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Chapter 5: Origin and Evolution of Life [ Exercise,Solutions,Notes ]

Origin and Evolution of Life [ Exercise,Solutions,Notes ]

Origin and Evolution of Life [ Exercise,Solutions,Notes ]

Multiple choice question,.

Exercise | Q 1.01 | Page 117
Who proposed that the first form of life could have come from per- existing nonliving organic molecules?
Alfred Wallace
Oparin and Haldane
Charles Darwin
Louis Pasteur
Solution:
Oparin and Haldane

Exercise | Q 1.02 | Page 117
The sequence of origin of life may be-
Organic materials- inorganic materials – Eobiont- colloidal aggregates- cell.
Inorganic materials – organic materials – colloidal aggregates – Eobiont- cell
Organic materials- inorganic materials – colloidal aggregates - cell
Inorganic materials- organic materials – Eobiont- colloidal aggregates - cell
Solution:
Inorganic materials – organic materials – colloidal aggregates – Eobiont- cell

Exercise | Q 1.03 | Page 117
In Hardy - Weinberg equation, the frequency of homozygous recessive individual is represented by:
p2
pq
q2
2pq
Solution:
q2 

Exercise | Q 1.04 | Page 117
Select the analogous organs.
Forelimbs of whale and bat
Flippers of dolphins and penguin
Thorn and tendrils of Bougainvillea and Cucurbita.
Vertebrates hearts or brains.
Solution:
Flippers of dolphins and penguin

Exercise | Q 1.05 | Page 117
Archaeopteryx is known as missing link because it is a fossil and share characters of both
Fishes and amphibians
Annelida and arthropoda
Birds and reptiles
Chordates and nonchordates
Solution:
Birds and reptiles


Exercise | Q 1.06 | Page 117
Identify the WRONG statement regarding evolution.
Darwin’s variations are small and directional
Mutations are random and nondirectional
Adaptive radiations leads to divergent evolution
Mutations are non - random and directional
Solution:
Mutations are non - random and directional

Exercise | Q 1.07 | Page 117
Gene frequency in a population remain constant due to –
Mutation
Migration
Random mating
Non- random mating
Solution:
Random mating

Exercise | Q 1.08 | Page 117
Which of the following characteristic is not shown by the ape?
Prognathous face
Tail is present
Chin is absent
Forelimbs are longer than hind limbs
Solution:
Tail is present

Exercise | Q 1.09 | Page 117
______ can be considered as a connecting link between ape and man.
Australopithecus
Homo habilis
Homo erectus
Neanderthal man
Solution:
Australopithecus can be considered as connecting link between ape and man.

Exercise | Q 1.1 | Page 117
 The cranial capacity of Neanderthal man was
600 cc
940 cc
1400 cc
1600 cc
Solution:
1400 cc

Very short answer question.

Exercise | Q 2.01 | Page 117
Define the Gene pool
Solution:
The total genetic information encoded in the sum total of genes in a Mendelian population is called gene pool.

Exercise | Q 2.01 | Page 117
Define the Gene frequency
Solution:
The proportion of an allele in the gene pool, to the total number of alleles at a given locus, is called gene frequency.

Exercise | Q 2.01 | Page 117
Define Organic evolution.
Solution:
Organic evolution can be defined as slow, gradual, continuous and irreversible changes through which the present-day complex forms of the life developed (or evolved) from their simple pre-existing forms.

Exercise | Q 2.01 | Page 117
Define Population.
Solution:
According to this theory all individuals of the same species constitute a population.

Exercise | Q 2.01 | Page 117
Define Speciation.
Solution:
The process of formation of a new species from the pre-existing species is called speciation.

Exercise | Q 2.02 | Page 118
What is adaptive radiation?
Solution:
The process of evolution which results in the transformation of original species to many different varieties is called adaptive radiation.

Exercise | Q 2.03 | Page 118
If variation occurs in a population by chance alone and not by natural selection and brings a change in frequencies of an allele. What is it called? 
Solution:
If the variation in a population occurs by chance alone and not by natural selection and brings about a change in frequencies of an allele, it is called genetic drift.


Exercise | Q 2.04 | Page 118
State the Hardy – Weinberg equilibrium.
Solution:
The Hardy-Weinberg equilibrium law states that ‘at the equilibrium point, both the gene (allele) frequency and genotypic frequency remain constant from generation to generation’.

