XII BIOLOGY
CH-5 - ORIGIN AND EVOLUTION OF LIFE
Organic Evolution-
i)Evolution
(Latin word, e = from; volvere = to roll) : The act of unrolling or unfolding
of nature. ii)Organic evolution
: Organic evolution is a slow, gradual, continuous and irreversible changes
through which the present day complex forms have descended from their simple,
pre-existing forms of the past
Lamarck’s theory (Theory of origin of acquired characters/inheritance of acquired characters and use and disuse of organs) :
The traits are acquired due to internal force,
changes in environment, new needs and the use and disuse of organs. This gives
rise to new species after several generations. Lamarckism was disproved by
August Weismann.
Weismann’s theory of Germplasm : Variations produced in somatic cells (somatoplasm) are not inherited while variations produced in germ cells (germplasm) are inherited to next generation.
Darwinism- Darwinism or theory of origin of species by Natural Selection.
Darwin’s book : ‘The origin of species by Natural Selection’, wrote in 1859 after observations of variations between the tortoises and finches on Galapagos islands.C. Lyell’s viewpoint on which Darwin’s theory was based : The natural forces that existed in the past are same as those existing at present.Wallace also made similar observations. R. Malthus provided the idea that increase in human population leads tocompetition and struggle for existence of human species. .
Five main postulates Darwinism :
i.Overproduction (Prodigality of nature).
iii.Organic variationsiv.
iv)Natural selection (H. Spencer termed natural selection as ‘survival of fittest’)
i. Over production or prodigality of production:
All
organisms have a natural tendency to over produce. If this tendency is not
checked, then even a single species of a plant or animal will occupy the entire
space available on the earth.
ii. Struggle for existence: Organisms multiply in geometric ratio, but space and food remain constant leading to competition for survival. Increase in the number of species leads to a competition called struggle for existence.
a.Intra-specific struggle: It is the competition among the individuals of the same species.. e.g. struggle between cow and cow, horse and horse, deer and deer, etc. for getting grass.
i.Nature selects the organisms with favourable variations.
ii. Organisms with favourable variations are fittest to survive and they get selected in the process of evolution.
iii. These organism transmits favourable characters to the next generation. iv. In succeding generation also natural selection occurs. Hence theory is known as natural selection.
v. Origin of new species: According Darwin, useful variations appear in every generation and are inherited from one generation to another.
Evidences Darwinism include :
Evolution of long-necked Giraffe came to existence. Black colour peppered moths evolved gradually as new species.DDT resistance
(i) Evolution
of long-necked Giraffe to pluck and eat
more leaves from
tall trees and
woody climbers. This adaptation became fixed in the life for survival.
The Giraffe borne tall could survive in famine heat areas. This adaptation was transmitted to their offspring. This is how, present long-necked Giraffe came to existence. (ii) Black colour peppered moths evolved gradually as new species.
(iii) DDT resistance in mosquitoes-intensive DDT
spraying destroyed all types
of mosquitoes.
However some mosquitoes developed resistance to DDT
and survived the
on slaught of
DDT spray.
Such resistant mosquitoes survived and reproduce giving rise to more resistant offspring.
Drawbacks and Objections to Darwnism :
• Darwin wrongly considered minute non- heritable fluctuating variation as principal factors.
They do not form part of evolution.
• He
did not distinguish somatic and germinal variation and considered all
variations are heritable.
• He did not explain the ‘arrival of the fittest’.
He did not explain the cause,
origin and inheritance of variations and of vestigial organs.
• He
could not explain the extinction of species.
•
Intermediate
form during evolution were not recognised.
•
He
could not explain existence of neutral flowers and the sterility of hybrids.
• Mutation
Theory-
• 1.
Hugo de Vries proposed mutation theory based on his observations on Oenothera
lamarckiana.
• 2.
Though offspring resemble their parents in many characters, some sudden and
spontaneous variations are seen in them, which is said to be mutations or
discontinuous variations.
• Main features of mutation theory :
•
Large,
sudden and discontinuous variations, inheritable changes in a population which
provide the raw material for organic evolution.
• Mutation
may be useful or harmful. Useful mutations are selected by nature.
•
Accumulation
of useful mutations over a period of time leads to the origin and establishment
of new species, while harmful mutations are eliminated by nature or can remain
in population.
Objections to Mutation Theory :
1.The
large and discontinuous variations were chromosomal aberrations which bring
about minor changes.
2.Rate
of mutation is very slow.
3.Chromosomal
aberrations are unstable and hence not important in evolution.
Speciation
(Formation of new species) :
Small
Darwinian variations are directional. Variations due to mutations are large,
sudden, random.
Darwin’s
opinion : Gradual, inheritable variations over a long period of time, lead to
speciation.
De
Vries’s opinion : Mutations cause speciation
Modern Synthetic Theory of Evolution-
R.
Fischer, J. B. S. Haldane, T. Dobzhansky, J. Huxley, E. Mayr, Simpson,
Stebbins, Fisher, Sewall Wright, Medel, T. H. Morgan, etc. together have
synthesised the modern theory of evolution.
Stebbins suggested five key factors for evolution : Gene mutations, chromosomal mutations, genetic recombinations, natural selection and reproductive isolation → together bring about evolution of new species.
Population : All individuals of the same species form population.
‘Mendelian
population’ : Small groups of interbreeding populations.
Gene
pool :
Gene pool is the total genetic information or sum total of genes of all
individuals in a Mendelian population.
Factors
affecting gene pool :
Migration, replacement of one generation by another in the Mendelian
population.
Gene
frequency :
The proportion of an allele in the gene pool, to the total number of alleles at
a given locus.
