Heredity and Evolution : Chapter Notes

Notes for heredity and evolution chapter of class 10 science. Dronstudy provides free comprehensive chapterwise class 10 Science notes with proper images & diagram.


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Heredity- Heredity is the phenomena of inheritance of traits/features from the parents to their offspring’s/progeny.

Trait is any characteristic that is transferred from parent to offspring. E.g. height and colour.

Evolution is the process of development of new type of living organism from an old one by gradual changes.

Variation during reproduction:

Variation is the minor differences which exist between individuals belonging to the same species.

We have already learnt that there are two types of reproduction- Asexual and sexual reproduction.

Variations arise during the process of reproduction. Offspring’s produced by asexual or sexual methods of reproduction resemble each other but they also differ to some extent which may be easily identifiable or at times difficult to observe or identify. But during asexual reproduction, there may be very little variations and many variations in case of sexual reproduction. In other words, the variation in sexually reproducing organisms like plants, animals and some microbes can be easily observed unlike asexually reproducing organisms. Therefore, sexual mode of reproduction is considered to be better than asexual mode of reproduction.


There are variations in every generation, in the figure given above, the original organism at the top will give rise to, say, two individuals, similar in body design, but with subtle differences. Each of them, in turn, will give rise to two individuals in the next generation. Each of the four individuals in the bottom row will be different from each other. While some of these differences will be unique, others will be inherited from their respective parents, who were different from each other.

Variation during reproduction is important because it is necessary for survival. For ex. If the temperature of earth increases suddenly, then most of the bacteria living on earth would vanish. Only few bacteria variants which can tolerate heat have better chances of survival in heat wave in comparison to non-variant bacteria having no capacity to tolerate heat wave. If these variants were not there, then the entire species of bacteria would have been destroyed. Therefore, variation becomes important.

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How traits are transferred from parents to offspring’s?

chromosome, showing gene as section of DNA

The nucleus of a cell contains chromosomes that carry genetic information. Chromosomes are made from long, coiled molecules of DNA. DNA contains genes and they are units of heredity and are responsible for inheritance. Genes control the expression of characteristics. Different genes are responsible for different traits. In other words, a gene carries the genetic code for a particular characteristic. For example, height of an organism, complexion, and shape of nose are all controlled by different genes.

Characters are transferred through genes present in the DNA molecules in the chromosomes present in the nucleus of the cell.

   The inheritance of characters is due to the fact that both the father and mother contributes equal amount of genetic material to the child. So for each trait there are two factors one from the father and one from the mother.


Most genes have two or more variations, called alleles. For example, the gene for height has two alleles – tall or short. An individual may inherit two identical or two different alleles from their parents. When two different alleles are present they interact in specific ways.

Dominant trait- The gene which decides the trait in the presence of other different gene. In other words, the trait that is expressed is called dominant and is denoted using the capital letter.

Recessive trait- The gene which decides the trait in the presence of other identical gene. In other words, the trait that is not expressed is called recessive trait and is denoted using small letter. Tall (TT) and short (Tt).

The traits due to dominant alleles are always expressed even when a recessive allele is present. Traits due to recessive alleles are only observed when two recessive alleles are present. For example, the allele for tallness is dominant and the allele for shortness is recessive.

If an individual inherits:

• Two tall alleles (both dominant-TT), the person will be tall

• One tall allele (dominant- T) and one short allele (recessive-t), the person will be tall (Tt)

• Two short alleles (recessive), the person will be short (tt).

  • GENOTYPE = the genes present in the DNA of an organism.  We will use a pair of letters to represent genotypes for one particular trait. Genotypes are inherited. For ex: Tt tall or tt for short etc.) There are always two letters in the genotype because (as a result of sexual reproduction) one code for the trait comes from male and the other comes from female, so every offspring gets two codes (two letters).

Therefore, there will be three possible genotypes. For example- for the trait height of an organism, the genotypes will be TT, Tt or tt.

  • Phenotype- Property shown in the organism. The observable character in an organism. Examples of phenotypes in humans- Blonde hair, grey eyes, attached earlobes etc.

