Heredity and Evolution by ACHARAYAZ Notes - Class 10 PDF Download

 Page 1


 
 
HEREDITY AND EVOLUTION
Heredity 
The transmission of characteristics (resemblances as well as 
variations) from parents to offsprings i.e., from one generation 
to the next is called heredity. 
 
Variation 
The differences shown by the individuals of a species, and also 
by the offsprings of the same parents are called variations. 
 
Genetics 
The study of heredity and variation is called genetics. 
 
Evolution 
The gradual changes taking place in the organisms which in 
turn causing the diversity of the living organisms over the long 
period of time is called evolution. 
 
Accumulation Of Variation During Reproduction 
In case of asexual reproduction in organisms, there are very 
minor differences that occur between the newly formed 
organisms due to the small inaccuracies in DNA copying. 
However if sexual reproduction is involved, greater diversity 
will be generated due to the inheritance of traits from the two 
parents. 
 
Do All The Variations In A Species Have Equal Chances 
Of Surviving In The Environment In Which They Find 
Themselves? 
Obviously not. Depending upon the nature of variations, 
different individuals would have different kinds of advantages 
and disadvantages. Selection of variants by environmental 
factors forms the basis of evolutionary processes. 
 
Heredity 
 
Rules Of Heredity 
The rules of heredity determine the process by which traits 
and characteristics are reliably inherited.  The rules are related 
to the fact that in human beings both the father and the 
mother contribute practically equal amounts of genetic 
material to the child. This means that each trait can be 
influenced by both the maternal and the paternal DNA. Thus, 
for each trait there will be two versions in each child. So for 
this kind of situation Mendel scientist worked out the main 
rules for such inheritance. 
 
Gregor Johann Mendel worked out the basic rules of such 
inheritance of traits more than a century ago. He studied the 
inheritance of contrasting characters (traits) such as 
tallness/dwarfness of plants, round/wrinkled form of seeds, on 
garden pea plant also called Pisum sativum. 
 
Mendel’s Experimental Plant 
Mendel selected garden pea plant (Pisum sativum) for series 
of hybridization experiments because of following special 
features:- 
(i) It had a short life cycle and, therefore, it was possible to 
study number of generations. 
(ii) Garden pea plant had distinct, easily detectable 
contrasting variants of features. For instance, some plants 
were tall and some dwarf, some had violet flowers and 
some had while flowers, some plants had round seeds 
and some had wrinkled seeds and so on. Mendel, in fact 
noted seven pairs of such contrasting characters in 
garden pea plant. 
 (iii) Each pea plant produced many seeds in one generation. 
 (iv) The garden pea plants could easily be raised, maintained 
and handled. 
 
Mendel’s Experimental Technique 
Mendel conducted breeding experiments in three steps 
(i) Selection of pure parent plants (i.e., plants producing 
similar traits in every generation 
(ii) Production of first generation of plants by crossbreeding 
(hybridization). 
(iii) Raising of second and subsequent generation by self-   
pollination (fertilization) of F
1
 generation hybrids. 
  
Mendel’s Monohybrid Cross 
Mendel first selected ‘pure line’ plants (i.e., the plants that 
produce similar traits generation after generation). He, then, 
cross pollinated plants having the contrasting traits, 
considering one trait at a time during one cross. 
 
Example 
(i) He crossbred garden pea plant having tallness trait with 
plant having dwarfness trait. In this monohybrid cross, 
the pollen grains from the flower of the tall plant were 
transferred over the flower of a dwarf plant or vice - 
versa.  
(ii) After the transfer of pollen grains, the cross pollinated 
flower was properly covered and were allowed to mature. 
All the plants of F
1
 generation were carefully observed. 
Mendel observed that all the plants of F
1
 generation were 
tall and there were no intermediate characteristics. 
 
Page 2


 
 
HEREDITY AND EVOLUTION
Heredity 
The transmission of characteristics (resemblances as well as 
variations) from parents to offsprings i.e., from one generation 
to the next is called heredity. 
 
Variation 
The differences shown by the individuals of a species, and also 
by the offsprings of the same parents are called variations. 
 
Genetics 
The study of heredity and variation is called genetics. 
 
Evolution 
The gradual changes taking place in the organisms which in 
turn causing the diversity of the living organisms over the long 
period of time is called evolution. 
 
Accumulation Of Variation During Reproduction 
In case of asexual reproduction in organisms, there are very 
minor differences that occur between the newly formed 
organisms due to the small inaccuracies in DNA copying. 
However if sexual reproduction is involved, greater diversity 
will be generated due to the inheritance of traits from the two 
parents. 
 
Do All The Variations In A Species Have Equal Chances 
Of Surviving In The Environment In Which They Find 
Themselves? 
Obviously not. Depending upon the nature of variations, 
different individuals would have different kinds of advantages 
and disadvantages. Selection of variants by environmental 
factors forms the basis of evolutionary processes. 
 
Heredity 
 
Rules Of Heredity 
The rules of heredity determine the process by which traits 
and characteristics are reliably inherited.  The rules are related 
to the fact that in human beings both the father and the 
mother contribute practically equal amounts of genetic 
material to the child. This means that each trait can be 
influenced by both the maternal and the paternal DNA. Thus, 
for each trait there will be two versions in each child. So for 
this kind of situation Mendel scientist worked out the main 
rules for such inheritance. 
 
Gregor Johann Mendel worked out the basic rules of such 
inheritance of traits more than a century ago. He studied the 
inheritance of contrasting characters (traits) such as 
tallness/dwarfness of plants, round/wrinkled form of seeds, on 
garden pea plant also called Pisum sativum. 
 
Mendel’s Experimental Plant 
Mendel selected garden pea plant (Pisum sativum) for series 
of hybridization experiments because of following special 
features:- 
(i) It had a short life cycle and, therefore, it was possible to 
study number of generations. 
(ii) Garden pea plant had distinct, easily detectable 
contrasting variants of features. For instance, some plants 
were tall and some dwarf, some had violet flowers and 
some had while flowers, some plants had round seeds 
and some had wrinkled seeds and so on. Mendel, in fact 
noted seven pairs of such contrasting characters in 
garden pea plant. 
 (iii) Each pea plant produced many seeds in one generation. 
 (iv) The garden pea plants could easily be raised, maintained 
and handled. 
 
Mendel’s Experimental Technique 
Mendel conducted breeding experiments in three steps 
(i) Selection of pure parent plants (i.e., plants producing 
similar traits in every generation 
(ii) Production of first generation of plants by crossbreeding 
(hybridization). 
(iii) Raising of second and subsequent generation by self-   
pollination (fertilization) of F
1
 generation hybrids. 
  
Mendel’s Monohybrid Cross 
Mendel first selected ‘pure line’ plants (i.e., the plants that 
produce similar traits generation after generation). He, then, 
cross pollinated plants having the contrasting traits, 
considering one trait at a time during one cross. 
 