Exercise | Q 2.05 | Page 118
What are homologous organs?
Solution:
Homologous organs are those organs, which are structurally similar but perform different functions.

Exercise | Q 2.06 | Page 118
What is vestigeal organ?
Solution:
Vestigeal organs or rudimentary organs are imperfectly developed and non-functional, degenerate structures that were functional in some related and other animals or in ancestors.

Exercise | Q 2.07 | Page 118
What is the scientific name of the modern man?
Solution:
The scientific name of the modern man is Homo sapiens.

Exercise | Q 2.08 | Page 118
What is coacervate?
Solution:
Coacervates are colloidal aggregations of hydrophobic proteins and lipids (lipoid bubbles).

Exercise | Q 2.09 | Page 118
Which period is known as “age of Reptilia”?
Solution:
Jurassic period is known as age of Reptilia.

Exercise | Q 2.1 | Page 118
Name the ancestor of human which is described as a man with an ape brain.
Solution:
Australopithecus is the ancestor of humans which is described as a man with an ape brain.

Short answer question,. 


Exercise | Q 3.1 | Page 118
Write a note on Genetic drift.
Solution:
1. Any alternation in allete frequency in the natural population by chance, is called genetic drift. e.g. Elimination of a particular allele from a population due to events like accidental death prior to mating of an organism.

2. The concept of genetic drift was first given Sewall Wright, and is hence also called as the Sewall Wright effect.

3. Genetic drifts are random or directionless.

4. The effect of genetic drift is more significant in small population than in large population.

5. Due to genetic drift, some alleles of a population are lost or reduced by chance and some others may be increased.

6. Sometimes, a few individuals become isolated from the large population and they produce a new population in a new geographical areas. The allele frequency of the new population becomes different. The original drifted population (i.e. colonizing ancestor/ pioneer) becomes ‘founders’ and the effect is called the founder effect.

7. A bottleneck effect is seen when much of a population is killed due to a natural disaster and only a few remaining individuals are left to begin a new population.

Exercise | Q 3.2 | Page 118
Enlist the different factors that are responsible for changing gene frequency.
Solution:
The four major factors that are responsible for changing gene frequency are as follows:

1. Gene flow (Migration):

Gene flow is the movement of genes into or out of a population. Gene movement may be in the form of migration of organism, or gametes (dispersal of pollens) or segments of DNA (transformation). Thus, gene flow alters gene frequency causing evolutionary changes.

2. Genetic drift:

Any random fluctuation (alteration) in allele frequency, occurring in the natural population by pure chance, is called genetic drift. For example, when the size of a population is severely reduced due to natural disasters like earthquakes, floods, fires, etc. cause the elimination of particular alleles from a population. Smaller populations have greater chances for genetic drift. Thus, genetic drift will result in the change in the gene frequency and has the potential to bring about evolutionary change.

3. Natural selection:

Natural selection is the process by which better adapted organisms grow and produce more offsprings in the population. It brings about evolutionary changes by favouring differential reproduction of genes that bring about changes in gene frequency from one generation to the next generation.

iv. Mutations:

Sudden permanent heritable changes are called mutations. Mutation can occur in the gene, in the chromosome and in the chromosome number. Mutation that occurs within the single gene, is called point mutation or gene mutation or in a larger segment of genes by chromosomal aberrations. Both point mutations and chromosomal aberrations can alter gene frequency. Mutation leads to the change in the phenotype of the organism, causing variation.

Exercise | Q 3.3 | Page 118
Draw a graph to show that natural selection leads to disruptive change.
Solution:
Origin and Evolution of Life [ Exercise,Solutions,Notes ]


Exercise | Q 3.4 | Page 118
Give the significance of fossils.
Solution:
1. Fossils are the dead remains of plants and animals that lived in past in various geological layers.

2. The study of fossils provides the most convincing and direct evidence of evolution.

3. Study of fossils tells us that life forms were not the same millions of years ago (mya).

4. The geological time's scale is based on fossil records.

5. From the fossil records we can trace the complete evolutionary history of animals.

6. Study of fossils is an important aspect of evolution since it can be used in paleontology and anthropology for determining age of the fossils and deducing information about their ancestors.


