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