Let us consider the example of hair colour

Situation 1

Colour of hair (H For black; h –blonde)

Parents- Blonde (Mother) Black (father)

Genotype- hh HH

Case 1 Zygote (offspring will have black hair as H is dominant)

Situation 2

Colour of hair(H For black; h –blonde)

Parents- Black (Mother) Blonde (father)

Genotype- Hh hh

Case 1 Zygote (offspring will have black hair as H is dominant)

Parents- Black (Mother) Blonde (father)

Genotype- Hh hh

Case 2 Zygote (offspring will have blonde hair as it is the only gene present)

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Situation 3

Colour of hair (H For black; h –blonde)

Four options are possible-

Situation 1

Parents- Black (Mother) Black (father)

Genotype- Hh Hh

Case 1 Zygote (offspring will have blonde hair as it is the only gene present)

Parents- Black (Mother) Black (father)

Genotype- Hh hh

Case 2 Zygote (offspring will have black hair as H is dominant)

Parents- Black (Mother) Black (father)

Genotype- Hh hh

Case 3 Zygote (offspring will have black hair as H is dominant)

Parents- Black (Mother) Black (father)

Genotype- Hh hh

Case 4 Zygote (offspring will have black hair as H is dominant)

Genetics is the branch of biology that deals with heredity and variations.

Rule of inheritance:

Mendelian laws of inheritance are statements about the way certain characteristics are transmitted from one generation to another in an organism. Gregor Johann Mendel conducted experiments with garden pea plants and determined the rules for the inheritance of traits. Among the traits that Mendel studied were the color of a plant's flowers, their location on the plant, the shape and color of pea pods, the shape and color of seeds, and the length of plant stems. His experiments consisted of crossing pea plants of distinct characteristics (size, color of the seeds, etc.), cataloguing the results and interpretingthem.

Mendel is considered the father of Genetics. He was a monk, biologist and botanist born in Austria in 1822 and who died in 1884.

Mendel selected the pea plant for his experiments because-

  • 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.
  • Self-fertilizing in nature and also cross bred experimentally.
  • Produce large number of offspring.
  • Pea plants grow very fast

Mendel's traits included:

a. Seed shape ---  Round (R) or Wrinkled (r)

b. Seed Color ---- Yellow (Y) or  Green (y)

c. Pod Shape --- Smooth (S) or wrinkled (s)

d. Pod Color ---  Green (G) or Yellow (g)

e. Seed Coat Color --- Grey (G) or White (g)

f. Flower position --- Axial (A) or Terminal (a)

g. Plant Height --- Tall (T) or Short (t)

h. Flower color --- Purple (P) or white (p)

Inheritance involves the passing of discrete units of inheritance, or genes, from parents to offspring. Initially, Mendel took pea plants with different characteristics – a tall plant and a short plant, produced progeny from them, and calculated the percentages of tall or short progeny.

He first ensured that he had pure-bred tall and pure-bred short plants by selecting seeds from plants that had been self-pollinating for many generations. He selected one tall and one short plant from the pure-bred. He called this the P or parental generation.

When pure-bred parent plants were cross-bred, the offspring of this cross were all hybrids showing only the dominant trait and were called the first filial or F1 generation. The dominant traits were always seen in the progeny, whereas recessive traits were hidden until the first-filial generation (F1) hybrid plants were left to self-pollinate.

The next generation was called the second filial or the F2 generation. Mendel then crossed a pure and a hybrid from his Fgeneration. When 2 hybrids were crossed, 75% (3/4) of the offspring showed the dominant trait & 25% (1/4) showed the recessive trait; always a 3:1 ratio.

This 3:1 ratio occurs in later generations as well.   Mendel realized that this underlying regularity was the key to understanding the basic mechanisms of inheritance.

This indicates that both the tallness and shortness traits were inherited in the F1 plants, but only the tallness trait was expressed. Thus, two copies of the trait are inherited in each sexually reproducing organism. These two may be identical, or may be different, depending on the parentage.

He concluded that traits were not blended but remained distinct in subsequent generations, which was contrary to scientific opinion at the time. Traits carried by genes do not mix.

The plants that are produced in the F1 generation are called hybrids as they have a mixture of traits of both the parents. Since, in this case only one trait, i.e., height was considered; this cross is called the monohybrid cross.

F1 cross Tt x Tt

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F2 generation TT (tall) Tt(Tall) Tt(Tall) tt(Short)

TT TT TT TT TT Tt Tt tt TT Tt Tt tt tt tt tt tt

Each of the F1 generation plants (shown above) inherited a ‘T’ allele from one parent and a‘t’ allele from the other.  When the f1 plants breed, each has an equal chance of passing on either T or t alleles to each offspring.

With all of the seven pea plant traits that Mendel examined, one form appeared dominant over the other, in other words, the dominant gene masked the presence of the other allele (recessive gene).  For example, when the genotype for height of the pea plant is Tt, the phenotype is tall.  However, the dominant tall allele does not alter the recessive short one in any way. Both alleles can be passed on to the next generation unchanged. Hybrids always show the dominant trait in their phenotype.