Exercise | Q 3.5 | Page 118
Write the objections to Mutation theory of Hugo de Vries.
Solution:
Objections to Mutation Theory are as follows:

1. The large and discontinuous variations observed by Hugo de Vries were actually due to chromosomal aberrations. Gene mutations usually bring about only minor changes.

2. Rate of mutation is very slow as compared to the requirement of evolution.

3. Chromosomal aberrations have little significance in evolution as they are quite unstable.

Exercise | Q 3.6 | Page 118
Give an example of disrruptive selection
Solution:
1. In this selection, nature selects extreme phenotypes and eliminate intermediates.

2. This results in the formation of two peaks in the distribution of traits.

3. This kind of selection is rare.

4. It ensures the effect on the entire gene pool of a population, considering all mating types or systems.

e.g. Disruptive selection was observed in the different beak sizes of African seed cracker finches. The birds have different sizes of the beak and they feed on seeds. The available seeds were of two kinds i.e., small and large-sized seeds. Large beak sized birds feed on large seeds while small beak sized birds feed on small seeds and their number was increased. Intermediate beak sized birds were unable to feed on either type of seeds so their population decreased gradually and then were eliminated by natural selection.

Exercise | Q 3.6 | Page 118
What is disrruptive selection?
Solution:
Disruptive Natural selection is a selection in which more number of individuals acquire peripheral character value at both ends of the distribution curve.

Exercise | Q 4 | Page 118
Match the following:
Column- IColumn- II
1. August Weismanna. Mutation theory
2. Hugo de vriesb. Germplasm theory
3. Charl Darwinc. Theory of acquired characters
4. Lamarkd. Theory of natural selection

Solution:
Column- IColumn- II
1. August Weismannb. Germplasm theory
2. Hugo de vriesa. Mutation theory
3. Charl Darwind. Theory of natural selection
4. Lamarkc. Theory of acquired characters
Long answer question,.

Exercise | Q 5.1 | Page 118
Would you consider wings of butterfly and bat as homologous or analogous and why?
Solution:
The wings of butterfly and bat are analogous but not homologous.

Examples of analogous structures are as follows:

1. Wings of butterfly (insects) and of birds look superficially alike but they are not anatomically similar structures though they perform similar functions.

2. Eye of the octopus (mollusca) and of mammals. They differ in their retinal position, the structure of lens, and the origin of different eye parts.

3. The flippers of penguins (birds) and dolphins (mammals).

4. Sweet potato (root modification) and potato (stem modification) store food in form of starch.

Exercise | Q 5.2 | Page 118
 What is adaptive radiation? Explain with suitable example.
Solution:
1. The process of evolution which results in the transformation of original species to many different varieties is called adaptive radiation.

2. Darwin’s Finches is one of the best examples of adaptive radiation. During his visit to Galapagos Islands, Charles Darwin also noticed a variety of small birds. These birds are now called Darwin’s finches. Darwin concluded that the American mainland species of the bird was the original one from which they migrated to the different islands of Galapagos. These birds adapted to the different environmental conditions of these islands. From original seed-eating features, many other forms with altered beaks evolved into insectivorous features.

3. Another example of adaptive radiation is Australian Marsupials. In Australia, there are many marsupial mammals who evolved from a common ancestor.

Exercise | Q 5.3 | Page 118
By taking industrial melanism as one example. Explain the concept of natural selection.
Solution:
  1. Natural selection encourages those genes or traits that assure the highest degree of adaptive efficiency between the population and its environment.
  2. Industrial melanism is one of the best examples of natural selection.
  3. In Great Britain, before industrialization (1845) grey white-winged moths (Biston betularia) were more in number than black-winged moth (Biston carbonara).
  4. These moths are nocturnal and during the day time they rest on a tree trunk.
  5. White-winged moths were camouflaged (hide in the background) well with the lichen-covered trees that helped them to escape from the predatory birds.
  6. However, the black-winged moth resting on lichen-covered tree trunks were easy victims for the predatory birds and their number was reduced.
  7. During the industrial revolution, large number of industries came up in Great Britain.
  8. The industries released black sooty smoke that covered and killed the lichens growing on a tree and turn the tree black due to pollution.
  9. This change became an advantage to the black-winged moths that camouflaged well with the black tree trunks and their number increased
  10. The white-winged moths however became victims to predatory birds due to which their number reduced. Thus, natural selection has resulted in the establishment of a phenotypic trait in the changing environmental conditions.

