ISC 12> ORIGIN AND EVOLUTION OF LIFE> 3.THEORIES OF EVOLUTION
Theories of evolution
Lamarckism: evidences in favour of Lamarckism (giraffe’s neck), criticism of Lamarckism;
Darwinism: basic postulates of Darwinism, drawbacks of Darwinism, Neo-Darwinism; Variations: causes of variation, Selected examples of natural selection (DDT resistance in mosquito, malaria in relation to G-6-P-D deficiency and sickle-cell anaemia); Artificial selection; adaptations (Lederberg’s replica plating experiment). Lederberg's replica plating experiment with Darwinian interpretation, mechanism of speciation, definition of gene pool;
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CLASS PRESENTATION |
Theories of evolution of life
Based on the evidences of evolution various theories have been put forth regarding the evolution of life. How organisms changed and new species developed. There were a number of theories put forth for this, However the most important are the theory postulated by
Lamarck and Darwin.
Lamarck and Darwin.
LAMARCKISM
Changes in the structure or function of any organ acquired during the life time of an individual in response to changes in the environment are inherited by the offspring and keep on adding over a period of time. These changes lead to the origin of new species.
This was given by JEAN LAMARCK as the theory of acquired characters. THEORY OF INHERITANCE OF ACQUIRED CHARACTERS.
LAMARKS POSTULATES:
1.New needs: There are changes occurring in the environment that creates new needs, these allow them to adapt better to the changed environment. These efforts lead to a change in habits or behaviour. 2.Acquisition of new characters or organs: the new characters can be acquired in two ways-
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JEAN LAMARCK
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EVIDENCES IN FAVOR OF LAMARCKISM1.Evolution of long neck in Giraffe.
According to Lamarck, Giraffes evolved from deer like ancestors. These organisms had short neck and fore limbs and grazed on grass. As the climatic conditions changed the rich vegetation changed to few trees. The leaves of these trees were the only source of food for these Giraffes. The ancestors of the Giraffe had to stretch their neck to obtain the leaves on the trees. This gradual stretching of the neck lead to increase in the neck and forelimbs. This was then transmitted to the next generation and thus Giraffes have long neck and fore limbs. 2.Evolution of feet of modern horse. They developed the power of fast running to protect themselves from enemies. Thus gradually there was lengthening of limbs and the middle digit with the reduction of other digits. 3.Evolution of snakes. According to Lamarck, the ancestors of snakes were limbed and lizard-like. They lived in thick jungles. Out of fear of mammals, these snakes started creeping on jungle floor and living in narrow crevices or burrows. For creeping among the vegetation or burrowing in the narrow crevices, they stretched their body, which gradually became elongated. The limbs were of no use and were a hindrance in creeping and burrowing, thus gradually disappeared. 4.Webbed feet in aquatic birds. Aquatic birds like ducks, swans and geese, etc., have arisen from terrestrial ancestor by developing web between the toes for wading in water (adaptation to aquatic mode of life-continuous use). Web developed because the ancestral forms had to spread their toes and stretch the skin between them to rest on water reduction in the size of wings due to their continuous disuse. |
CRITICISM OF LAMARCKISM
1.The tendency to increase in size can be seen in many forms, however evolution also shows reduction in size. For example, in Angiosperms, the trees are primitive forms whereas shrubs, herbs and grasses evolved later and are more advanced. Moreover, persons constantly using an organ does not necessarily strengthen it, for example people busy in reading and writing and using their eyes more than others, often develop impaired sight.
2.Inheritance of acquired characters: For example, if any of the parent becomes blind or deaf or lame before producing the offspring, they do not produce blind, deaf or lame offspring. Mutilations and wounds of parents do not appear in the offsprings. Small feet in Chinese women, piercing of nose and ears in Indian women are age old customs, yet these have no hereditary effect. The strong muscles of a wrestler are not inherited by his children.