Example 
(i) He crossbred garden pea plant having tallness trait with 
plant having dwarfness trait. In this monohybrid cross, 
the pollen grains from the flower of the tall plant were 
transferred over the flower of a dwarf plant or vice - 
versa.  
(ii) After the transfer of pollen grains, the cross pollinated 
flower was properly covered and were allowed to mature. 
All the plants of F
1
 generation were carefully observed. 
Mendel observed that all the plants of F
1
 generation were 
tall and there were no intermediate characteristics. 
 
 
                    
(iii) Then he self pollinated the plants obtained by F
1
 
generation to produce F
2
 generation or F
2
 progeny. As a 
result he observed that the F
2 
progeny of the F
1
 tall 
plants are not all tall. Instead one quarter of them are 
short. 
 
This indicates that both tallness and dwarfness traits were 
inherited by the F
1
 generation plants but only the tallness trait 
was expressed. Thus two copies of the trait are inherited in 
each sexually reproducing organism. The two may be identical 
or may be different. 
 
Conclusion 
Mendel concluded that sexually reproducing individuals have 
two copies of genes for the same trait. If the copies are not 
identical, the trait that gets expressed is called dominant 
trait and the other is called recessive trait. The characters 
are not lost even when they are not expressed. 
 
2. Mendel’s Dihybrid Cross 
Mendel also studied the inheritance of two characters 
simultaneously. In one such cross, Mendel considered shape 
as well as colour of the seeds simultaneously. 
(i) He selected pure line plants and then cross pollinated 
flowers raised from seeds of round shape and yellow 
colour with those from wrinkled seeds and green colour. 
(ii) Mendel observed that in F
1
 generation all seeds had the 
features of only one parental type, i.e., round shape and 
yellow colour. He raised plants from F1 generation seeds 
and allowed the flowers to self pollinate to produce the 
seeds of F
2
 generation. These flowers were kept covered 
from the beginning. 
 
 
(iii) In F
2
 generation, Mendel observed the appearance of four 
types of combination. These included two parental types 
(round shaped and yellow coloured seeds, and wrinkled 
shaped and green coloured seeds) and two new 
combinations (round shaped and green coloured seeds, 
and wrinkled shaped and yellow coloured seeds) in 
approximately same proportion.  
 
Conclusion 
 Traits in one individual may be inherited separately, giving 
rise to new combinations of traits in the offspring of 
sexual reproduction. 
 
How Do The Traits Get Expressed? 
To explain this let us take an example of the tallness as a 
characteristic of the garden pea plant. We know that the 
hormones in the plant trigger their growth. The plant height 
depends on the amount of the particular hormone synthesized.  
The amount of the synthesized plant hormone depends on the 
efficiency of the process of making it. If the specific protein 
needed for this process is synthesized and work properly, a lot 
of hormone will be made. This will support more growth and 
the plant will be tall. If the gene for the tallness trait will be 
altered, the protein now synthesized is less efficient and 
therefore, the amount formed also will be less. As a result the 
growth of plant will be less and the plant will be dwarf. 
 
Sex Determination  
 
Sex Determination 
It is a method to determine different organisms that the young 
one formed after the sexual reproduction is a male or a 
female. 
 
Method Of Sex Determination 
Different species use very different strategies for this.  
(i) Some rely entirely on environmental cues. Thus, is some 
animals, the temperature at which fertilized eggs are kept 
determines whether the animals developing in the eggs 
will be male or female. 
(ii) In other animals, such as snails, individuals can change 
sex indicating that sex is not genetically determined  
(iii) In human beings, the sex of the individual is largely 
genetically determined. In other words, the genes 
inherited from our parents decide whether we will be boys 
or girls.  
 
Sex Determination In Human Beings 
 
Page 3


 
 
HEREDITY AND EVOLUTION
Heredity 
The transmission of characteristics (resemblances as well as 
variations) from parents to offsprings i.e., from one generation 
to the next is called heredity. 
 
Variation 
The differences shown by the individuals of a species, and also 
by the offsprings of the same parents are called variations. 
 
Genetics 
The study of heredity and variation is called genetics. 
 
Evolution 
The gradual changes taking place in the organisms which in 
turn causing the diversity of the living organisms over the long 
period of time is called evolution. 
 
Accumulation Of Variation During Reproduction 
In case of asexual reproduction in organisms, there are very 
minor differences that occur between the newly formed 
organisms due to the small inaccuracies in DNA copying. 
However if sexual reproduction is involved, greater diversity 
will be generated due to the inheritance of traits from the two 
parents. 
 
Do All The Variations In A Species Have Equal Chances 
Of Surviving In The Environment In Which They Find 
Themselves? 
Obviously not. Depending upon the nature of variations, 
different individuals would have different kinds of advantages 
and disadvantages. Selection of variants by environmental 
factors forms the basis of evolutionary processes. 
 
Heredity 
 
Rules Of Heredity 
The rules of heredity determine the process by which traits 
and characteristics are reliably inherited.  The rules are related 
to the fact that in human beings both the father and the 
mother contribute practically equal amounts of genetic 
material to the child. This means that each trait can be 
influenced by both the maternal and the paternal DNA. Thus, 
for each trait there will be two versions in each child. So for 
this kind of situation Mendel scientist worked out the main 
rules for such inheritance. 
 
Gregor Johann Mendel worked out the basic rules of such 
inheritance of traits more than a century ago. He studied the 
inheritance of contrasting characters (traits) such as 
tallness/dwarfness of plants, round/wrinkled form of seeds, on 
garden pea plant also called Pisum sativum. 
 
Mendel’s Experimental Plant 
Mendel selected garden pea plant (Pisum sativum) for series 
of hybridization experiments because of following special 
features:- 
(i) It had a short life cycle and, therefore, it was possible to 
study number of generations. 
(ii) Garden pea plant had distinct, easily detectable 
contrasting variants of features. For instance, some plants 
were tall and some dwarf, some had violet flowers and 
some had while flowers, some plants had round seeds 
and some had wrinkled seeds and so on. Mendel, in fact 
noted seven pairs of such contrasting characters in 
garden pea plant. 
 (iii) Each pea plant produced many seeds in one generation. 
 (iv) The garden pea plants could easily be raised, maintained 
and handled. 
 
Mendel’s Experimental Technique 
Mendel conducted breeding experiments in three steps 
(i) Selection of pure parent plants (i.e., plants producing 
similar traits in every generation 
(ii) Production of first generation of plants by crossbreeding 
(hybridization). 
(iii) Raising of second and subsequent generation by self-   
pollination (fertilization) of F
1
 generation hybrids. 
  
Mendel’s Monohybrid Cross 
Mendel first selected ‘pure line’ plants (i.e., the plants that 
produce similar traits generation after generation). He, then, 
cross pollinated plants having the contrasting traits, 
considering one trait at a time during one cross. 
 
