Subject - Biology
Notes of XII Science
CH.5 Origin and Evolution of Life
5.6 Modern Synthetic Theory of Evolution :
Main Contributors of Modern Synthetic Theory of Evolution :
R. Fischer, J.B.S.Haldane, T.Dobzhansky, J. Huxley, E.Mayr, Simpson, Stebbins, Sewall Wright, Mendel, T.H. Morgan etc.
Five key factors : Stebbins in his book discussed five key factors as gene mutation, mutation in chromosome structure & number, genetic recombinations, natural selection and reproductive isolation contributed in evulution of species.
Q. Define Population ( 1 Mark)
Def : All individuals of the same species constitute a population. e.g. Human being population.
Q.Define Mendelian population ( 1 Mark)
Small interbreeding group of a population is called Mendelian population.
Q. Define Gene Pool ( 1 Mark)
Def Gene Pool : The sum total of genes of all individuals of interbreeding population is called Gene Pool.
Q.Define Gene frequency (1 Mark)
The proportion of an allele in the gene pool as compared with other alleles at the same locus is called Gene frequency.
Q.Which are the concepts of Modern synthetic theory (1 Mark)
Three main concepts are a. Genetic variation b. Natural Selection c. Isolation
Q. Define Genetic variation ( 1 Mark)
The change in gene and gene frequencies is known as Genetic variation.
Q. What is Genetic variation? Explain the different factors responsible for genetic variations. (3 Marks)
The change in gene and gene frequencies is known as Genetic variation.
Factors causing Genetic Variation :
A. Gene Mutation :
Def : Changes in the chemical make up of a gene is called Gene mutation.
Point Mutation : When mutation occurs within the single gene is called point mutation.
This leads to change in the phenotype of organism causing
variation.
B. Genetic recombination : Exchange of genetic material occur between non-sister chromatids of homologus chromosomes during gamete formation in sexually reproducing organisms is called Genetic recombination.
It produces new genetic combinations which result in variation.
C. Gene flow : Transfer of genes between genetically different populations is called Gene flow.
Gene movement occur in the form of migration of organism or gametes.
Gene flow also changes gene frequency causing evolutionary changes.
D. Genetic drift :
i.Any random fluctuation in allele frequency occuring in natural population by pure chance is called Genetic drift.
ii. Smaller populations have greater chances for genetic drift.
iii.e.g. When the size of a population is severely reduced due to natural disasters as earthquakes, floods,fires which cause elimination of particular alleles from a population causing Genetic drift.
iv.Genetic drift is important factor for evolutionary change.
E. Chromosomal aberrations : The structural morphological change in chromosome due to rearrangement is called chromosomal aberrations.
Chromosomal aberrations occur due to deletion, duplication, inversion and translocation
Q.Explain how chromosomal aberrations occur. (3 Marks)
Chromosomal aberrations occur due to following factors
a. Deletion : Loss of genes from chromosome.
b. Duplication : Genes are repeated or doubled in number on chromosome.
c. Inversion : A particular segment of chromosome is broken and gets reattached to the same chromosome in inverted position. Here no loss or gain of genes.
d. Translocation : Transfer of a part of chromosome or set of genes to a nonhomologus chromosome is called translocation.
(Refer figures of Deletion, Duplication, Inversion & Translocation in Textbook page 101)
Natural selection:
Q. What is natural selection (1 Mark)
The process by which better adapted organisms grow & produce more number of offsprings in the population is called Natural selection.
Q. By talking industrial melanism as one example explain the concept of natural selection ( LA Que.)
OR
Q.Explain Natural selection with example of Industrial melanism
(LA Que.)
1. Industrial melanism is one of the best example of natural selection.
2.It is related to development of dark coloured pepperered moth due to air pollution during industrial revolution in Great Britain.
3. In Great Britain grey white winged moths ( Biston betularia )
were more in number than black winged moth ( Biston carbonaria)
4. White winged moth well adapted with lichen covered trees & protected from predatory birds.
5.Black winged moth resting on lichen covered tree trunks became easy victims for predatory birds & their number reduced.
6. During industrial revolution due to black sooty smoke released by industries lichen killed & tree trunk became black due to air pollution.