3. Another major criticism was based on the experiment performed by AUGUST WEISMANN, he had cut off the tails of rats for about 80 generations, but tailless offsprings were never born.
4.Theory of germplasm– ‘Theory of continuity of germplasm’ proposed by Weismann (1892) and Mendel’s laws of Inheritance were hard blow to Lamarck’s theory of inheritance of acquired characters. According to germplasm theory, each organism has two types of cells:
(a) Germ cells, which pass on their hereditary material to next generation and are found in gonads.
(b) Somatic cells, which form the body and do not pass their hereditary material to the offsprings.
2.Inheritance of acquired characters: For example, if any of the parent becomes blind or deaf or lame before producing the offspring, they do not produce blind, deaf or lame offspring. Mutilations and wounds of parents do not appear in the offsprings. Small feet in Chinese women, piercing of nose and ears in Indian women are age old customs, yet these have no hereditary effect. The strong muscles of a wrestler are not inherited by his children.
3. Another major criticism was based on the experiment performed by AUGUST WEISMANN, he had cut off the tails of rats for about 80 generations, but tailless offsprings were never born.
4.Theory of germplasm– ‘Theory of continuity of germplasm’ proposed by Weismann (1892) and Mendel’s laws of Inheritance were hard blow to Lamarck’s theory of inheritance of acquired characters. According to germplasm theory, each organism has two types of cells:
(a) Germ cells, which pass on their hereditary material to next generation and are found in gonads.
(b) Somatic cells, which form the body and do not pass their hereditary material to the offsprings.
DARWINISM
1.CHARLES ROBERT DARWIN– origin of species by natural selection.
2.Theory of natural selection was announced by Charles Darwin and Alfred Russel Wallace. 3.Theory was based on– animals and plants in Galapagos islands, observation on animal distribution, analysis from data in essay of human population and controlled breeding and artificial selection in domestic animals. An interesting article Obesity? Big Feet? Blame Darwin Darwin's postulates |
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1.Overproduction: All living beings have an inherent tendency to produce
offspring of their own kind in large number for the perpetuation of their race. The number of their offspring is much more
than can be supported by a particular environment and can possibly survive. For
example: A Paramecium divides three times in 48 hours. If all its offspring survive and multiply, in five years the mass of paramecia will be equal to ten thousand times the mass of earth.
2.There is limited food and space. The space in the universe remains constant. The ultimate source of food for plants and animals also remains constant. Thus, the carrying capacity of the environment does not allow the population to grow indefinitely. Inspite of enormous reproductive potential of living beings under natural conditions the number of individuals of each species remains nearly constant over a long period of time.
3.Struggle for existence–According to Darwin individuals multiply in geometric ratio whereas space and food availability remains constant, so there is an intense competition and three fold struggle for existence. The struggle is intraspecific, interspecific and with the environment.
(a) Intraspecific struggle : This is struggle between the individuals of the same species. This is the toughest form of struggle as the needs of the individuals of the same species are identical.
(b) Interspecific struggle: This is the struggle between the individuals of different species.
(c) Struggle with the environment: This struggle of living beings is with the changes of environment such as heat, cold, drought, flood, storm, famine, light, food and shelter, etc.
4.Variations: The struggle for existence results in competition among the organisms. Thus the organisms adapt and differ from each other in shape, size, behaviuor etc. Variation is the law of nature. This leads them to change in accordance with the conditions to utilise the natural resources and survive successfully.
5.Natural selection and survival of the fittest. According to Darwin nature selects only those individuals which are with more favourable variations and are best adapted to the environment. The less fit and unfit organisms are left out by selection. This sorting out of individuals with useful variations was called natural selection by Darwin and survival of the fittest by Wallace.
Darwin explained evolution of long neck of modem giraffe by stating the existence of long necked and short necked giraffe in the ancestral population. After the change in the climate, resulting in presence of trees and no grasslands, the longer-necked giraffe with longer forelegs were more successful in reaching the soft leaves of trees for feeding. Therefore, natural selection favoured longer necked progeny generation after generation. Selection of longer neck through a long series of generations resulted in the evolution of such long-necked modem giraffe.