After considering the character pair singly, Mendel then began his experiments with two pairs of characters simultaneously and thus obtained the di-hybrid ratio.

Di-hybrid cross- The cross in which two pairs of characters are studied is called di-hybrid cross. In his second experiment, Mendel used di-hybrid cross. In the di-hybrid crosses, it was found that the two traits were inherited independent of each other. The dominant alleles for each of the two traits asserted their dominance independent of the other. For example, round seeds (R) are dominant to wrinkled seeds (r), and yellow seeds (Y) are dominant to green seeds (y). Following the convention for notation, these would be RRYY for round and yellow and rryy for wrinkled and green.

Colour and shape of pea plants

Colour Y= allele for yellow seeds y= allele for green seeds
Shape R=allele for round seeds r=allele for wrinkled seeds

The plants with yellow and round seeds (pure) were crossed with those having green and wrinkled seeds (pure). In the P1 cross, RRYY x rryy, all of the F1 offspring showed only the dominant form for both traits; all hybrids, RrYy. The F1seeds were yellow and round.

When these F1 seeds were grown into plants, F2 seeds were obtained which showed all the four possible combinations i.e. (i) yellow and round seeds, (YyRr) (ii) yellow and wrinkled seeds (Yyrr), (iii) green and round seeds (yyRr), and (iv) green and wrinkled seeds (yyrr). Of these, 315 seeds were round and yellow, 108 were round and green, 101 were wrinkled and yellow, and 32 were wrinkled and green. Hence, their phenotype ratio was about 9:3:3:1ratio.

Parents: RRYY x rryy

( Round seeds, yellow cotyledons) (Wrinkled seeds and green cotyledons)


F1 generation:

All round seeds and yellow cotyledons

F1 generation allowed to self-pollinate:


RY Ry rY ry RY Ry rY ry

RRYY RRYy RrYY RrYy RRYy Rryy RrYy Rryy RrYY RrYy rrYY rrYy RrYy Rryy rrYy rryy

F2 generation: RRYY, RRYy, RrYY, RrYy : RRYY,Rryy : rryy, rrYy : rryy

F2 generation: Round, yellow : Round, green: Wrinkled yellow: Wrinkled green

F2 generation: 9:3:3:1

From this dihybrid cross, we know that there are four different phenotypes but these are produced by nine different genotypes.

The conclusion of dihybrid cross is that “two or more pairs of alleles segregate independently of each other as a result of meiosis”. This is known as Mendel’s second law of independent assortment.

Similarly, a cross between tall, round seeded plant (TTRR) and a dwarf wrinkled seeded plant(ttrr) was carried out by Mendel.

Parents: TTRR x rryy

( Tall, Round seeded plant) (dwarf Wrinkled seeded plant)


F1 generation:

All are tall , rounded seeds

F1 generation allowed to self-pollinate:


TR Tr tR tr TR Tr tR tr

TTRR TTRr TtRR TtRr TTRr TTrr TtRr Ttrr TtRR TtRr ttRR ttRr TtRr TtRr Ttrr ttRr ttrr

F2 generation: TTRR,TTRr,TtRR,TtRr : TTrr,Ttrr: ttRR, ttRr : ttrr

F2 generation: tall, round: Tall, wrinkled: dwarf rounded: dwarf, wrinkled

F2 generation: 9:3:3:1

The gametes have an equal chance of having RY or Ry. RY and ry do not have to sort together into gametes. Again, probability determines the genotype and phenotype for the F2 generation.

Mendel performed experiments on a number of different characters and always found similar results and ratios.

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Results of Mendel's Experiments:

  • Inheritable factors or genes are responsible for all heritable characteristics
  • Phenotype is based on Genotype
  • Each trait is based on two genes, one from the mother and the other from the father
  • True-breeding individuals are homozygous ( both alleles) are the same
  • When different alleles for a characteristic are inherited, the trait of only one (the dominant one) will be expressed. The recessive trait's phenotype only appears in true-breeding (homozygous) individuals
Trait: Pod Color
Genotypes: Phenotype:
GG Green Pod

Gg Green Pod

gg Yellow Pod

  • Each genetic trait is produced by a pair of alleles which separate during reproduction
R r
  • Each factor (gene) is distributed (assorted) randomly and independently of one another in the formation of gametes
RY Ry rY Ry

Mechanism of heredity:

How do traits get expressed?

