could be established by the appearance of the character of shortness in the F2 generation; in which most of the plants were tall and some of the plants were short. This experiment showed that the character of shortness of recessive in F1 generation and hence could not be observed. The ratio of number of tall plants to that of short plants in F2 generation was 3 : 1.
Mendel's First Law: The Law of Segregation: Every individual possesses a pair of alleles for a particular trait. During gamete formation, a gamete receives only one trait from the alleles. A particular trait can be dominant or recessive in a particular generation.
Dihybrid Cross: The cross in which two pairs of characters are studied is called dihybrid cross. In his second experiment, Mendel used dihybrid cross.
Let us take example between plants with round and green seeds and those with wrinkled and yellow seeds. The genotype of round and green seeds is shown by RRyy and that of wrinkled and yellow seeds is shown by rrYY. In the F1 generation, all plants produced round and yellow seeds; which means that wrinkled texture was the recessive character and so was the green colour of seeds. When plants of F1 generation were allowed to self pollinate; it was observed that most of the plants in F2 generation produced round and yellow seeds. Some plants produces round green seeds, some produced wrinkled yellow seeds and some produced wrinkled green seeds. The ratio was 9 : 3 : 3 : 1; as shown in the figure.
Mendel's Second Law: Law of Independent Assortment: Alleles of different characters separate independent from each other during gamete formation.
In the above example; alleles of texture were assorted independently from those of seed colour.
Sex Determination in Humans:
Somatic cells in human beings contain 23 pairs of chromosomes. Out of them the 23rd pair is composed of different types of chromosomes which are named as X and Y chromosomes. The 23rd pair contains one X and one Y chromosome in a male. On the other hand, the 23rd pair in a female contains X chromosomes. This means that all the eggs would have X chromosome as the 23rd chromosome, while a sperm may have either X or Y chromosome as the 23rd chromosome. When a sperm with X chromosome fertilizes the egg, the resulting zygote would develop into a female child. When a sperm with Y chromosome fertilizes the egg, the resulting zygote would develop into a male child.
The change in inherited traits in biological population over subsequent generations is called evolution. Scientists have proven that life evolved in the form of simple unicellular organisms on this earth; and all the organisms which are present today have evolved from a common ancestor. The idea of evolution is based on the premise of a common ancestry.
To understand how evolution takes place, let us take some imaginary examples.
Situation 1:- A group of red ants is living in a bush. As hunting birds can easily spot red ants in the green background, they enjoy feasting on ants. Because of some error in DNA replication, some blue ants come into origin. Hunting birds cannot spot blue ants against a green background. As a result, blue ants survive and red ants become extinct over a period of time. The origin of blue ants happened by chance but it gave survival benefit to the ants. Finally, blue ants could survive and proliferated in the surrounding.
Situation 2:- In the same group; some blue ants came into origin. Ants of both colours were almost equal in population. One day, an elephant cam and trampled the bushes. All the red ants perished in the accident; leaving only the blue ants. This resulted in extinction of red ants but blue ants could continue their race. The survival of blue ants was because of an accident and the accident was the cause of natural selection.
Situation 3:- A group of red ants was living in a bush. Due to draught like conditions, availability of food became a problem for the ants. All the ants became weak and underweight. Subsequent generations comprised of smaller ants and the trend continued for a few generations. Situations changed and plenty of food became available. Ants once again developed to their normal size. In this case, the change in size was a change in phenotype and hence was not inheritable. The change in size could not produce variation and evolution in the species.
Darwin's Theory of Evolution:
Charles Darwin wrote his famous book 'Origin of Species'. He threw new insights on evolution of species. Some salient points of Darwin's theory are as follows:
Organisms have unlimited capacity to reproduce:- Organisms can reproduce offspring at a fast pace. This is necessary for survival, because a higher number of offspring ensures that at least some of them could survive. Each organism has to struggle for its day to day survival. For example; a frog lays thousands of eggs at one go. The spawn is released in water and it is left to fend for itself. Most of the eggs are either washed away or are eaten by predators. However, some portion of eggs from the spawn develops into tadpoles. Once again, many tadpoles are eaten up by predators; leaving a few which develop into adults. It is evident, that a large number of eggs is needed to ensure that at least some of them develop into adults.
Natural Selection:- Different individuals of a particular species have different traits. Those with more suitable traits are selected by the nature. Each organism needs a particular trait for finding food and finding a mate. Those with better traits are finally able to pass on their traits to the next generation.
Survival of the Fittest: Those organisms which are the fittest are able to survive, while others perish. That is how many species become extinct and some species continue to evolve over a period of time.