6.Inheritance of useful variations: The individuals which survive the struggle for existence transmit their useful variations to the offspring, which will also prove to be fit. Thus offsprings of selected individual are born fit to the environment.
7.Formation or origin of new species. Darwin suggested the appearance of new variations in every generation. These variations keep on accumulating and after a number of generations, the offsprings become markedly different forming a new species. Origin of new species by gradual modification of older ones is called speciation.
2.There is limited food and space. The space in the universe remains constant. The ultimate source of food for plants and animals also remains constant. Thus, the carrying capacity of the environment does not allow the population to grow indefinitely. Inspite of enormous reproductive potential of living beings under natural conditions the number of individuals of each species remains nearly constant over a long period of time.
3.Struggle for existence–According to Darwin individuals multiply in geometric ratio whereas space and food availability remains constant, so there is an intense competition and three fold struggle for existence. The struggle is intraspecific, interspecific and with the environment.
(a) Intraspecific struggle : This is struggle between the individuals of the same species. This is the toughest form of struggle as the needs of the individuals of the same species are identical.
(b) Interspecific struggle: This is the struggle between the individuals of different species.
(c) Struggle with the environment: This struggle of living beings is with the changes of environment such as heat, cold, drought, flood, storm, famine, light, food and shelter, etc.
4.Variations: The struggle for existence results in competition among the organisms. Thus the organisms adapt and differ from each other in shape, size, behaviuor etc. Variation is the law of nature. This leads them to change in accordance with the conditions to utilise the natural resources and survive successfully.
5.Natural selection and survival of the fittest. According to Darwin nature selects only those individuals which are with more favourable variations and are best adapted to the environment. The less fit and unfit organisms are left out by selection. This sorting out of individuals with useful variations was called natural selection by Darwin and survival of the fittest by Wallace.
Darwin explained evolution of long neck of modem giraffe by stating the existence of long necked and short necked giraffe in the ancestral population. After the change in the climate, resulting in presence of trees and no grasslands, the longer-necked giraffe with longer forelegs were more successful in reaching the soft leaves of trees for feeding. Therefore, natural selection favoured longer necked progeny generation after generation. Selection of longer neck through a long series of generations resulted in the evolution of such long-necked modem giraffe.
6.Inheritance of useful variations: The individuals which survive the struggle for existence transmit their useful variations to the offspring, which will also prove to be fit. Thus offsprings of selected individual are born fit to the environment.
7.Formation or origin of new species. Darwin suggested the appearance of new variations in every generation. These variations keep on accumulating and after a number of generations, the offsprings become markedly different forming a new species. Origin of new species by gradual modification of older ones is called speciation.
Evidences in support of Darwinism
1.Artificial Selection: By controlled breeding and selection for
several generations, many new varieties of plants and new species, races or
breeds of domestic animals have been developed. New races of dogs, horses,
pigeons, poultry, sheeps, pigs, goats, etc., have been developed by man by
artificial selection. Thus there is a close parallelism between natural selection and artificial selection.
2.Mimicry and protective coloration: Mimicry and protective colouration as seen in certain animals can be achieved only by gradual changes occuning side by side both in the model and mimic occupying the same area.
3.Correlation between position of nectaries in flowers and length of Proboscis of pollinating insects: This relation between two different organisms evolve gradually and can be explained by natural selection.
2.Mimicry and protective coloration: Mimicry and protective colouration as seen in certain animals can be achieved only by gradual changes occuning side by side both in the model and mimic occupying the same area.
3.Correlation between position of nectaries in flowers and length of Proboscis of pollinating insects: This relation between two different organisms evolve gradually and can be explained by natural selection.
Criticism of Darwinism
1.Darwin was not clear about the sources of variations and mechanisms of natural selection.