Example 
(i) He crossbred garden pea plant having tallness trait with 
plant having dwarfness trait. In this monohybrid cross, 
the pollen grains from the flower of the tall plant were 
transferred over the flower of a dwarf plant or vice - 
versa.  
(ii) After the transfer of pollen grains, the cross pollinated 
flower was properly covered and were allowed to mature. 
All the plants of F
1
 generation were carefully observed. 
Mendel observed that all the plants of F
1
 generation were 
tall and there were no intermediate characteristics. 
 
 
                    
(iii) Then he self pollinated the plants obtained by F
1
 
generation to produce F
2
 generation or F
2
 progeny. As a 
result he observed that the F
2 
progeny of the F
1
 tall 
plants are not all tall. Instead one quarter of them are 
short. 
 
This indicates that both tallness and dwarfness traits were 
inherited by the F
1
 generation plants but only the tallness trait 
was expressed. Thus two copies of the trait are inherited in 
each sexually reproducing organism. The two may be identical 
or may be different. 
 
Conclusion 
Mendel concluded that sexually reproducing individuals have 
two copies of genes for the same trait. If the copies are not 
identical, the trait that gets expressed is called dominant 
trait and the other is called recessive trait. The characters 
are not lost even when they are not expressed. 
 
2. Mendel’s Dihybrid Cross 
Mendel also studied the inheritance of two characters 
simultaneously. In one such cross, Mendel considered shape 
as well as colour of the seeds simultaneously. 
(i) He selected pure line plants and then cross pollinated 
flowers raised from seeds of round shape and yellow 
colour with those from wrinkled seeds and green colour. 
(ii) Mendel observed that in F
1
 generation all seeds had the 
features of only one parental type, i.e., round shape and 
yellow colour. He raised plants from F1 generation seeds 
and allowed the flowers to self pollinate to produce the 
seeds of F
2
 generation. These flowers were kept covered 
from the beginning. 
 
 
(iii) In F
2
 generation, Mendel observed the appearance of four 
types of combination. These included two parental types 
(round shaped and yellow coloured seeds, and wrinkled 
shaped and green coloured seeds) and two new 
combinations (round shaped and green coloured seeds, 
and wrinkled shaped and yellow coloured seeds) in 
approximately same proportion.  
 
Conclusion 
 Traits in one individual may be inherited separately, giving 
rise to new combinations of traits in the offspring of 
sexual reproduction. 
 
How Do The Traits Get Expressed? 
To explain this let us take an example of the tallness as a 
characteristic of the garden pea plant. We know that the 
hormones in the plant trigger their growth. The plant height 
depends on the amount of the particular hormone synthesized.  
The amount of the synthesized plant hormone depends on the 
efficiency of the process of making it. If the specific protein 
needed for this process is synthesized and work properly, a lot 
of hormone will be made. This will support more growth and 
the plant will be tall. If the gene for the tallness trait will be 
altered, the protein now synthesized is less efficient and 
therefore, the amount formed also will be less. As a result the 
growth of plant will be less and the plant will be dwarf. 
 
Sex Determination  
 
Sex Determination 
It is a method to determine different organisms that the young 
one formed after the sexual reproduction is a male or a 
female. 
 
Method Of Sex Determination 
Different species use very different strategies for this.  
(i) Some rely entirely on environmental cues. Thus, is some 
animals, the temperature at which fertilized eggs are kept 
determines whether the animals developing in the eggs 
will be male or female. 
(ii) In other animals, such as snails, individuals can change 
sex indicating that sex is not genetically determined  
(iii) In human beings, the sex of the individual is largely 
genetically determined. In other words, the genes 
inherited from our parents decide whether we will be boys 
or girls.  
 
Sex Determination In Human Beings 
 
 
In case of human beings out of the 23 pairs of the 
chromosomes which are present in the human body cells, one 
pair of chromosomes also called sex chromosome or allosome 
decides the type of the sex in human beings. All these pairs of 
chromosomes in human female are perfect pairs whereas in 
human males one pair called the sex chromosomes, is odd in 
not always beings a perfect pair. Females have perfect pair of 
sex chromosomes, both called X. But men have a mismatched 
pair in which one is normal-sized X while the other is a short 
one called Y. So women are XX, while men are XY. During the 
gamete formation in males half of the gametes will carry X 
chromosomes and the other half will carry Y chromosomes. 
Whereas in females all the gametes will carry the same X 
chromosomes. Therefore, all children will inherit an X 
chromosomes from their mother regardless of whether they 
are boys or girls whereas, if male gamete carrying X 
chromosome fuses with the female gamete a female child is 
formed and if male gamete carrying Y chromosome fuses with 
female gamete a male child is formed.  
        
Thus, the sex of the children will be determined by what they 
inherit from their father. A child who inherits X chromosomes 
from her father will be a girl, and one who inherits as Y 
chromosomes from him will be a boy. 
 
Evolution 
Evolution is defined as slow changes that are taking place in 
the species of an organism over a period of million of years. 
 
An Illustration  
Consider a group of twelve red beetles. They live let us 
assume, in some bushes with green leaves. Their population 
will grow by sexual reproduction, and therefore, can generate 
variations. Let us imagine also that crows eat these beetles. 
The more beetles the crows eat, the fewer beetles are 
available to reproduce. Now, let us think about some different 
situations that can develop in this beetle population. 
(i) In the first situation, a colour variation arises during 
reproduction, that there is one beetle that is green in 
colour instead of red. This beetle, moreover, can pass the 
colour on to its progeny, so that all its progeny beetles 
are green. Crows cannot see green coloured beetles on 
the green leaves of the bushes, and therefore cannot eat 
them. Therefore the progeny of green beetles is not 
eaten, while the progeny of red beetles continues to be 
eaten. As a result, there are more and more green beetles 
than red ones in the beetle population. 
(ii) 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. This beetle can also pass the 
colour on its progeny, so that all its progeny beetles are 
blue. 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, in the population, as 
it expands, there are a few blue beetles, but most are red. 
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. 
 
Now the first case is a case of natural selection. In the first 
case, the variation became common because it gave a survival 
advantage. In other words, it was naturally selected. We can 
see that the natural selection is applied by the crows. The 
more crows there are, the more red beetles would be eaten, 
and the more the proportion of green beetles in the population 
would be. Thus, natural selection is directing evolution in the 
beetle population. It results in adaptation in the beetle 
population to fit their environment better. 
 
Now the second case is the case of genetic drift (accidental 
survival). The colour change gave no survival advantage. 
Instead, it was simply a matter of accidental survival of beetles 
of one colour that changed the common characteristic of the 
resultant population. The elephant would not have caused 
such major havoc in the beetle population, if the beetle 
population had been very large. So, accidents in small 
populations can change the frequency of some genes in a 
population, even if they give no survival advantage. This is the 
notion of genetic drift, which provides diversity without 
any adaptations. 
 
(iii) 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. The beetles are poorly nourished as a 
result the average weight of adult beetles decreases from 
what it used to be when leaves were plentiful, but there is 
not 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. At this time the 
beetles regain their health and their occurred no effects 
on their progeny. 
 