Factors of Modern synthesis theory:
Factors of Modern synthesis theory:
a)Genetic variation-
i.
Gene Mutation
: Sudden permanent heritable change in the genetic material is called mutation.
Single gene mutations are called point mutations. Chromosomal aberrations and
ploidy too cause mutations. All mutations lead to variations.
iii
. Gene flow :
Movement of genes into or out of a population. Migration of organism, or
gametes (dispersal of pollens) or segments of DNA (transformation).
iv.
Genetic drift :
Any
random fluctuation (alteration) in allele frequency, occurring in the natural
population by pure chance. Smaller populations may show genetic drift.
v.
Chromosomal aberrations :
The
structural, morphological change in chromosome due to rearrangement cause
chromosomal aberrations.
Types of aberrations :
a.Deletion
:
Loss of genes from chromosome.
b.Duplication
:
Repetitions of genes or doubling of chromosome number.
c.Inversion
:
Sequence of the genes get inverted due to 180° twist.
d.Translocation
:
Transfer (transposition) of a part of chromosome or a set of genes to a
non-homologous chromosome.
Chromosomal aberrations
b. Natural selection :
•
Main
driving force behind the evolution. It brings about evolutionary changes by
favouring differential reproduction of genes that bring about changes in gene
frequency from one generation to next generation.
•
The
‘fittest’ (well adapted) survives and leaves more progeny. E.g. Industrial
melanism in peppered moth (Biston betularia) is example of natural selection in
action.
•
It
brings about evolutionary
changes by favouring differential
reproduction of genes that
bring about changes
in gene frequency from one generation to next
generation.
•
Selection against harmful mutations leads to a
mutation balance in which allele frequency of harmful
recessives remain constant generation after generation.
•
Natural
selection encourages those
genes or traits that assure highest degree of adaptive efficiency between
population and its environment.
•
Industrial
melanism is one of
the best example for natural selection. In Great Britain, before
industrilisation (1845) grey white
winged moths (Biston
betularia) were more in
number than black-winged moth (Biston carbonaria).
•
These moths are nocturnal and during day time they
rest on tree
trunk. White-winged moth can
camouflaged (hide in the background) well with the lichen covered trees that
helped them to escape
from the predatory
birds.
•
on other hand, the black-winged moth resting on
lichen covered tree
trunks were easy
victims for the predatory birds and their number was reduced.
•
During industrial
revolution, large number of
industries came up in Great Britain. The industries released black sooty smoke
that covered and killed the lichens growing on tree and turn the tree black due
to pollution.
•
This change become an advantage to the black-winged
moth that camoflaged well with the black tree trunks and their number increased
while the white-winged moth become victims to predatory birds due to which
their number reduced.
•
Thus natural selection has resulted in the establishment
of a phenotypic
traits in changing the
environmental conditions.
c.Isolation :
•
The
separation of the population of a particular species into smaller units which
prevents interbreeding between them is isolation.
• Isolating mechanisms : Barrier which prevents gene flow or exchange of genes between isolated populations.
It is also
called as physical
isolation. It occurs when an
original population is divided into
two or more
groups by geographical barriers such as river, ocean,
mountain, glacier etc.
These
barriers prevent interbreeding between isolated groups.
The
separated groups are
exposed to different kinds
of environmental factors
and they acquired new
traits by mutations.
The separated
populations develop distinct
gene pool and they
do not interbreed.
Thus, new species have
been formed by
geographical isolation. E.g. Darwin’s Finches.
II.
Reproductive Isolation :
Reproductive
isolations occurs due
to change in genetic
material, gene pool
and structure of genital
organs.
It prevents interbreeding between population.
Types of Isolating Mechanisms :
A) Pre-mating or pre-zygotic
isolating mechanism :
This mechanism prevent fertilization and zygote
formation.
i.
Habitat isolation or (Ecological isolation)
:
Members of a
population living in the
same geographic region
but occupy separate habitats so
that potential mates do not meet.
ii.
Seasonal or temporal isolation :
Members of a population living in the same
geographic region but are sexually mature at different years or different times
of the year.
iii. Ethological isolation : Due to specific mating
behaviour the members of population do not mate.
iv.
Mechanical Isolation : Members of two population
have difference in the structure of reproductive organs.
B). Post-mating or Post-zygotic barriers :
i.Gamete
mortality –
Gametes have a limited life span. Due to one or the
other reasons, if union of the two gametes does not occur in the given time, it
results in the gamete mortality.
ii.
Zygote mortality –
Here, egg is
fertilized but zygote dies due to one or the other reasons.
iii.
Hybrid sterility –
Hybrids develop to maturity but become sterile due to failure of proper gametogenesis (meiosis).
e.g. Mule is an intergeneric hybrid which is sterile.
Mechanism of organic evolution-
Type of Natural selection :
•
Stabilizing selection : (Balancing selection)
•
Here more individuals of a population acquired a
mean character value.
•
It tends to favour the intermediate forms and
eliminate both the phenotypic extreme. For e.g. More number of infants with
intermediate weight survive better as compare to those who are over-weight or
under-weight.
•
It reduces variations.
•
It does not lead to evolutionary change but tend to
maintain phenotypic stability within population, therefore, it is described as
stabilizing selection.
Genetically stabilizing selection represents a situation where a population is adapted to its
•
In this type, more individuals acquired value other
than the mean character value.
•
Naural selection acts to eliminate one of the
extremes of the phenotypic range and favour the other. e.g. systematic
elimination of homozygous recessives.
•
Directional selection operates for many generations,
it results in an evolutionary trend within a population and shifting a peak in
one direction.
•