2.Inheritance of small variations: Darwin stated that small variations formed the raw materials for evolution. However this can happen in only fully functional organs, for ex. the inheritance of the wing of bird in its initial stage when it was not of any advantage to the organism could not be explained.
3.Existence of vestigial organs: According to Darwin, only useful organs are selected and inherited. Then presence of vestigial organs that are functionless could not be explained.
4.Overspecialization: Overspecialisation of some structures like antlers and tusks of elephants becomes a hindrance to these organisms. This fact that these structures that were hindrance to the organism being inherited could not be explained.
5. Darwin explained the survival of the fittest and not the arrival of the fittest.
2.Inheritance of small variations: Darwin stated that small variations formed the raw materials for evolution. However this can happen in only fully functional organs, for ex. the inheritance of the wing of bird in its initial stage when it was not of any advantage to the organism could not be explained.
3.Existence of vestigial organs: According to Darwin, only useful organs are selected and inherited. Then presence of vestigial organs that are functionless could not be explained.
4.Overspecialization: Overspecialisation of some structures like antlers and tusks of elephants becomes a hindrance to these organisms. This fact that these structures that were hindrance to the organism being inherited could not be explained.
5. Darwin explained the survival of the fittest and not the arrival of the fittest.
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DE VRIES’ MUTATION THEORY
New species appear by sudden heritable changes. This theory was given by HUGO DE VRIES, a Dutch botanist, director of botanical garden Amsterdan.
He observed different types of Primrose (Oenothera lamarckiana)
From the observation he postulated the mutation theory
Postulates
Evidences in favour of mutation theory
Appearance of new variety from normal parents –
Evidences against mutation theory
Significance of De Vries theory of mutation
He observed different types of Primrose (Oenothera lamarckiana)
From the observation he postulated the mutation theory
Postulates
- New species appear by sudden changes called mutations.
- All organisms have tendency to mutate.
- Individuals showing mutations are called mutants.
- Mutations are indeterminate and can be useful or harmful.
- Can occur in different members of the same species.
- Can appear again and again generation after generation.
Evidences in favour of mutation theory
Appearance of new variety from normal parents –
- Short legged Ancon sheep,
- Hornless Hereford cattle,
- Hairless cats and dogs.
- Seedless variety of banana
Evidences against mutation theory
- Cannot explain mimicry i.e. co-evolution
- Rate of mutation is slow.
- Evolution is gradual but mutation is abrupt.
Significance of De Vries theory of mutation
- Mutations are important for evolution
- Mutations introduce variations that are then selected by nature
- Mutations provide the raw materials on which natural selection can operate
NEODARWINISM
Neo-Darwinism, also called the modern evolutionary synthesis, generally denotes the integration of Charles Darwin's theory of evolution by natural selection, Gregor Mendel's theory of genetics as the basis for biological inheritance, and mathematical population genetics.
Major figure in the development of the modern synthesis include Thomas Hunt Morgan
MODERN SYNTHETIC THEORY OF EVOLUTION
(1) Neo–Darwin is a modification of the original theory of Darwin to remove its short–comings.
(2) Instead of continuous variations, mutations are believed to help form new species.
(3) Variations accumulate in the gene pool and not in the individuals.
(4) Neo–Darwinism incorporates isolation as an essential component of evolution.
(5) The theory can explain the occurrence of unchanged forms over millions of years.
(6) Normally only those modifications are transferred to next generation which influence germ cells or where somatic cells give rise to germ cells.
Major figure in the development of the modern synthesis include Thomas Hunt Morgan
MODERN SYNTHETIC THEORY OF EVOLUTION
(1) Neo–Darwin is a modification of the original theory of Darwin to remove its short–comings.
(2) Instead of continuous variations, mutations are believed to help form new species.
(3) Variations accumulate in the gene pool and not in the individuals.
(4) Neo–Darwinism incorporates isolation as an essential component of evolution.
(5) The theory can explain the occurrence of unchanged forms over millions of years.
