Transcript heredity and evolution

PREPARED BY:JEHANGEER HAMEED
(10th class)
Variations in organisms mainly occur during sexual reproduction
Because of mutations caused by errors in DNA replication. Thus no two
Offspring's are identical to each other .Other causes of such variations
are crossing over during meiosis separation of chromosomes during
gametogenesis from both parents at the time of fertilization.
Hence we see that each generation provides the next
Generation with some variations though the body design remains the same.
Some new variations are also produced in the new generation. The
variations keep on accumulating and passing to more individuals in each
generation. Thus as we proceed to later generation the frequency of
variations goes on increasing.
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Heridity means continuity of features from one generation to
another.Heridity can be defined as “resemblances among
individuals related by descent” or “ the transmission of traits
from parents to offspring's”.
From biological point of view variation can be defined as “
the occurrence of differences among the individuals”Heridity
and variation are important aspects of the branch of biology
which is known as GENETICS. That’s why Genetics is called
the ‘SCIENCE OF HERIDITY ANDVARIATIONS.’
The concept of heredity is not new. Selective breeding of
horses and cattle used to be done during the ancient
civilization of Babylon and Assyria. Ancient Chinese writings
mention about creating better varieties of paddy. But the
scientific principles, which governs patterns of inheritance
,were unknown till the experiments and findings of GREGOR
JOHANN MENDEL(1822-1884).His observations regarding the
occurrence of contrasting characters in various generation of
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Mendel conducted breeding experiments in three steps:
Selection of pure parent plants
Production of first generation of plants by cross breeding
Raising of second and subsequent generations by self fertilization of hybrids.
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MENDELS MONOHYBRID CROSS:
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A breeding experiment dealing with a single character is called a monohybrid
cross
Mental first selected ‘pure line’ plants(i.e., the plants that produced similar
traits generation after generation). He, then, cross pollinated such plants
having the contrasting traits, considering one trait at a time. For instance, in
one such cross breeding experiment, he cross bred garden pea plant having
round seeds with plant having wrinkled seeds. In this monohybrid cross, the
pollen grains from the flower of the desired plant raised from round seeds
were transferred over the previously emasculated flower of a plant raised
from wrinkled seeds or vice-versa. After the transfer of pollen grains, the
cross pollinated flower was properly covered and seeds produced were
allowed to mature. All the seeds F1 generation were carefully observed.
Mental observed that all the seeds of F1 generation were of round type and
there were no intermediate characteristics.
He raised plants from F1 seeds and allowed the flowers to selfpollinate to produce the seeds of F2 generation. The flowers were kept
covered from the beginning to avoid unwanted pollens to reach these
flowers. In F2 generation, Mendel observed the appearance of both round
and wrinkled seeds in approximately 3:1 proportion.
 STRUCTURE
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OF DNA
The expanded form of DNA is deoxyribonucleic acid. It was
first isolated by the scientist FREDERICK MEISHER from the
nucleus of the puss cells in 1869. He named it as NUCLEIN or
nucleic acid because of its acidic nature. DNA is the carrier of
the genetic information from generation to generation. It
transmits the hereditary characters in a coded language from
parents to the offspring's.
DNA is a macromolecule or polymer. It is made of very
large number of nucleotide units and hence is termed
polynucleotide.
Each nucleotide unit in a DNA molecule is made of
three components:
(a)Deoxyribose sugar:It is a pentose sugar represented as “S”
(b)Nitrogenous base: Each nucleotide unit has a nitrogen
containing base.
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In a DNA molecule, nitrogenous bases are of two types:
(a) PURINES: The purines in a DNA molecule are Adenosine(A)
and Guanine(G).
(b) PYRIMIDINES: The pyrimidines in a DNA molecule are
Cytosine(C)) and Thymine(T)
3.PHOSPHATE GROUP: The phosphate group contains one
phosphorus atom and four specifically linked oxygen atoms.
Thus , there are four types of nucleotides in a DNA molecule
depending upon the kind of nitrogenous base present in each
nucleotide.
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HOW DO TRAITS GET EXPRESSED
According to Mendel's interpretations, both parents are
contribute equally to the DNA of the progeny during sexual
reproduction.Also,both parents help in determining the traits
in the progeny. In other words, both parents contribute a
copy each of the same genes and both male and female
gamets,therefore have only one set of genes.