Exercise | Q 5.4 | Page 118
 Describe the Urey and Miller experiment.
Solution:
1. Apparatus and Procedure:

They designed a glass-apparatus called spark-discharge apparatus. The apparatus was first sterilized and evacuated. Methane, ammonia, and hydrogen gases were pumped in the proportion of 1:2:2 into the glass chamber. A tube carrying water vapour was also connected to the chamber. The lightning effect was mimicked by the action of electric discharge in the chamber.
The process of evaporation and precipitation was also stimulated by the use of heating mantle and condenser respectively.
The mixture of CH4, NH3, H2 was exposed continuously to electric discharge for several days causing the gases to interact, after which these were condensed.

2. Observation:

It was observed that the liquid collected in the U-tube turned brown.

3. Results and Conclusions:

Chemical analysis of this liquid reported the presence of simple organic compounds. (urea, amino acids, lactic acid, etc.). This experiment strongly supports that the simple molecules present in the Earth’s early atmosphere combined to form the organic building blocks of life.

Exercise | Q 5.5 | Page 118
Describe the different types of reproductive isolations.
Solution:
A number of isolating mechanisms are operated in nature and therefore divergence and speciation may occur.
The isolating mechanisms are of two types namely, geographical isolation and reproductive isolation.

1. Geographical Isolation:

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 rivers, oceans, mountains, glaciers, 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.

2. Reproductive Isolation:

Reproductive isolation occurs due to change in genetic material, gene pool and structure of genital organs. It prevents interbreeding between populations. Types of Isolating Mechanisms:

a. Pre-mating or pre-zygotic isolating mechanism:

This mechanism prevents fertilization and zygote formation.

i. Habitat isolation or (Ecological isolation): Members of a population living in the same geographic region but occupying separate habitats in such a way that potential mate 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 behavior the members of the population do not mate.

iv. Mechanical Isolation: Members of two populations have a difference in the structure of reproductive organs.

2. Post-mating or Post-zygotic barriers:

i. Gamete mortality: Gametes have a limited life span. Due to one or the other reasons, if the union of the two gametes does not occur in the given time, it results in gamete mortality.

ii. Zygote mortality: Here, egg is fertilized but the zygote dies due to one or the other reasons.

iii. Hybrid sterility: Hybrids develop to maturity but become sterile due to the failure of proper gametogenesis (meiosis).
e.g. Mule is an inter-generic hybrid that is sterile.

Exercise | Q 5.5 | Page 118
What is Isolation?
Solution:
It is the separation of the population of a particular species into smaller units which prevents interbreeding between them. A barrier that prevents gene flow or exchange of genes between isolated populations, is called isolating mechanism.

Exercise | Q 5.6 | Page 118
Explain the different factors responsible for genetic variations.
Solution:
Definition: Genetic variations are caused due to various aspects of mutation, recombination, and migration. The change in gene and gene frequencies is known as genetic variation.

Genetic variations are caused by the following factors:

a. Gene Mutation:
Sudden permanent heritable change is called a mutation. Mutation can occur in the gene, in the chromosome, and in the chromosome number. The mutation that occurs within the single gene is called point mutation or gene mutation.
Mutation leads to the change in the phenotype of the organism, causing variation.

b. Genetic recombination:
In sexually reproducing organisms, during gamete formation, the exchange of genetic material occurs between non-sister chromatids of homologous chromosomes. This is called crossing over. It produces new genetic combinations that result in variations. Fertilization between opposite mating gametes leads to various recombinations resulting in the phenotypic variations causing a change in the frequencies of alleles.

c. Gene flow:
Gene flow is the movement of genes into or out of a population. Gene movement may be in the form of migration of organism, or gametes (dispersal of pollens) or segments of DNA (transformation). Gene flow also alters gene frequency causing evolutionary changes.

d. Genetic drift:
Any random fluctuation (alteration) in allele frequency, occurring in the natural population by pure chance, is called genetic drift.
e.g. When the size of a population is severely reduced due to natural disasters like earthquakes, floods, fires, etc., it causes the elimination of particular alleles from a population. Smaller populations have greater chances for genetic drift. It will result in a change in the gene frequency. Genetic drift is also an important factor for evolutionary change.

e. Chromosomal aberrations:
The structural and morphological change in chromosome due to rearrangement is called chromosomal aberrations. It changes the arrangement of the genes (order or sequence) that results in the variation.
Chromosomal aberrations occur due to the following reasons:

1. Deletion: Loss of genes from the chromosome.

2. Duplication: Genes are repeated or doubled in number on the chromosome.

3. Inversion: A particular segment of the chromosome is broken and gets reattached to the same chromosome in an inverted position due to 180° twists. There is no loss or gain of the gene complement of the chromosome.