7. Black winged moth well adapted and their number increased & white winged moth become victim & their number reduced.
8. Thus natural selection established phenotypic trait relating with changing environmental conditions.
Isolation :
Q. What is isolation (1 Mark)
Separation of a single population into several subunits which prevent interbreeding between them is called Isolation.
Q. Enlist types of Isolation (1 Mark)
Isolation is of two types as
A. Geographical isolation
B. Reproductive isolation.
Q. Write a note on Geographical isolastion (2 Mark)
1. Def : The isolation occur when an original population
divided into two or more groups by geographical barriers such as river, ocean,mountain, glacier is called Geographical isolation.
2. These barriers prevent interbreeding between isolated groups.
3. The separated groups are exposed to different kinds of environmental factors.
4. They acquired new traits by mutation.
5. The separated population develop distinct gene pool & they do not interbreed.
6. Thus new species are formed by geographical isolation.
e.g. Darwin’s Finches.
Reproductive isolation :
Q.What is Reproductive isolation? Describe different types of
Reproductive isolation. ( LA )
Isolation which occur due to change in genetic material, gene pool and structure of genital organs is called Reproductive isolation.
Types of Reproductive isolation :
A. Pre-mating or pre- zygotic isolation: This mechanism prevent fertilization & zygote formation. It is of four types as follows.
i. Habitat or Ecological isolation : Members of a population living in same geographic region but occupy separate habitat which prevent interbreeding.
ii. Seasonal or temporal isolation : Members of a population attain sexual maturity at different time which prevent interbreeding.
iii. Ehological isolation :Members of two populations have different mating behaviour which prevent interbreeding.
iv. Mechanical isolation : Members of two populations have difference in the structure of reproductive organs which prevent interbreeding.
B. Post mating or Post zygotic barriers :
It is of three types as follows
i. Gamete mortality : Sperms transfer occur but egg is not fertilized due to gametes have limited life span.
ii. Zygote mortality : Here egg is fertilized but zygote dies due to one or other reasons.
iii. Hybrid sterility :
Hybrids develop upto maturity but become sterile due to
failure of proper gametogenesis.
e.g. Mule is sterile intergeneric hybrid.
5.13 Human Evolution :
Q.Enlist Special charecters acquired by man in course of evolution.(2 Marks)
i. Increase in size & complexity of brain.
ii. Increase in cranial capacity with increase intelligence.
iii. Erect posture, bipedal locomotion & opposable thumb.
iv. Shorter forelimbs & larger hindlimbs.
v. Distinct chin
vi. Broad pelvic girdle vii. Stereoscopic vision viii. Social & cultural development.
Q. Complete the chart (2 Marks)
Appearance Human evolution
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Ape like Dryopithecus
Man like Ramapithecus
Connecting link between Australopithecus
Ape & Man
Handy man like Homo habilis
Ape man Homo erectus
Advanced prehistoric man Neanderthal man
Modern man Homo sapiens
Q. Complete the chart ( 2 Marks)
Site of fossil record Human evolution
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Lake victoria of Africa Dryopithecus
Shivalik Hills in India Ramapithecus
Toung in South Africa Australopithecus
Olduvai Gorge Tanzania Homo habilis
Java and peking Homo erectus
Neanderthal valley in Germany Neanderthal man
Africa Homo sapiens
Q. Complete the chart (2 Marks)
Period Human evolution
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Miocene epoch 20-25 mya Dryopithecus
Miocene & early pliocene Ramapithecus
epoch 14 - 12 mya
Late pliocene or early pleistocene Australopithecus
epoch about 4 - 1.8 mya
Late pliocene or early pleistocene Homo habilis
2.5 to 1.4 mya
Middle pleistocene epoch Homo erectus
1.5 mya ago
Late pleistocene epoch Neanderthal man
100000 to 40000 yrs ago
Q. Complete the chart (2 Marks)
Posture Human evolution
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Semi erect Dryopithecus
erect Ramapithecus
upright Australopithecus
erect Homo habilis
erect Homo erectus
erect Neanderthal man
erect Homo sapiens
Q.Complete the chart (2 Marks)
Cranial capacity Human evolution
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----- Dryopithecus
----- Ramapithecus
450 to 600 cc Australopithecus
650 to 800 cc Homo habilis
900 cc Homo erectus
1400 cc Neanderthal man
1450 cc Homo sapiens
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(According to reduce syllabus during 2020 - 2021)
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