In this situation it is seen that the poor health of the beetles is 
only an acquired trait of the beetles which has not altered 
their DNA and therefore is not transferred to their progenies. 
Hence this situation does not lead to evolution. 
 
From the above discussion we conclude that there are two 
types of traits: 
(i) Acquired traits                      (ii) Inherited traits 
 
Acquired Traits 
These are the changes that occur in the non reproductive 
tissues and cannot be passed on to the DNA of the germ cells. 
Hence these traits are not inheritable. Therefore these traits 
do not direct evolution. 
Example 
Health of an organism. 
 
Inherited Traits 
These are the changes that occur in the reproductive tissues 
and can be passed on to the DNA of the germ cells. Hence 
these traits are inheritable. Therefore these traits direct 
evolution. 
Example 
Colour of the beetle. 
 
 
Page 4


 
 
HEREDITY AND EVOLUTION
Heredity 
The transmission of characteristics (resemblances as well as 
variations) from parents to offsprings i.e., from one generation 
to the next is called heredity. 
 
Variation 
The differences shown by the individuals of a species, and also 
by the offsprings of the same parents are called variations. 
 
Genetics 
The study of heredity and variation is called genetics. 
 
Evolution 
The gradual changes taking place in the organisms which in 
turn causing the diversity of the living organisms over the long 
period of time is called evolution. 
 
Accumulation Of Variation During Reproduction 
In case of asexual reproduction in organisms, there are very 
minor differences that occur between the newly formed 
organisms due to the small inaccuracies in DNA copying. 
However if sexual reproduction is involved, greater diversity 
will be generated due to the inheritance of traits from the two 
parents. 
 
Do All The Variations In A Species Have Equal Chances 
Of Surviving In The Environment In Which They Find 
Themselves? 
Obviously not. Depending upon the nature of variations, 
different individuals would have different kinds of advantages 
and disadvantages. Selection of variants by environmental 
factors forms the basis of evolutionary processes. 
 
Heredity 
 
Rules Of Heredity 
The rules of heredity determine the process by which traits 
and characteristics are reliably inherited.  The rules are related 
to the fact that in human beings both the father and the 
mother contribute practically equal amounts of genetic 
material to the child. This means that each trait can be 
influenced by both the maternal and the paternal DNA. Thus, 
for each trait there will be two versions in each child. So for 
this kind of situation Mendel scientist worked out the main 
rules for such inheritance. 
 
Gregor Johann Mendel worked out the basic rules of such 
inheritance of traits more than a century ago. He studied the 
inheritance of contrasting characters (traits) such as 
tallness/dwarfness of plants, round/wrinkled form of seeds, on 
garden pea plant also called Pisum sativum. 
 
Mendel’s Experimental Plant 
Mendel selected garden pea plant (Pisum sativum) for series 
of hybridization experiments because of following special 
features:- 
(i) It had a short life cycle and, therefore, it was possible to 
study number of generations. 
(ii) Garden pea plant had distinct, easily detectable 
contrasting variants of features. For instance, some plants 
were tall and some dwarf, some had violet flowers and 
some had while flowers, some plants had round seeds 
and some had wrinkled seeds and so on. Mendel, in fact 
noted seven pairs of such contrasting characters in 
garden pea plant. 
 (iii) Each pea plant produced many seeds in one generation. 
 (iv) The garden pea plants could easily be raised, maintained 
and handled. 
 
Mendel’s Experimental Technique 
Mendel conducted breeding experiments in three steps 
(i) Selection of pure parent plants (i.e., plants producing 
similar traits in every generation 
(ii) Production of first generation of plants by crossbreeding 
(hybridization). 
(iii) Raising of second and subsequent generation by self-   
pollination (fertilization) of F
1
 generation hybrids. 
  
Mendel’s Monohybrid Cross 
Mendel first selected ‘pure line’ plants (i.e., the plants that 
produce similar traits generation after generation). He, then, 
cross pollinated plants having the contrasting traits, 
considering one trait at a time during one cross. 
 
Example 
(i) He crossbred garden pea plant having tallness trait with 
plant having dwarfness trait. In this monohybrid cross, 
the pollen grains from the flower of the tall plant were 
transferred over the flower of a dwarf plant or vice - 
versa.  
(ii) After the transfer of pollen grains, the cross pollinated 
flower was properly covered and were allowed to mature. 
All the plants of F
1
 generation were carefully observed. 
Mendel observed that all the plants of F
1
 generation were 
tall and there were no intermediate characteristics. 
 
 
                    
(iii) Then he self pollinated the plants obtained by F
1
 
generation to produce F
2
 generation or F
2
 progeny. As a 
result he observed that the F
2 
progeny of the F
1
 tall 
plants are not all tall. Instead one quarter of them are 
short. 
 
This indicates that both tallness and dwarfness traits were 
inherited by the F
1
 generation plants but only the tallness trait 
was expressed. Thus two copies of the trait are inherited in 
each sexually reproducing organism. The two may be identical 
or may be different. 
 
Conclusion 
Mendel concluded that sexually reproducing individuals have 
two copies of genes for the same trait. If the copies are not 
identical, the trait that gets expressed is called dominant 
trait and the other is called recessive trait. The characters 
are not lost even when they are not expressed. 
 
2. Mendel’s Dihybrid Cross 
Mendel also studied the inheritance of two characters 
simultaneously. In one such cross, Mendel considered shape 
as well as colour of the seeds simultaneously. 
(i) He selected pure line plants and then cross pollinated 
flowers raised from seeds of round shape and yellow 
colour with those from wrinkled seeds and green colour. 
(ii) Mendel observed that in F
1
 generation all seeds had the 
features of only one parental type, i.e., round shape and 
yellow colour. He raised plants from F1 generation seeds 
and allowed the flowers to self pollinate to produce the 
seeds of F
2
 generation. These flowers were kept covered 
from the beginning. 
 
 
(iii) In F
2
 generation, Mendel observed the appearance of four 
types of combination. These included two parental types 
(round shaped and yellow coloured seeds, and wrinkled 
shaped and green coloured seeds) and two new 
combinations (round shaped and green coloured seeds, 
and wrinkled shaped and yellow coloured seeds) in 
approximately same proportion.  
 
Conclusion 
 Traits in one individual may be inherited separately, giving 
rise to new combinations of traits in the offspring of 
sexual reproduction. 
 
How Do The Traits Get Expressed? 
To explain this let us take an example of the tallness as a 
characteristic of the garden pea plant. We know that the 
hormones in the plant trigger their growth. The plant height 
depends on the amount of the particular hormone synthesized.  
The amount of the synthesized plant hormone depends on the 
efficiency of the process of making it. If the specific protein 
needed for this process is synthesized and work properly, a lot 
of hormone will be made. This will support more growth and 
the plant will be tall. If the gene for the tallness trait will be 
altered, the protein now synthesized is less efficient and 
therefore, the amount formed also will be less. As a result the 
growth of plant will be less and the plant will be dwarf. 
 