(i)Every eukaryotic cell contains nucleus which have chromosomes.

(ii) Chromosomes present in nucleus contain hereditary information. Chromosomes occur in pair, one comes from mother and others from father.

(iii)Chromosomes are made up of DNA and protein. Its important component is DNA.

(iv)The DNA molecule carries a code that instructs the cell about which kind of protein it will make.

(v)Each chromosome carries instruction for making many different proteins.

(vi) Part of DNA responsible for a trait is called gene.

(vii) There are many genes present on one chromosome, which control the characteristic of an organism.

(vii) A gene is expressed in the form of proteins. As the cells duplicate, they pass this genetic information to the new cells.

(viii) But genes do not perform the actual work. Rather, they serve as instruction books for making functional molecules such as ribonucleic acid (RNA) and proteins, which perform the chemical reactions in our bodies. It is these proteins which carry out the actual function of performing different functions and controlling various characteristics. So genes produce the proteins which in turn controls the characteristics, or traits.

E.g. for tallness of a plant if proteins work efficiently a lot of hormone will be produced and the plant will be tall.

Let us take the example of tallness as a characteristic. We know that plants have hormones that can trigger growth. Plant height can thus depend on the amount of a particular plant hormone. The amount of the plant hormone made will depend on the efficiency of the process for making it. Consider now an enzyme that is important for this process. If this enzyme works efficiently, a lot of hormone will be made, and the plant will be tall. If the gene for that enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less, and the plant will be short. Thus, genes control characteristics, or traits.

Sex Determination

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How is the sex of a newborn individual determined?

Different species use very different strategies for this. Some rely entirely on environmental cues. Thus, in some animals, the temperature at which fertilised eggs are kept determines whether the animals developing in the eggs will be male or female. In other animals, such as snails, individuals can change sex, indicating that sex is not genetically determined.

Sex determination in human beings:

In human beings, the sex of the individual is largely genetically determined.

All human cells contain 23 pairs of chromosomes. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair, called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes, both called X. But men have a mismatched pair in which one is a normal-sized X while the other is a short one called Y. So women are XX, while men are XY.

In females, all egg cells receive X chromosome during meiosis. But in males, half of the sperms receive X chromosome and half of the sperms receive Y chromosome.

During the time of fertilization, the egg cell may be fertilized by a sperm carrying an X chromosome, thereby producing a zygote that has 2 X chromosomes which will in turn develop into a female child. On the other hand, if the egg cell is fertilized by a sperm that is carrying a Y chromosome, then a zygote containing one X and one Y chromosome is produced which in turn develops into a male child.

Therefore, the sex of the offspring’s in humans and in mammals is determined by which chromosome the sperm is carrying. We can therefore say that an equal number of male and female offspring’s will be produced.

Parental phenotype Female Male

Parental genotype XX XY

Gametes X X X Y

Off spring genotype XX XY

Off spring phenotype Female Male


An illustration of evolution:

To understand how evolution takes place, let us consider some imaginary examples-

i) Consider a group of twelve red beetles. They live in some bushes with green leaves. Their population will grow by sexual reproduction, and therefore, can generate variations.

ii) Let us imagine also that crows eat these beetles. The more beetles the crows eat, the fewer beetles are available to reproduce.

Let us consider three different situations that can develop in this population of beetles.

a) Appearance of favourable variations

b) Appearance of unfavourable variations

c) Appearance of a plant disease.

  • In the first situation, a colour variation arises during reproduction, so that there is one beetle that is green in colour instead of red. The number of green beetles increases after reproduction. Crows cannot see green-coloured beetles on the green leaves of the bushes, and therefore cannot eat them. The progeny of green beetles is not eaten, while the progeny of red beetles continues to be eaten. As a result, the population of green beetles increases than the red ones in the beetle population. The origin of green beetles happened by chance but it gave survival benefits to the beetles. Finally, the green beetles could survive and multiplied in the surroundings.
  • In a second situation, again, a colour variation arises during reproduction, but now it results in a beetle that is blue in colour instead of red. The number of blue beetles increases after reproduction. Crows can see blue-coloured beetles in the green leaves of the bushes as well as they can see red ones, and therefore can eat them. Initially, the number of red beetles is more than the number of blue beetles. But at this point, an elephant comes by, and stamps on the bushes where the beetles live. This kills most of the beetles. By chance, the few beetles that have survived are mostly blue. The beetle population slowly expands again, but now, the beetles in the population are mostly blue. The survival of the blue beetles was because of an accident and accident was the cause of natural selection.
  • Now consider a third situation. In this, as the beetle population begins to expand, the bushes start suffering from a plant disease. The amount of leaf material for the beetles is reduced. Therefore, the beetles are poorly nourished. The average weight of adult beetles decreases due to non-availability of leaves, but there is no genetic change occurring. After a few years and a few beetle generations of such scarcity, the plant disease is eliminated. There is a lot of leaf food and weight of the beetles also increases. Here, the change in size was a change in the phenotype and hence was not inheritable. Change in size could not produce any variation and evolution in the species.