(6) Normally only those modifications are transferred to next generation which influence germ cells or where somatic cells give rise to germ cells.
Variations and its causes
Variation is a process whereby closely related organisms differ among themselves.
No two individuals look alike, this is called variation.
CAUSES OF VARIATIONS
1.Environmental conditions or Somatogenic variations : Changes in expression of genes of somatic cells, these are non-inheritable.
2.Changes in gene pattern or Germinal or blastogenic variations: Changes in the germplasm or chromosomes of gametes, these are heritable.
No two individuals look alike, this is called variation.
CAUSES OF VARIATIONS
1.Environmental conditions or Somatogenic variations : Changes in expression of genes of somatic cells, these are non-inheritable.
2.Changes in gene pattern or Germinal or blastogenic variations: Changes in the germplasm or chromosomes of gametes, these are heritable.
SOURCES OF VARIATIONS
1.MUTATIONS
2.RECOMBINATION
3.MIGRATION- genetic drift, gene flow
4.HYBRIDISATION
5.SELECTION
2.RECOMBINATION
3.MIGRATION- genetic drift, gene flow
4.HYBRIDISATION
5.SELECTION
Sources of variation: Mutation
A sudden hereditary change in the genetic make-up of an organism other than which occurs by the simple recombination of genes.
This results in introduction of new genetic information into a population by modifying genes that are already present. All alleles for a particular trait originated as a result of mutations, and have been maintained in the gene pool as a result of sexual reproduction. A gene pool is the complete set of unique alleles in a species or population. |
Sources of variation: Recombination
Recombination results due to sexual reproduction bringing about genetic diversity.
Each individual has a unique gene pool half donated from the mother and half by the father. The gene mixing that occurs during sexual reproduction is termed as genetic recombination. Recombination does not change the frequency of alleles directly, but a new member has a unique combination of genes. Thus having unique characteristics superior to others. |
Sources of variation: Migration
Migration or movement of individuals from one place or region to the other results in alleles being added or subtracted from a local population.
When an organism leaves a population and enters another it results in loss of genetic information from one population and gain in the other. If the organism contains rare alleles, it may significantly affect the allele frequency of both populations. |
Genetic drift and Gene flow
Variations in gene frequencies within population can occur by chance. In each generation, some individuals may, just by chance, leave behind a few more descendents (and genes, of course!) than other individuals. The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals. This is genetic drift. Genetic drift can thus contribute to the process of speciation.
Gene flow is the movement of alleles from one population to another as a result of interbreeding between the members of the two populations. The random introduction of new alleles into the recipient population and their removal from the donor population affects the allele frequency of both populations. This leads to increased genetic variations. |
Genetic drift
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Sources of variation: Hybridisation
It is the intermingling of genes of two populations of a species which are normally separate. This occurs by migration or artificial cross breeding.
Sources of variation:Selection
Selection is the process where the organisms that are better adapted to the environment survive and reproduce. These organisms then pass on their successful characteristics to the next generation.
When population increases, there is limited space and food, this results in competition. Organisms better adapted for this competition survives.
When population increases, there is limited space and food, this results in competition. Organisms better adapted for this competition survives.
Selection is differential or non-random reproduction of successful genotypes.
Differential reproduction: changes are brought about by favoring differential reproduction of genes. Gene pool of better adapted individual are naturally selected.
Environmental limiting factors and population size operate together to produce a selection pressure, that can vary in intensity.
Thus selection pressure can be considered as a means of increasing or decreasing the spread of an allele within the gene pool that can lead to evolutionary changes.
Differential reproduction: changes are brought about by favoring differential reproduction of genes. Gene pool of better adapted individual are naturally selected.
Environmental limiting factors and population size operate together to produce a selection pressure, that can vary in intensity.
Thus selection pressure can be considered as a means of increasing or decreasing the spread of an allele within the gene pool that can lead to evolutionary changes.
Types of selection: There are three different types of results produced:
Stabilizing selection: eliminates extreme variation favoring average or normal phenotypes.