Sex Determination  
 
Sex Determination 
It is a method to determine different organisms that the young 
one formed after the sexual reproduction is a male or a 
female. 
 
Method Of Sex Determination 
Different species use very different strategies for this.  
(i) Some rely entirely on environmental cues. Thus, is some 
animals, the temperature at which fertilized eggs are kept 
determines whether the animals developing in the eggs 
will be male or female. 
(ii) In other animals, such as snails, individuals can change 
sex indicating that sex is not genetically determined  
(iii) In human beings, the sex of the individual is largely 
genetically determined. In other words, the genes 
inherited from our parents decide whether we will be boys 
or girls.  
 
Sex Determination In Human Beings 
 
 
In case of human beings out of the 23 pairs of the 
chromosomes which are present in the human body cells, one 
pair of chromosomes also called sex chromosome or allosome 
decides the type of the sex in human beings. All these pairs of 
chromosomes in human female are perfect pairs whereas in 
human males one pair called the sex chromosomes, is odd in 
not always beings a perfect pair. Females have perfect pair of 
sex chromosomes, both called X. But men have a mismatched 
pair in which one is normal-sized X while the other is a short 
one called Y. So women are XX, while men are XY. During the 
gamete formation in males half of the gametes will carry X 
chromosomes and the other half will carry Y chromosomes. 
Whereas in females all the gametes will carry the same X 
chromosomes. Therefore, all children will inherit an X 
chromosomes from their mother regardless of whether they 
are boys or girls whereas, if male gamete carrying X 
chromosome fuses with the female gamete a female child is 
formed and if male gamete carrying Y chromosome fuses with 
female gamete a male child is formed.  
        
Thus, the sex of the children will be determined by what they 
inherit from their father. A child who inherits X chromosomes 
from her father will be a girl, and one who inherits as Y 
chromosomes from him will be a boy. 
 
Evolution 
Evolution is defined as slow changes that are taking place in 
the species of an organism over a period of million of years. 
 
An Illustration  
Consider a group of twelve red beetles. They live let us 
assume, in some bushes with green leaves. Their population 
will grow by sexual reproduction, and therefore, can generate 
variations. Let us imagine also that crows eat these beetles. 
The more beetles the crows eat, the fewer beetles are 
available to reproduce. Now, let us think about some different 
situations that can develop in this beetle population. 
(i) In the first situation, a colour variation arises during 
reproduction, that there is one beetle that is green in 
colour instead of red. This beetle, moreover, can pass the 
colour on to its progeny, so that all its progeny beetles 
are green. Crows cannot see green coloured beetles on 
the green leaves of the bushes, and therefore cannot eat 
them. Therefore the progeny of green beetles is not 
eaten, while the progeny of red beetles continues to be 
eaten. As a result, there are more and more green beetles 
than red ones in the beetle population. 
(ii) 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. This beetle can also pass the 
colour on its progeny, so that all its progeny beetles are 
blue. 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, in the population, as 
it expands, there are a few blue beetles, but most are red. 
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. 
 
Now the first case is a case of natural selection. In the first 
case, the variation became common because it gave a survival 
advantage. In other words, it was naturally selected. We can 
see that the natural selection is applied by the crows. The 
more crows there are, the more red beetles would be eaten, 
and the more the proportion of green beetles in the population 
would be. Thus, natural selection is directing evolution in the 
beetle population. It results in adaptation in the beetle 
population to fit their environment better. 
 
Now the second case is the case of genetic drift (accidental 
survival). The colour change gave no survival advantage. 
Instead, it was simply a matter of accidental survival of beetles 
of one colour that changed the common characteristic of the 
resultant population. The elephant would not have caused 
such major havoc in the beetle population, if the beetle 
population had been very large. So, accidents in small 
populations can change the frequency of some genes in a 
population, even if they give no survival advantage. This is the 
notion of genetic drift, which provides diversity without 
any adaptations. 
 
(iii) 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. The beetles are poorly nourished as a 
result the average weight of adult beetles decreases from 
what it used to be when leaves were plentiful, but there is 
not 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. At this time the 
beetles regain their health and their occurred no effects 
on their progeny. 
 
In this situation it is seen that the poor health of the beetles is 
only an acquired trait of the beetles which has not altered 
their DNA and therefore is not transferred to their progenies. 
Hence this situation does not lead to evolution. 
 
From the above discussion we conclude that there are two 
types of traits: 
(i) Acquired traits                      (ii) Inherited traits 
 
Acquired Traits 
These are the changes that occur in the non reproductive 
tissues and cannot be passed on to the DNA of the germ cells. 
Hence these traits are not inheritable. Therefore these traits 
do not direct evolution. 
Example 
Health of an organism. 
 
Inherited Traits 
These are the changes that occur in the reproductive tissues 
and can be passed on to the DNA of the germ cells. Hence 
these traits are inheritable. Therefore these traits direct 
evolution. 
Example 
Colour of the beetle. 
 
 
 
Speciation 
Speciation is defined as the formation of a new species from 
the existing ones. New species are said to be formed from the 
existing ones when one species splits into two such 
populations that cannot reproduce with each other. When this 
happens, they can be called two independent species.  
Following factors lead the speciation to take place: - 
(i) Natural selection 
(ii) Genetic drift 
(iii) Geographical isolation 
Out of which geographical isolation plays the most important 
role in the speciation to take place. 
 
How Speciation Takes Place? 
What we have seen so far is micro-evolution. That means that 
the changes are small, even though they are significant. Also, 
they simply change the common characteristics of a particular 
species But this does not properly explain how new species 
come into existence.  
 
Example 
Consider the bushes the beetles feed on are spread widely 
over a mountain range. The beetle population becomes very 
large as a result. But individual beetles feed mostly on a few 
nearby bushes throughout their lifetime. They do not travel 
far. So, in this huge population of beetles, there will be sub-
population in neighborhoods. Since male and female beetles 
have to meet for reproduction to happen, most reproduction 
will be within these sub-populations. Of course, an occasional 
adventurous beetle might go from one site to another. Or a 
beetle is picked up by a crow from one site and dropped in the 
other site without being eaten. In either case, the migrant 
beetle will reproduce with the local population. This will result 
in the genes of the migrant beetle entering in a new 
population. This kind of gene flow is bound to happen 
between populations that are partly, but not completely 
separated. If, however, between two such sub-populations a 
large river comes into existence, the two populations will be 
further isolated. The levels or gene flow between them will 
decrease even further. 
Over generations, genetic drift will accumulate different 
changes in each sub-population. Also, natural selection may 
also operate differently in these different geographic locations. 
Thus, for example, in the territory of one sub-population, 
crows are eliminated by eagles, but this does not happen for 
the other sub-population, where crow numbers are very high. 
As a result, the green variation will not be selected at the first 
site, while it will be strongly selected at the second. 
 
Together, the processes of genetic drift and natural selection 
will result in these two isolated sub-populations of beetles 
becoming more and more different from each other. Even the 
number of chromosomes can also be changed. Eventually, 
members of these two groups will be incapable of reproducing 
with each other even if they happen to meet. Then in this case 
we will say that two different species have been formed. 
 