Inherited traits :- Inherited traits are traits in an organism due to changes in the genetic composition and it can be passed from one generation to the next and it results in evolution. Inherited traits include things such as hair color, eye color, muscle structure, bone structure, and even features like the shape of a nose.

Acquired traits :- Acquired traits are traits which are acquired by an organism during its lifetime and it cannot be passed from one generation to the next and it does not result in evolution. Acquired traits include things such as calluses on fingers, larger muscle size from exercise or from avoiding predators. Behaviors that help an organism survive would also be considered acquired characteristics most of the time. Things like where to hide, what animals to hide from and other behavior like that. For plants acquired characteristics might include bending because of wind or growths resulting from insect bites.

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Species- A collection of organisms which can reproduce among themselves. E.g., the species homo sapiens

Speciation- The process by which new species develop from the existing species is known as speciation. It is an event that splits a population of the same species into two independent species which cannot reproduce among them.

Causes of speciation-

Genetic drift: It occurs due to changes in the frequencies of particular genes by chance alone. e.g. If a hurricane strikes the mainland, and bananas with beetle eggs on them are washed away to an island. This is called a genetic drift. Genetic drift is the evolution of new species because of geographical segregation.

• Natural selection: These are the variations caused in individuals due to natural selection which lead to the formation of a new species. e.g. If the ecological conditions are slightly different on the island as compared to the mainland, it leads to a change in the morphology and food preferences in the organisms over the course of generations.

Splitting of population: A population splits into different sub-populations due to geographical isolation that leads to the formation of a new species.

Geographical isolation- Speciation starts because populations are prevented from inter-breeding by geographical isolation. In this a group of individuals of a species may become geographically isolated from other members or species. So, both these groups grow in different environment and hence evolve to form a new species. Geographic isolation is a common way for the process of speciation to begin.-

-Continent drift

-Mountain rise

-Rivers change course

-Organisms migrate

Few factors that affect speciation are

  • Hybridization- Hybridization is a process by which new types of species come into existence as a result of mating of two distinct species. This process may lead to the existence of new species which is different from the parents. Hence, hybridization may result in speciation.

Tracing evolutionary relations

The modern system of classification is based on evolutionary relationships. The degree of similarity and dissimilarity shows that all animals have evolved from a common ancestor.

  • Homologous organs- Organs having same design but dissimilar in shape, size and function. They suggest the presence of a common ancestor. Example: Fore limbs of frog, reptile, birds and mammals.

Organs such as bat's wing, wings of birds, forelimb of a horse, and human arm have a common underlying anatomy that was present in their last common ancestors; therefore their forelimbs are homologous organs. The shape and the size of the bones are not similar, there is a similarity in their structure that is, and they have the same set of bones. Man uses his hands to grasp and perform tasks, bats and birds use their wings for flight, horses use their forelimbs for running and whales use their flippers for swimming.

Animals like lizard, rabbit, frog and birds have forelimbs of different shapes according to their lifestyle. But they all have the same set of bones namely humerus, una and radius. Such homologies reveal the common ancestry of all these animals. Such differences are due to divergent evolution or adaptation for varied conditions.

  • Analogous Organs: Organs similar in shape and function but their origin, basic plan and development are dissimilar. They suggest different ancestors. Example. Wings of butterfly, bird and bat.

This happens because sometimes animals belonging to different groups happen to live in the same habitat. This forces them to lead a similar type of life. This leads to the development of superficially similar structure.

Fossils- Fossils are the preserved remains or impressions of living organisms (plants and animals) that lived in the past.

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Usually, when organisms die, their bodies will decompose and be lost. But, once in a while, the body or at least some parts may be in an environment that does not let it decompose completely. If a dead insect gets caught in hot mud, for example, it will not decompose quickly, and the mud will eventually harden and retain the impression of the body parts of the insect. All such preserved traces of living organisms are called fossils.