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Directional selection: Operates in response to gradual changes in environmental conditions. It results in selection in one direction in relation to one specific character.
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Disruptive selection: favours both extremes thus two peaks are seen.
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Examples of natural selection
1.Industrial melanism: The Peppered moth seen before 1845 was motted grey (Biston betularia) , that would easily blend with the lichen covered trees. After 1845 black coloured moths were noticed. This variety was called Biston betularia carbonaria. In the next 50 years, the frequency of these black moths showed a considerable increase. This increase from 1% to almost 99% was noticed in the industrial areas. Due to inductrial revolution the tree trunks were covered with soot from the industries. This was a disadvantage for the grey moths that could now be spotted easily by the predators, whereas the blacks moths were camouflaged in the soot covered trees.
This was observed first and worked out by H.B.D. KETTLEWELL and E.B. FORD. Thus here environmental factors played a key role in decrease of grey variety and increase in the black or melanic variety. PEPPERED MOTH STIMULATION |
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2. D.D.T resistant mosquitoes: DDT initially proved to be a very effective insecticide, however it then became ineffective. The explanation is given as follows- The original population had some DDT resistant variety. Since DDT was not sprayed, they had no additional advantages over the DDT sensitive mosquitoes. When DDT was sprayed, natural selection favored the DDT resistant mosquitoes. Gradually the entire population became DDT resistant type.
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3. Malaria & G-6 PD deficiency:
G-6 PD is an enzyme glucose-6-phosphate dehydrogenase. It decomposes hydrogen per oxide formed during metabolism. The deficiency of this enzyme is an inborn error of metabolism controlled by specific gene. When persons suffering from malaria are given primaquin, it causes hemolysis as hydrogen per oxide is not decomposed due to G-6 PD deficiency. RBC are deformed and in such distorted RBC, malarial parasite fails to survive and multiply. Thus these individuals are resistant to malaria and are favored by natural selection.
Further reading
glucose6phosphatedehydrogenase
G-6 PD is an enzyme glucose-6-phosphate dehydrogenase. It decomposes hydrogen per oxide formed during metabolism. The deficiency of this enzyme is an inborn error of metabolism controlled by specific gene. When persons suffering from malaria are given primaquin, it causes hemolysis as hydrogen per oxide is not decomposed due to G-6 PD deficiency. RBC are deformed and in such distorted RBC, malarial parasite fails to survive and multiply. Thus these individuals are resistant to malaria and are favored by natural selection.
Further reading
glucose6phosphatedehydrogenase
4. Sickle cell anemia and malaria:
This is a genetic disorder in tropical Africa, where malaria is widespread. in this disorder the normal HbA gene is replaced by HbS causing sickle cell anemia. The oxygen carrying capacity of hb gets reduced. The RBCs become sickle shaped causing severe hemolytic anemia. Malarial parasite is unable to survive in the sickle shaped RBC. Heterozygous individuals are able to cope up with malaria. This type of gene is maintained along with the normal gene as it has survival value in malaria infested areas.
This is a genetic disorder in tropical Africa, where malaria is widespread. in this disorder the normal HbA gene is replaced by HbS causing sickle cell anemia. The oxygen carrying capacity of hb gets reduced. The RBCs become sickle shaped causing severe hemolytic anemia. Malarial parasite is unable to survive in the sickle shaped RBC. Heterozygous individuals are able to cope up with malaria. This type of gene is maintained along with the normal gene as it has survival value in malaria infested areas.
Natural and Artificial selection
Natural selection | Artificial selection | |
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1 | The selection is excerted by nature | The selection is exreted by man |
2 | Process operates in natural populations | The process operates in domestic populations |
3 | It is slow process that can take thousands of years | It is a fast process and shows immediate results. |
4 | This is responsible for the biological diversity | Produces new varieties from starting generations |
ADAPTATION
Any character that is advantageous to the organism is called as ADAPTATION. These are adjustments occurring in the living organism by which they can match with the changing environment.