4. Translocation: Transfer (transposition) of a part of a chromosome or a set of genes to a non-homologous chromosome is called translocation. It is affected naturally by the transposons present in the cell.

Exercise | Q 5.6 | Page 118
What is Genetic variations?
Solution:
Genetic variations are caused due to various aspects of mutation, recombination, and migration. The change in gene and gene frequencies is known as genetic variation. 

Exercise | Q 6 | Page 118
Complete the chart.
EraDominating group
of animal
1. Cenozoic____________
2. _________Reptiles
3. Palaeozic____________
4. _________Invertebrates
Solution:
EraDominating group
of animal
1. CenozoicMammals
2. MesozoicReptiles
3. PalaeozicAmphibians
4. PalaeozoicInvertebrates


origin and evolution of life notes pdf

5.1 Origin of life : (Protobiogenesis) The living matter shows attributes or characters like responsiveness, growth, metabolism, energy transformations and reproduction. As far as origin of life is considered, it has remained an enigma for intellectuals at all times. Despite of advancements in various fields like biochemistry, astrobioloy, geography, molecular biology, etc. scientists are unable to ascertain the truth. Various theories and hypotheses have been proposed to find the probable answer to this question. 

a. Theory of special creation : It is the oldest theory and is based on religious belief without any scientific proof. It states that all living organisms are created by a super-natural power.

b. Cosmozoic theory/Theory of Panspermia: This theory advocates that life did not arise on the planet Earth. It may have descended to the earth from other planets in the form of spores or micro-organisms, called cosmozoa/ panspermia. Recently, NASA has reported fossils of bacteria-like organisms on a piece of Martian rock recovered from Antarctica

c. Theory of spontaneous generation (Abiogenesis) : According to this theory, life originated from non-living material spontaneously. This theory was disproved by Louis Pasteur.

d. Theory of biogenesis : According to this theory, living organisms are always produced from pre-existing living forms, by process called reproduction. Theory of biogenesis however could not explain origin of first life on earth but could explain only the continuity of life.

5.2 Chemical Evolution of Life (Self assembly theory of origin of life ) : According to this theory, life originated on earth by combinations of several chemicals through constant chemical reactions over a long period of time. This theory is also called self assembly theory of origin of life or biochemical origin of life. This theory was first formulated by Haeckel but later developed by the Russian scientist Alexander I. Oparin (1924) and British biologist J. B. S. Haldane (1929). The process of chemical evolution can be divided into following steps :

a. Origin of Earth and Primitive atmosphere: The origin of universe was explained by the Big-Bang theory of Georges Lemaitre (1931). According to this theory the Universe originated about 20 billion years ago by a single huge titanic explosion. As the universe expanded, the temperature decreased and various galaxies of solid objects were formed. Milky Way is one such galaxy of which our solar system is one small part. Earth is one of the planets of solar system and originated about 4.6 billion year ago. When formed, it was a rotating cloud of hot gases and cosmic dust called Nebula. The condensation and cooling resulted in stratification with heavier elements like nickel and iron passing to the core and lighter ones like helium, hydrogen, nitrogen, oxygen, carbon, etc. remaining on the surface. They formed the atmosphere of the earth. The primitive atmosphere of the earth was quite different from the present one and it was of a reducing type, devoid of free oxygen.

b. Formation of ammonia, water and methane: Primitive atmosphere was very hot. As it slowly cooled, the lighter elements started to react with each other. The early atmosphere was rich in hydrogen, carbon, nitrogen and sulphur of which hydrogen being more active, it reacted with other elements to form chemicals on earth like CH4 , NH3 , H2 O and H2 S. 