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THEORIES ON ORIGIN OF LIFE;
1.Theory of special creation: It stated that life was created by
the wishes of a divine being or some supernatural power, THE
CREATOR OR GOD out of nothing at a particular time in the
remote past. According to CHRISTIAN belief, God created the
universe,plants,animals and human beings in six natural
days. Adam and Eve were the first man and woman created by
God. According to Hindu mythology, Lord Brahma creates the
world with his wish.
The theory of special creation is purely a religious
concept. It is not based on scientific principles.
2.THEORY OF SPONTANEOUS GENERATION: It stated that life
originated spontaneously from non- living materialsabiogenesis.For example, fly larvae were believed to develop
from rotten meat; frogs, snakes and crocodiles could form
from the moist soil of the river. However , this idea was laid
to rest by LOUIS PASTEUR who experimentally proved that
microorganisms, such as protists and bacteria, arise from the
pre-existing micro organisms of their kind.
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ABIOGENIC OR NATUALISTIC THEORY OR
CHEMICAL ORIGIN OF LIFE;
It is now well accepted that life originated from inanimate
matter by a series of chemical reactions on the primitive
earth. This concept was put forward by ALEXANDER.I.OPARIN
J.B.S.HALDANE INDEPENDENTLY. This theory is now known as
primary a biogenesis.
J.B.S.HALDANE,a British scientist suggested in 1929
that life must have arisen on the primitive earth from
collection of chemicals through a progressive series of
chemical reactions in which atoms combined into inorganic
molecules, inorganic molecules into simple organic
compounds, simple organic compounds into the living
matter. The primary abiogenesis is thought to have occurred
about 3.6 billion years ago. HALDANE speculated that the
conditions on the primitive earth were far from the conditions
we see today. The atmosphere was highly reducing and
oxygen in molecular form was not available.
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EVOLUTION
The term evolution has been derived from the Latin word
“evolvere” which means to “unroll” or unfold. It refers to a
gradual change from one form to another. Such a change in
elements with time is called inorganic evolution. However
gradual change in living organisms with time since beginning
of life is termed organic evolution’
The concept of organic evolution was first given by
CHARLES DARWIN which simply means “DESCENT WITH
MODIFICATION". This doctrine of organic evolution states that
the present day complex organisms have originated from the
earlier simpler forms of life during the course of ages by the
process of gradual change.
So, evolution is the constant process of gradual
change occurring in the organisms since the origin of life that
has given rise to enormous variety of complex organisms on
this earth.
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SPECIATION
Origin of new species from the existing one is called
speciation.
(REFER TEXT BOOK)
REPRODUCTIVE ISOLATION:
Reproductive isolation refers to the m mechanisms which
check the populations of two different groups from
interbreeding.
Reproductive isolation may happen in no. of ways:
(a) TEMPORAL ISOLATION: It is the failure of two groups to
interbreed because they breed at different times(seasons) of
the year.
(b)BEHVIOURAL ISOLATION: It prevents interbreeding of two
groups due to differences in courtship and mating behavior.
©PHYSIOLOGICAL ISOLATION: It is the functional
incompatibility in mating or in production and survival of
gametes.
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(d)MECHANICAL ISOLATION: It is the difference in size or
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EVIDENCES OF EVOLUTION:
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structure of genitalia between two groups that make
interbreeding difficult or impossible.
1.MORPHOLOGICAL AND ANATOMICAL EVIDENCES:
These evidenes are based on the similarities in the external and
internal features of the different kinds of organisms. These
features are:
(a)HOMOLOGOUS ORGANS: The organs which perform different
functions in different species but have similar basic structure
and similar embryonic origin are called homologous organs.
For example the forelimbs of a frog, a lizard, a bird and a human
being show similarity in basic structure. The forelimbs in all
these different vertebrates also develop in similar fashion.
However ,these organs perform entirely different
functions:theforelimbs of a frog help the animal to prop up the
front end of the body at rest and also act as shock absorbers
when the animal lands on the ground after a leap; the forelimbs
of a lizard are modified for creeping movements; the forelimbs
of a bird are modified for flying purposes, while forelimbs of a
human being are used for grasping.
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(b).ANALOGOUS ORGANS
The organs which are quite different in fundamental structure
and embryonic origin but performs same function and may
superficially look alike in entirely different species are called
analogous organs.