Thus adaptation can be defined as the characteristics of living forms to develop over a period of time which enable them to survive and reproduce within the limits of a particular environment.
The process of adaptation involves three important forces, Mutation, Selection, Genetic drift
Selection of the better genotype –E.g. Peppered moth
Thus adaptation can be defined as the characteristics of living forms to develop over a period of time which enable them to survive and reproduce within the limits of a particular environment.
The process of adaptation involves three important forces, Mutation, Selection, Genetic drift
Selection of the better genotype –E.g. Peppered moth
LEDERBERG’S replica plating experiment
JOSHUA LEDERBERG and ESTHER LEDERBERG bacteria grown in culture to produce several colonies. Then grew the bacteria on plates with an antibiotic such as penicillin. Most colonies could not survive, but the ones that survived were resistant to penicillin.
Thus this strain acquired ability to survive in new environment. The experiment- 1.Bacteria are spread out on a plate, called the “original plate.” 2.They are allowed to grow into several different colonies 3.This layout of colonies is stamped from the original plate onto a new plate that contains the antibiotic penicillin. 4.Colonies X and Y on the stamped plate survive. They must carry a mutation for penicillin resistance. 5. The Lederbergs set out to answer the question, did the colonies on the new plate evolve antibiotic resistance because they were exposed to penicillin? The answer is no: When the original plate is washed with penicillin, the same colonies (those in position X and Y) live—even though these colonies on the original plate have never encountered penicillin before. Two possible explanations: 1.Lamarck’s view– penicillin induced change enabling bacteria to grow in penicillin. 2. Darwin’s view– original culture had this variety and it was favoured thus only the selected survived. |
Concept of species
Geographically localized group of individuals of the same kind at a particular time represents a population and the sum of all the populations of the same kind form a species. Species consists of individuals that share the same gene pool.
Speciation or origin of new SPECIES
Speciation is the process which new species are formed.
An interbreeding population which is reproductively isolated from other similar but morphologically distinguishable population is called as species.
Mechanisms that reduce the chances of interbreeding in related species of living organisms is called isolating mechanisms.
Speciation or origin of new SPECIES
Speciation is the process which new species are formed.
An interbreeding population which is reproductively isolated from other similar but morphologically distinguishable population is called as species.
Mechanisms that reduce the chances of interbreeding in related species of living organisms is called isolating mechanisms.
Temporal isolation (different breeding season)
Species may breed at different times of the day, different seasons, or different years, and this can prevent them from mating. ex.Three tropical orchid species of the genus Dendrobium each flower for a single day; the flowers open at dawn and wither by nightfall. |
Other isolating mechanisms also include physiological isolation (gametes do not meet & fuse) and Genetic isolation (zygote not possible, hybrid is sterile).
Allopatric and Sympatric speciations
Allopatric are those which inhabit different areas, whereas sympatric are those which occupy the same geographical area.
Allopatric speciation is usually shown by the animal population.
Allopatric speciation is usually shown by the animal population.
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Sympatric speciation | Allopatric speciation | |
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1 | It occurs in population that occupy the same geographical area. | It occurs in population that occurs in different geographical area |
2 | It occurs by sudden reproductive isolations | It occurs in different populations by gradual accumulation of variations and mutations |
3 | These are isolated by ecological, ethological or genetic isolations | These are isolated by geographical barriers |
4 | Sympatric speciation is sudden | Allopatric speciation is gradual |
MICROEVOLUTION
changes in the gene pool of a population over time which result in relatively small changes to the organisms in the population — changes which would not result in the newer organisms being considered as different species. Examples of such microevolutionary changes would include a change in a species’ coloring or size.
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MACROEVOLUTION
changes in organisms which are significant enough that, over time, the newer organisms would be considered an entirely new species. In other words, the new organisms would be unable to mate with their ancestors, assuming we were able to bring them together.
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