Molecular Origin of Life: Stanley L. Miller and Harold C. Urey, conducted the Miller-Urey experiment in 1953 to demonstrate how the life would have originated on the earth. They created an environment in laboratory which mimicked the environment of earth as it was during the time of origin of life. Water, methane, ammonia and hydrogen were used in that experiment. The liquid was heated to initiate evaporation and electrodes were used to create electric discharge. At the end of two weeks, some organic molecules were formed in the setup. Some amino acids and sugar were also formed. This proved the hypothesis of J. B. S. Haldane that life originated from inorganic raw materials.
Speciation: The process of origin of a new species is called speciation. A species is a group of organisms in which most of the characters are similar and members of a species are able to breed among themselves. Speciation can happen if two groups of the same species are somehow prevented from interbreeding for several generations. This can happen because of geographical segregation or because of some genetic changes. Evolution of new species, because of geographical segregation is called genetic drift.
Evolution and Classification
The modern system of classification is based on evolutionary relationship. Due to this, this is also known as phylogenetic classification. The kingdom is the highest taxa, while the species is the lowest taxa. Members of a species have a higher number of common characters, than members of a kingdom. For example; all human beings belong to the species Homo sapiens. Human beings can interbreed; irrespective of their race or skin colour. All human beings come under the class mammalia; as do the monkeys, elephants and cows. Apparently, each species of the class mammalian is quite different yet they have certain common characters; like hairs on the body and mammary glands in females. Similarly, all animals are eukaryotes and cell wall is absent in their cells. The degree of similarity or dissimilarity shows that all animals have evolved from a common ancestor.
Homologous Organs: Organs which have common design but serve different functions in different animals are called homologous organs. For example; the forelimbs of all tetrapods are composed of humerus, radio-ulna, tarsals and metatarsals. Yet, the forelimbs of frogs are adapted to a jumping movement, those of birds are used for flying and those of humans are used for handling tools. This shows that frogs, birds and humans have evolved from a common ancestor.
Analogous Organs: Organs which have different design but serve a common function in different animals are called analogous organs. Wings of birds and wings of bat are good examples of a pair of analogous organs. Wings of birds are composed of all the bones of forelimb and are covered with feathers. Wings of bats are mainly composed of the digital bones and a thin membrane covering the structure. Yet wings in both the organisms are used for flying.
Fossils: The preserved remains of animals or plants or other organisms from the distant past are called fossils. The term distant is a key term in this definition. Scientists usually take 10,000 years as the minimum age for the remains to be categorized as fossil. Many fossils have been discovered till date. These fossils tell us about many extinct animals and also give insights into how the evolution could have taken place.
Evolution by Stages:
Evolution of complex organs and thus of complex organisms has happened in stages. Let us take the example of evolution of eyes. Planaria is the first animal which shows 'eye' like structure. The dark spots on planaria are light sensitive spots but a planaria cannot distinguish between two different objects. Eyes of insects are compound eyes which are made up of thousands of optical surfaces. Eyes of higher animals are simple eyes which are composed of a single lens. Most of the animals cannot differentiate among colours. Depth perception is also weak in many animals. Human eyes are the most advanced; because humans can recognize colours and have very good depth perception.
Most of the tetrapods have to use all the four limbs for locomotion. Some apes can walk up to smaller distances by using just the hind limbs. Humans have finally evolved the bipedal walking.
Evolution Vs Progress:
Evolution does not mean progress in every case. This can be proved by example of bacteria. Bacteria are the simplest and one of the oldest organisms on the earth. Their simple body design does not make them weak from any angle. Bacteria are known to survive some of the harshest climates; like craters of volcanoes and sulfur springs. Many animals have certain features which hamper even their routine activities. For example; the branch-like horns of antelope are a handicap for them. When an antelope runs for its life; there are times when its horns get entangled in branches or bushes. This results in the death of the antelope. Colourful feathers of a male peacock are very good when it comes to attract a female. But because of its conspicuous feathers, it can be easily spotted by a predator. Because of its bulky feather it cannot fly away to safety.
The modern humans are called Homo sapiens. Many scientific investigations have shown that the modern humans evolved in Africa. They migrated towards north; in due course of time and settled near what is known as the Mediterranean Sea. When the ice age ended, melting of ice resulted in the in water level. The humans migrated in different directions from that area. One branch went to the western Asia, then to the Indian Peninsula and finally to Australia. From the Indian Peninsula, branch migrated towards China and subsequently to the North America. From North America, the humans migrated to the South America. From the Mediterranean Sea, the second branch migrated towards Europe; where they are believed to replace the Neanderthals.
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