Finding age of fossils:

i) Radiocarbon dating- Age of the fossils can be found by detecting the ratios of different isotopes of the same element in the fossil material

ii) Depth of fossils- Age of the fossils can be found by estimation of the depth of the layer of rocks in which it is found. E.g., The fossils we find closer to the surface are more recent than the fossils we find in deeper layers

How fossils give information about evolution?

Some invertebrates on the sea-bed die, and are buried in the sand around 100 million years ago. More sand accumulates, and sandstone forms under pressure. Millions of years later, dinosaurs living in the area die, and their bodies, too, are buried in mud. This mud is also compressed into rock, above the rock containing the earlier invertebrate fossils.

Again millions of years later, the bodies of horse-like creatures dying in the area are fossilised in rocks above these earlier rocks.

Much later, by erosion or water flow wears away some of the rock and exposes the horse-like fossils. As we dig deeper, we will find older and older fossils.

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Evolution by stages:

Evolution of complex organisms has taken place through a series of DNA changes (mutations) created bit by bit over generations.

For example, evolution of eyes.

Eyes were not present in the initial stages of life on earth. It evolved slowly through a series of changes. Eyes would not have evolved through a single DNA change.

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.


A characteristic of a particular animal may, post-evolution be useful for performing a totally different function.

For example:,Feathers

Long feathers were considered to provide insulation in cold weather. Some reptiles like the dinosaur had feathers but very few were adapted for flying. In the present day, birds use feathers for flight, which is an example of adaptation.

Artificial selection

This is the usage of plants with desirable characteristics to produce new varieties.

Broccoli, cauliflower, cabbage, kohlrabi and kale are produced from its ancestor wild cabbage by artificial selection.

Humans cultivated wild cabbage for over 2000 years and produced different vegetables from it by artificial selection.

Eg :- Cabbage – by selecting short distance between the leaves.

Cauliflower – by selecting sterile flowers.

Kale – by selecting large leaves

Kholrabi – by selecting the swollen stem

Broccoli – by  arresting flower growth

Molecular phylogeny- Evolution occurs due to changes in DNA. Therefore by studying the differences in DNA of organisms, we can learn about evolution (which organism evolved from whom). This approach is based on the idea that organisms which are more distantly related will accumulate a greater number of differences in their DNA and vice versa. Such studies trace the evolutionary relationships and it has been highly gratifying to find that the relationships among different organisms shown by molecular phylogeny match the classification scheme.


New organisms formed from evolution are not always better. Moreover, old organisms were not always inefficient.

Example, 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.

Evolution basically leads to more complex organisms due to genetic variation and natural selection.

In evolution, old species need not disappear.

One species may evolve into there can be many branches of evolution.E,g,., chimpanzee and man

They would have had a common ancestor from which both would have evolved. We have not descended from chimpanzee

Evolution of human:

Humans have evolved like any other organisms. There is nothing special about us. Humans have evolved among themselves. Human beings evolved in Africa. Some of them stayed there but some migrated to different parts of the world. Then due to genetic variations and the environmental changes in different geographical regions they developed changes in their forms and features.

different parts of the world.

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.

Origin of life on earth:

In 1929, a British scientist J.B.S. Haldane (who became a citizen of India later) suggested that life must have developed from simple inorganic molecules which were present on earth soon after it came into existence. Haldane said that conditions on earth at that time which were very different from the conditions we see today, could have converted inorganic molecules into complex organic molecules that were necessary for life. The first primitive organisms would arise from further chemical synthesis. He also suggested from theoretical considerations that living beings were formed in the oceans.

Long time ago, life on earth were very different from the conditions we see today in the sense-

-Earth did not have oxygen in the atmosphere. Atmosphere was supposed to contain gases such as ammonia, methane, hydrogen sulphide etc.

- There used to be frequent lightning.

-Mostly, inorganic compounds were present on earth. There were no organic compounds.

-Lightning triggered chemical reaction between inorganic compounds. This formed new organic compounds like amino acids.

The theory of life on earth was proposed by Haldane and approved by the experiments conducted by 1953, Stanley. L. Miller and Harold C. Urey in 1953. They assembled an atmosphere similar to that thought to exist on early earth (this had molecules like ammonia, methane and hydrogen sulphide, but no oxygen) over water. This was maintained at a temperature just below 100°C and sparks were passed through the mixture of gases to simulate lightning. At the end of a week, 15% of the carbon (from methane) had been converted to simple compounds of carbon including amino acids which make up protein molecules. These protein molecules are found in living organisms. Thus, these experiments provide the evidence that life originated from non-living matter like inorganic molecules.

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