c. Formation of simple organic molecules : As temperature of the earth decreased, steam condensed into water that resulted in heavy rain fall and the earth gradually cooled. Rain water got accumulated on the land to form rivers, streams, lakes, seas and oceans. The atmosphere then did not contain ozone layer and thus ultra-violet radiations reached the surface of earth directly. Under the influence of available energy sources such as ultra-violet

rays, radiations, lightening and volcanic activities, the early molecules of hydrocarbons, ammonia, methane and water underwent reactions like condensation, polymerisation, oxidation and reduction. These reactions resulted in formation of simple molecules like monosaccharides, amino acids, purines, pyrimidines, fatty acids, glycerol, etc. All these simple organic molecules accumulated at the bottom of water bodies. Haldane described it as the ‘‘hot dilute soup’’ or ‘‘primitive broth’’. It did not show any degradation due to absence of free oxygen and enzymes.

d. Formation of complex organic molecules: The primitive broth was neutral and free from oxygen. Polymerisation took place and simple organic molecules aggregated to form new complex organic molecules like polysaccharides, fats, proteins, nucleosides and nucleotides. Polymerisation of amino acids formed protoproteins which later formed proteins. Formation of protein molecules is considered as landmark in the origin of life. Proteins (enzymes) accelerated the rate of other chemical reactions.

e. Formation of Nucleic acids : Nucleotides may have been formed by the reaction between phosphoric acid, sugar and nitrogenous bases (purines and pyrimidines). Number of nucleotides join together to form nucleic acids (RNA, DNA). Nucleic acids acquired self-replicating ability which is a fundamental property of living form.


f. Formation of Protobionts or Procells : Nucleic acids along with inorganic and organic molecules formed the first form of life called protobionts. Protobionts are the prebiotic chemical aggregates having some properties of living system.

I. Geographical Isolation : 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.

5.7 Mechanism of organic evolution : One has to give the importance to the population while considering the mechanism of evolution. It is the population that evolves and not its individual members. Individual’s role is to pass its genetic variation to its offspring. The following are the basic processes which bring about evolution viz. Mutations, gene recombination, gene flow (migration), genetic drift, natural selection, isolation and speciation.

Mutations - These are permanent heritable changes in the genetic material of an organism. Mutations are already described earlier in this chapter. Gene mutations produce new alleles which are added to gene pool. 

Gene recombination - These are variation produce due to coming together of alleles during sexual reproduction. Gene recombinations occur due to random union of gametes, anaphasic separation of chromosomes and crossing over

Gene flow - It is the transfer of gene during interbreeding of populations that are genetically different. As explained earlier in this chapter gene flow is due to emigration and imigration. Its brings about changes in the allele frequency. Genetic drift - Any alternation in allete frequency in the natural population by chance, is called genetic drift. Concept of genetic drift was first given Sewall wright, hence, called as Sewall wright effect. For example, elimination of a particular allele from a population due to events like accidental death prior to mating of an organism. 

Genetic drifts are random or directionless. The effect of genetic drift is more significant in small population than in large population. Due to genetic drift, some alleles of a population are lost or reduced by chance and some others may be increased. Some time, a few individuals become isolated from the large population and they produce new population in new geographical area. The allele frequency of new population become different. The original drifted population (i.e. colonizing ancestor/ pioneer) becomes ‘founders’ and the effect is called founder effect.

A bottle neck effect is seen when much of a population is killed due to a natural disaster and only a few remaining individuals are left to begin a new population. Natural selection - It is a process by which better adapted individuals with useful variations are selected by nature and leave greater or more number of progenies (Differential reproduction)

 

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Balbharati solutions for Biology 12th Standard HSC for Maharashtra State Board
Chapter 1: Reproduction in Lower and Higher Plants
Chapter 2: Reproduction in Lower and Higher Animals
Chapter 3: Inheritance and Variation
Chapter 4: Molecular Basis of Inheritance
Chapter 5: Origin and Evolution of Life
Chapter 6: Plant Water Relation
Chapter 7: Plant Growth and Mineral Nutrition
Chapter 8: Respiration and Circulation
Chapter 9: Control and Co-ordination
Chapter 10: Human Health and Diseases
Chapter 11: Enhancement of Food Production
Chapter 12: Biotechnology
Chapter 13: Organisms and Populations
Chapter 14: Ecosystems and Energy Flow
Chapter 15: Biodiversity, Conservation and Environmental Issues

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