Evolution And Classification 
 
Characteristic 
Characteristics of the organisms are the details of the external 
and internal appearance or behaviour that distinguish them 
from one another. These characteristics also form the basis for 
the classification of the organism. 
Therefore by identifying the hierarchies of 
characteristics between the species, we can work out 
the evolutionary relationships of the species that we 
see around us. 
The evolutionary relationships among the organisms allow us 
to make classification groups. In other words we can also say 
that the classification of species is in fact the reflection of their 
evolutionary relationships. The more characteristics two 
species have in common the more closely they are related. 
Also the more closely they are related, the more recently they 
will have had a common ancestor. Thus, this way we can form 
small groups of species with recent common ancestors, then 
super groups of these groups with more distant common 
ancestors, and so on. Theoretically we can go on moving 
backwards until we reach a common ancestor at the very 
beginning of the evolutionary time. Ultimately this would lead 
to the fact that all living organisms have arise from non living 
matter. 
 
Tracing Evolutionary Relationships 
The evolutionary relationship of the organisms can be traced 
by studying the following attributes about them: - 
(i) Homologous Organs     
(ii) Analogous Organs 
(iii) Fossils 
(iv) Comparison Of The DNAs Of Different Species 
 
Homologous Organs 
Those organs in the different organisms which have same 
structure, shape and size are called homologous organs.  
Example 
Forelimbs of a frog, a lizard, a bird and a human being. All 
these organs have the same basic structure but perform 
different functions. 
 
By studying the homologous organs of the various organisms 
we can come to the conclusion that all these organisms belong 
to the same ancestors or common ancestors. 
 
 
 
These organs help us in tracing evolution in a way that it 
shows that as all these organisms have belong to the same 
ancestor therefore the ancestral vertebrate must have been 
modified according to the special needs of the further 
generations during the course of evolution. 
 
Analogous Organs 
Those organs of different organisms which have different 
shape, structure and size but perform similar functions are 
called analogous organs. 
Example 
Membranous wings of an insect and fleshy, bony wings of a 
bird. 
 
Page 5


 
 
HEREDITY AND EVOLUTION
Heredity 
The transmission of characteristics (resemblances as well as 
variations) from parents to offsprings i.e., from one generation 
to the next is called heredity. 
 
Variation 
The differences shown by the individuals of a species, and also 
by the offsprings of the same parents are called variations. 
 
Genetics 
The study of heredity and variation is called genetics. 
 
Evolution 
The gradual changes taking place in the organisms which in 
turn causing the diversity of the living organisms over the long 
period of time is called evolution. 
 
Accumulation Of Variation During Reproduction 
In case of asexual reproduction in organisms, there are very 
minor differences that occur between the newly formed 
organisms due to the small inaccuracies in DNA copying. 
However if sexual reproduction is involved, greater diversity 
will be generated due to the inheritance of traits from the two 
parents. 
 
Do All The Variations In A Species Have Equal Chances 
Of Surviving In The Environment In Which They Find 
Themselves? 
Obviously not. Depending upon the nature of variations, 
different individuals would have different kinds of advantages 
and disadvantages. Selection of variants by environmental 
factors forms the basis of evolutionary processes. 
 
Heredity 
 
Rules Of Heredity 
The rules of heredity determine the process by which traits 
and characteristics are reliably inherited.  The rules are related 
to the fact that in human beings both the father and the 
mother contribute practically equal amounts of genetic 
material to the child. This means that each trait can be 
influenced by both the maternal and the paternal DNA. Thus, 
for each trait there will be two versions in each child. So for 
this kind of situation Mendel scientist worked out the main 
rules for such inheritance. 
 
Gregor Johann Mendel worked out the basic rules of such 
inheritance of traits more than a century ago. He studied the 
inheritance of contrasting characters (traits) such as 
tallness/dwarfness of plants, round/wrinkled form of seeds, on 
garden pea plant also called Pisum sativum. 
 
Mendel’s Experimental Plant 
Mendel selected garden pea plant (Pisum sativum) for series 
of hybridization experiments because of following special 
features:- 
(i) It had a short life cycle and, therefore, it was possible to 
study number of generations. 
(ii) Garden pea plant had distinct, easily detectable 
contrasting variants of features. For instance, some plants 
were tall and some dwarf, some had violet flowers and 
some had while flowers, some plants had round seeds 
and some had wrinkled seeds and so on. Mendel, in fact 
noted seven pairs of such contrasting characters in 
garden pea plant. 
 (iii) Each pea plant produced many seeds in one generation. 
 (iv) The garden pea plants could easily be raised, maintained 
and handled. 
 
Mendel’s Experimental Technique 
Mendel conducted breeding experiments in three steps 
(i) Selection of pure parent plants (i.e., plants producing 
similar traits in every generation 
(ii) Production of first generation of plants by crossbreeding 
(hybridization). 
(iii) Raising of second and subsequent generation by self-   
pollination (fertilization) of F
1
 generation hybrids. 
  
Mendel’s Monohybrid Cross 
Mendel first selected ‘pure line’ plants (i.e., the plants that 
produce similar traits generation after generation). He, then, 
cross pollinated plants having the contrasting traits, 
considering one trait at a time during one cross. 
 
Example 
(i) He crossbred garden pea plant having tallness trait with 
plant having dwarfness trait. In this monohybrid cross, 
the pollen grains from the flower of the tall plant were 
transferred over the flower of a dwarf plant or vice - 
versa.  
(ii) After the transfer of pollen grains, the cross pollinated 
flower was properly covered and were allowed to mature. 
All the plants of F
1
 generation were carefully observed. 
Mendel observed that all the plants of F
1
 generation were 
tall and there were no intermediate characteristics. 
 
 
                    
(iii) Then he self pollinated the plants obtained by F
1
 
generation to produce F
2
 generation or F
2
 progeny. As a 
result he observed that the F
2 
progeny of the F
1
 tall 
plants are not all tall. Instead one quarter of them are 
short. 
 
This indicates that both tallness and dwarfness traits were 
inherited by the F
1
 generation plants but only the tallness trait 
was expressed. Thus two copies of the trait are inherited in 
each sexually reproducing organism. The two may be identical 
or may be different. 
 
Conclusion 
Mendel concluded that sexually reproducing individuals have 
two copies of genes for the same trait. If the copies are not 
identical, the trait that gets expressed is called dominant 
trait and the other is called recessive trait. The characters 
are not lost even when they are not expressed. 
 
2. Mendel’s Dihybrid Cross 
Mendel also studied the inheritance of two characters 
simultaneously. In one such cross, Mendel considered shape 
as well as colour of the seeds simultaneously. 
(i) He selected pure line plants and then cross pollinated 
flowers raised from seeds of round shape and yellow 
colour with those from wrinkled seeds and green colour. 
(ii) Mendel observed that in F
1
 generation all seeds had the 
features of only one parental type, i.e., round shape and 
yellow colour. He raised plants from F1 generation seeds 
and allowed the flowers to self pollinate to produce the 
seeds of F
2
 generation. These flowers were kept covered 
from the beginning. 
 