The wings of an insect and a bird are analogous
organs. It is so because both these organs in entirely different
animals perform similar functions,ie,they are used for flying
in the air. However, they are very different in structure .An
insect wing is a fold of membrane which is supported with
few muscles. On the other hand, wings of a bird are formed
of limb bones covered with flesh, skin and feathers
(c)VESTIGIAL ORGANS: The organs which occur in reduced
form and are useless to the possessor, but are homologous
to the fully developed, functional organs in the ancestors or
related forms are called vestigial organs. For instance
vermiform appendix of the large intestine and nictitating
membrane in the eye of human beings are vestigial organs.
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2.EMBRYOLOGICAL EVIDENCES:
Embryology refers to the study of development of an embryo
of organisms from fertilized egg to young one. Study of
embryology of different animal groups shows striking
structural similarities between them. The early embryos of all
the vertebrates resemble in shape and structure. The
resemblance of early embryos
offish,salamander,tortoise,chick,rabbit and man is so close
that it is very difficult to distinguish them from each other.
This similarity among the early embryos shows that all the
vertebrates have evolved from a common ancestor. Above
observations led EARNST HAECKEL to propose a law which is
popularly known as the biogenetic law. This law can be briefly
put in three words ontogeny repeats phylogeny or ontogeny
recapitulates phylogeny. It simply means that an organisms
repeats its ancestral history during its embryonic
development.
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BIOGENETIC LAW
The biogenetic law states that an animal in its individual
embryonic development from egg to adult repeats or
recapitulates in condensed form the stages through which its
ancestors have passed in the course of their evolution.
PALAEONTOLOGICAL EVIDENCES
Paleontology is the study of past life based on the fossil
record.
Normally, when the organisms die, their bodies are
decomposed and lost.However,if the body or some of its
parts remain in an environment that does not let it
decompose completely, the body or its parts will eventually
be preserved leave the impressions. For instance, if a dead
insect gets caught in mud, it will not decompose quickly; the
mud will eventually harden and retain the impressions of the
body parts of the insect.Thus,the fossils are the impressions
of the dead animals and plants that lived in the remote past.
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SOLVED NCERT TEXT BOOK INTEXT
QUESTIONS:
1.If a trait exists in 10% of a population of an asexually
reproducing species and a trait B exists in 60% of the same
population, whish trait is likely to have arisen earlier.
Ans.Trait B
In asexual reproduction, there is no reshuffling of
traits.Thus,appearance of few new traits in the population
occurs due to mild inaccuracy in DNA copying
process.Hence,few new traits occur than the already existing
traits.
2.How does creation of various in species ensure survival?
Ans.The process of natural selection selects the species with
favorable variations according to the surrounding conditions
and prevailing environment. The individuals whose variations
are useful, increase in number through differential reproduction
and species survival is ensured.
3.A man with blood group A marries a woman with blood
group O.Their daughter has blood group O.Is this
information enough to tell you which of the traits blood
group A or O is dominant? why or why not?
Ans.This information is not enough. This is because each
individual is carrying two allels.The recessive trait can occur
only when two alleles are similar. If blood group A is
dominant and O is recessive, then daughter can have blood
group O only when both recessive alleles occur together in
mother, and father has one allele of O and other of A.
4.Why are traits acquired during the life time of an individual
not inherited?
Ans.It is so because acquired characters involve changes in the
non-reproductive tissues only and do not bring any change in
the DNA. Any alteration in the DNA,in fact, is passed through
germ cells to the next generation.
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5.Why are small numbers of surviving tigers a cause of worry
from the point of view of genetics'?
Ans.Tigers are surviving in limited numbers. If some natural
calamity kills these small population of tigers, they will
suddenly become extinct as per genetic drift phenomenon.
6.Will geographical isolation be a major factor in the speciation
of a self pollinating plant species? Why or why not?
Ans.Geographical isolation will not be a major factor for the
speciation of a self-pollinating plant species because the
pollination is occurring on the same plant and a natural
barrier can occur between different organisms.
7.What are fossils? What do they tell us about the process of
evolution?
Ans.Fossils are the remains or impressions of the dead animals
and plants that lived in the remote past. They are the direct
evidences in support of organic evolution and help in
determining evolutionary relationship between organisms.
8.Can the wing of a butterfly and the wing of a bat be
considered homologous organs? Why or why not?
Ans.No.It is so because the wings of a butterfly and a wing of a
bat perform similar functions,ie,they are used for flying in the
air.However,they are very different in fundamental structure
and embryonic origin. In other words, they are analogous
organs. The superficial similarity of these organs is due to
adaptation to flying rather than to inheritance from a
common ancestor.
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