 
(iii) In F
2
 generation, Mendel observed the appearance of four 
types of combination. These included two parental types 
(round shaped and yellow coloured seeds, and wrinkled 
shaped and green coloured seeds) and two new 
combinations (round shaped and green coloured seeds, 
and wrinkled shaped and yellow coloured seeds) in 
approximately same proportion.  
 
Conclusion 
 Traits in one individual may be inherited separately, giving 
rise to new combinations of traits in the offspring of 
sexual reproduction. 
 
How Do The Traits Get Expressed? 
To explain this let us take an example of the tallness as a 
characteristic of the garden pea plant. We know that the 
hormones in the plant trigger their growth. The plant height 
depends on the amount of the particular hormone synthesized.  
The amount of the synthesized plant hormone depends on the 
efficiency of the process of making it. If the specific protein 
needed for this process is synthesized and work properly, a lot 
of hormone will be made. This will support more growth and 
the plant will be tall. If the gene for the tallness trait will be 
altered, the protein now synthesized is less efficient and 
therefore, the amount formed also will be less. As a result the 
growth of plant will be less and the plant will be dwarf. 
 
Sex Determination  
 
Sex Determination 
It is a method to determine different organisms that the young 
one formed after the sexual reproduction is a male or a 
female. 
 
Method Of Sex Determination 
Different species use very different strategies for this.  
(i) Some rely entirely on environmental cues. Thus, is some 
animals, the temperature at which fertilized eggs are kept 
determines whether the animals developing in the eggs 
will be male or female. 
(ii) In other animals, such as snails, individuals can change 
sex indicating that sex is not genetically determined  
(iii) In human beings, the sex of the individual is largely 
genetically determined. In other words, the genes 
inherited from our parents decide whether we will be boys 
or girls.  
 
Sex Determination In Human Beings 
 
 
In case of human beings out of the 23 pairs of the 
chromosomes which are present in the human body cells, one 
pair of chromosomes also called sex chromosome or allosome 
decides the type of the sex in human beings. All these pairs of 
chromosomes in human female are perfect pairs whereas in 
human males one pair called the sex chromosomes, is odd in 
not always beings a perfect pair. Females have perfect pair of 
sex chromosomes, both called X. But men have a mismatched 
pair in which one is normal-sized X while the other is a short 
one called Y. So women are XX, while men are XY. During the 
gamete formation in males half of the gametes will carry X 
chromosomes and the other half will carry Y chromosomes. 
Whereas in females all the gametes will carry the same X 
chromosomes. Therefore, all children will inherit an X 
chromosomes from their mother regardless of whether they 
are boys or girls whereas, if male gamete carrying X 
chromosome fuses with the female gamete a female child is 
formed and if male gamete carrying Y chromosome fuses with 
female gamete a male child is formed.  
        
Thus, the sex of the children will be determined by what they 
inherit from their father. A child who inherits X chromosomes 
from her father will be a girl, and one who inherits as Y 
chromosomes from him will be a boy. 
 
Evolution 
Evolution is defined as slow changes that are taking place in 
the species of an organism over a period of million of years. 
 
An Illustration  
Consider a group of twelve red beetles. They live let us 
assume, in some bushes with green leaves. Their population 
will grow by sexual reproduction, and therefore, can generate 
variations. Let us imagine also that crows eat these beetles. 
The more beetles the crows eat, the fewer beetles are 
available to reproduce. Now, let us think about some different 
situations that can develop in this beetle population. 
(i) In the first situation, a colour variation arises during 
reproduction, that there is one beetle that is green in 
colour instead of red. This beetle, moreover, can pass the 
colour on to its progeny, so that all its progeny beetles 
are green. Crows cannot see green coloured beetles on 
the green leaves of the bushes, and therefore cannot eat 
them. Therefore the progeny of green beetles is not 
eaten, while the progeny of red beetles continues to be 
eaten. As a result, there are more and more green beetles 
than red ones in the beetle population. 
(ii) 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. This beetle can also pass the 
colour on its progeny, so that all its progeny beetles are 
blue. 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, in the population, as 
it expands, there are a few blue beetles, but most are red. 
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. 
 
Now the first case is a case of natural selection. In the first 
case, the variation became common because it gave a survival 
advantage. In other words, it was naturally selected. We can 
see that the natural selection is applied by the crows. The 
more crows there are, the more red beetles would be eaten, 
and the more the proportion of green beetles in the population 
would be. Thus, natural selection is directing evolution in the 
beetle population. It results in adaptation in the beetle 
population to fit their environment better. 
 
Now the second case is the case of genetic drift (accidental 
survival). The colour change gave no survival advantage. 
Instead, it was simply a matter of accidental survival of beetles 
of one colour that changed the common characteristic of the 
resultant population. The elephant would not have caused 
such major havoc in the beetle population, if the beetle 
population had been very large. So, accidents in small 
populations can change the frequency of some genes in a 
population, even if they give no survival advantage. This is the 
notion of genetic drift, which provides diversity without 
any adaptations. 
 
(iii) 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. The beetles are poorly nourished as a 
result the average weight of adult beetles decreases from 
what it used to be when leaves were plentiful, but there is 
not 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. At this time the 
beetles regain their health and their occurred no effects 
on their progeny. 
 
In this situation it is seen that the poor health of the beetles is 
only an acquired trait of the beetles which has not altered 
their DNA and therefore is not transferred to their progenies. 
Hence this situation does not lead to evolution. 
 
From the above discussion we conclude that there are two 
types of traits: 
(i) Acquired traits                      (ii) Inherited traits 
 
Acquired Traits 
These are the changes that occur in the non reproductive 
tissues and cannot be passed on to the DNA of the germ cells. 
Hence these traits are not inheritable. Therefore these traits 
do not direct evolution. 
Example 
Health of an organism. 
 
Inherited Traits 
These are the changes that occur in the reproductive tissues 
and can be passed on to the DNA of the germ cells. Hence 
these traits are inheritable. Therefore these traits direct 
evolution. 
Example 
Colour of the beetle. 
 
 
 
Speciation 
Speciation is defined as the formation of a new species from 
the existing ones. New species are said to be formed from the 
existing ones when one species splits into two such 
populations that cannot reproduce with each other. When this 
happens, they can be called two independent species.  
Following factors lead the speciation to take place: - 
(i) Natural selection 
(ii) Genetic drift 
(iii) Geographical isolation 
Out of which geographical isolation plays the most important 
role in the speciation to take place. 
 
How Speciation Takes Place? 
What we have seen so far is micro-evolution. That means that 
the changes are small, even though they are significant. Also, 
they simply change the common characteristics of a particular 
species But this does not properly explain how new species 
come into existence.  
 
Example 
Consider the bushes the beetles feed on are spread widely 
over a mountain range. The beetle population becomes very 
large as a result. But individual beetles feed mostly on a few 
nearby bushes throughout their lifetime. They do not travel 
far. So, in this huge population of beetles, there will be sub-
population in neighborhoods. Since male and female beetles 
have to meet for reproduction to happen, most reproduction 
will be within these sub-populations. Of course, an occasional 
adventurous beetle might go from one site to another. Or a 
beetle is picked up by a crow from one site and dropped in the 
other site without being eaten. In either case, the migrant 
beetle will reproduce with the local population. This will result 
in the genes of the migrant beetle entering in a new 
population. This kind of gene flow is bound to happen 
between populations that are partly, but not completely 
separated. If, however, between two such sub-populations a 
large river comes into existence, the two populations will be 
further isolated. The levels or gene flow between them will 
decrease even further. 
Over generations, genetic drift will accumulate different 
changes in each sub-population. Also, natural selection may 
also operate differently in these different geographic locations. 
Thus, for example, in the territory of one sub-population, 
crows are eliminated by eagles, but this does not happen for 
the other sub-population, where crow numbers are very high. 
As a result, the green variation will not be selected at the first 
site, while it will be strongly selected at the second. 
 
Together, the processes of genetic drift and natural selection 
will result in these two isolated sub-populations of beetles 
becoming more and more different from each other. Even the 
number of chromosomes can also be changed. Eventually, 
members of these two groups will be incapable of reproducing 
with each other even if they happen to meet. Then in this case 
we will say that two different species have been formed. 
 
Evolution And Classification 
 
Characteristic 
Characteristics of the organisms are the details of the external 
and internal appearance or behaviour that distinguish them 
from one another. These characteristics also form the basis for 
the classification of the organism. 
Therefore by identifying the hierarchies of 
characteristics between the species, we can work out 
the evolutionary relationships of the species that we 
see around us. 
The evolutionary relationships among the organisms allow us 
to make classification groups. In other words we can also say 
that the classification of species is in fact the reflection of their 
evolutionary relationships. The more characteristics two 
species have in common the more closely they are related. 
Also the more closely they are related, the more recently they 
will have had a common ancestor. Thus, this way we can form 
small groups of species with recent common ancestors, then 
super groups of these groups with more distant common 
ancestors, and so on. Theoretically we can go on moving 
backwards until we reach a common ancestor at the very 
beginning of the evolutionary time. Ultimately this would lead 
to the fact that all living organisms have arise from non living 
matter. 
 
Tracing Evolutionary Relationships 
The evolutionary relationship of the organisms can be traced 
by studying the following attributes about them: - 
(i) Homologous Organs     
(ii) Analogous Organs 
(iii) Fossils 
(iv) Comparison Of The DNAs Of Different Species 
 
Homologous Organs 
Those organs in the different organisms which have same 
structure, shape and size are called homologous organs.  
Example 
Forelimbs of a frog, a lizard, a bird and a human being. All 
these organs have the same basic structure but perform 
different functions. 
 
By studying the homologous organs of the various organisms 
we can come to the conclusion that all these organisms belong 
to the same ancestors or common ancestors. 
 
 
 
These organs help us in tracing evolution in a way that it 
shows that as all these organisms have belong to the same 
ancestor therefore the ancestral vertebrate must have been 
modified according to the special needs of the further 
generations during the course of evolution. 
 
Analogous Organs 
Those organs of different organisms which have different 
shape, structure and size but perform similar functions are 
called analogous organs. 
Example 
Membranous wings of an insect and fleshy, bony wings of a 
bird. 
 
 
 
The analogous organs of course tell us that the organisms do 
not belong to the same ancestors but they explain about the 
evolution in some other way. The presence of analogous 
organs provides evidence because it tells us that the 
organisms with different structure can adapt to perform similar 
functions for their survival under adverse environmental 
conditions. 
 
Fossils 
All preserved traces of the living organisms are called fossils. 
In other words we can say that the fossils are the remains or 
impressions of the dead animals and plants that lived in the 
remote past.  
                
How Fossils Are Formed? 
Usually when organism die, there bodies will decompose and 
be lost. But every once in a while, the body of the organism or 
at least some parts may be present in an environment that 
does not let it completely decomposed. If a dead insect gets 
caught in hot mud, for example, it will not get decomposed 
quickly, and the mud will eventually harden and retain the 
impressions of the body parts of the insect. This preserved 
trace of the insect is called the fossil of the insect. 
 
It is true that analysis of the organ structure in fossils allows 
us to make estimates of how far back evolutionary relationship 
go. 
 
How Do We Know The Age Of The Fossils? 
There are two methods: - 
 
Relative Method 
In this method on digging into the earth, it is supposed that 
the fossils we find closer to the surface are more recent than 
the fossils we find in the deeper layers of the earth. As we dig 
deeper we find older and older fossils. It just gives a rough 
comparison of the age of the fossil. 
 
Carbon – 14 Dating Method 
In this method the ratios of the different isotopes of the same 
element are detected in the fossils. By calculating the half life 
of the isotope the exact age of the fossil can be found. In this 
method the right age of the fossil can be found. 
 
Comparison Of The DNAs Of Different Species 
We know that the change in the DNA during reproduction is 
the basic events in evolution. If that is the case then 
comparing the DNA of different species should give us a direct 
estimate of how much the DNA has changed during the 
formation of these species. This method is now extensively 
used to define evolutionary relationships. 
 
Evolution By Stages 
During evolution the formation of the complex organs in the 
organisms is formed bit by bit over generations. They have not 
been generated by a single DNA change. For example from 
the rudimentary eyes of planaria, to the insects, octopus and 
finally to the vertebrates, the complexity in the structure of the 
eye has come in stages over a long period of time. And the 
structure of the eye in each of these organisms is different 
enough for them to have separate evolutionary origin. 
         
Also a change that is useful for one property to start with can 
become useful later for quite a different function. Feathers, for 
example can start out as providing insulation in cold weather. 
But later they might become useful for flight. In fact some 
dinosaurs had feathers, although they could not fly using the 
feathers. Birds seem to have later adapted the feathers to 
flight. This means that birds are closely related to reptiles. 
Artificial Selection 
It is a process by which man selects trait(s) useful to him for 
improving the qualities of domesticated plants and animals. 
Man selects individuals having the desired traits and separates 
them from those which do not possess such characters. The 
selected individuals are interbred. This process of artificial 
selection when repeated for several times for many 
generations produces a new breed with desired traits.  
Example  
The wild cabbage plant is a good example. Humans have over 
more than two thousand years, cultivated wild cabbage as a 
food plant, and generated different vegetables from it by 
artificial selection. Some farmers have wanted to select for 
very short distances between the leaves and have bred the 
cabbage we eat. Some have wanted to select for arrested 
flower development and have bred broccoli, or for sterile 
flowers, and have made the cauliflower. Some have selected 
for swollen parts, and come up with kohlrabi. Some have 
looked for slightly larger leaves and come up with a leafy 
vegetable called kale.