Heredity and Evolution

1. Basic Terms

Chromosome is a thread-like structure in the nucleus of a cell formed of DNA which carries the genes. Different organisms have different number of chromosomes in their nuclei.
• A gene is a unit of DNA on a chromosome which governs the synthesis of one protein that controls a specific characteristic of an organism.
• Genes are actually units of heredity which transfer characteristics from parents to their offsprings during reproduction.
• Genes work in pairs. Genes are represented by letters.
• Genes controlling the same characteristics are given the same letters. For example, the gene for tallness is represented by the letter T whereas the gene for dwarfness is represented by the
letter t.
• The gene which decides the appearance of an organism even in the presence of an alternative gene is known as a dominant gene. It dominates the recessive gene for the same characteristic on the other chromosome of the pair.
• The gene which can decide the appearance of an organism only in the presence of another identical gene is called a recessive gene. A single recessive gene cannot decide the appearance of an organism.
• The dominant gene is represented by a capital letter and the corresponding recessive gene is represented by the corresponding small letter.
Genotype is the description of genes present in an organism.
• Genotype is always a pair of letters such as TT, Tt or tt (where T and t are the different forms of the same gene). Thus, the genotype of a tall plant could be TT or Tt whereas that of a dwarf plant is tt.
• The characteristic (or trait) which is visible in an organism is called its phenotype. For example, being ‘tall’ or ‘dwarf (short) are phenotypes of a plant because these traits can be seen by us or they are visible to us.
• When two parents cross to produce progeny. then their progeny is called First Filial Generation or F1 generation.
• When the first generation progeny cross (or breed) among themselves to produce second generation progeny, then this progeny is called Second Filial Generation or F2 generation. In other words, the generation produced by crossing two F1 progeny is called F2 generation.
2. Genetics
• The work of Mendel and other workers gave us an idea of inheritance patterns
• The factors (now called genes) represent the genetic basis of inheritance.
• The major contributions to molecular biology are from Watson, Crick, Nirenberg, Khorana, Kornberg, Benzer, Monad, Brenner etc.
• Gregor Johann Mendel, an Austrian monk discovered the basic principles of heredity. He conducted experiments on pea plant.
• He conducted research on the transmission of hereditary traits in plant hybrids.
• He published the results of his studies under the title ” Experiments on plant hybrids“.
• The observations he made while growing peas in his monastery garden became the foundation of modern genetics and the study of heredity.
Inheritance is the process by which characters are passed on from parents to progeny. Inheritance is the basis of heredity.
Variation is the degree by which progeny differ from their parents as well as among themselves.
• Humans knew from as early as 8000-1000 BC that one of the causes of variations was hidden in sexual reproduction.
• They exploited the variations that were naturally present in the wild populations of plants and animals to selectively breed and choose organisms that possessed desirable characters.
• Through artificial selection and domestication from ancestral wild cows we got well-known Indian breeds like Sahiwal cows in Punjab and Ongole bulls in Andhra Pradesh.
• Though our ancestors know about the inheritance of characters and variation they had very little idea about the scientific basis of these phenomena.
3. Mendels’ Experiments
• Mendel conducted hybridization experiments on garden peas for seven years and proposed the laws of inheritance in living organisms.
• During Mendel’s investigations into inheritance patterns, statistical analysis and mathematical logic were applied to problems in biology for the first time.
• Mendel investigated characters in the garden pea plant that manifested as two opposing traits eg. tall or dwarf plants, yellow or green seeds etc.
• Mendel conducted artificial pollination experiments using several true breeding pea lines.
• A true breeding line is obtained by continuous self-pollination and shows the stable trait inheritance and expression for several generations.
• Mendel selected 14 true breeding pea plant varieties as pairs which were similar except for one character with contrasting traits.
• Pea plant is an annual plant with well-defined charactersics
• It can be grown and crossed easily
• It has bisexual flowers and can be self-fertilized conveniently.
• It has a short life cycle and produces large number of offspring.
4. Monohybrid cross
• In monohybrid crosses Mendel studied the inheritance of one gene. In one of the monohybrid crosses, he crossed tall and dwarf pea plants.
• The first hybrid generation is called first filial progeny or the F1. Mendel found that F1 always resembled one of the parents and the trait of the other parent was not seen in them.
• The character that was not seen in the F1 generation expressed in the F2 generation
• When tall and dwarf plants were crossed F1 progeny plants were tall but some dwarf plants appeared in F2 generation
• In the F2 generation 3/4th of the plants were tall and 1/4 were dwarf plants.

Tall[TT]+Dwarf[tt][PARENTALGENERATION]cross breeding Tall[Tt]+Tall[Tt]+Tall[Tt]+Tall[Tt][ F1 GENERATION]

Tall[TT]+Dwarf[Tt][F1 GENERATION]cross breeding Tall[Tt]+Tall[Tt]+Tall[Tt]+Tall[tt][ F2 GENERATION]

• The tall and dwarf traits were identical to their parental types and did not show any blending.
• Mendel proposed that factors now called genes were being stably passed down unchanged from parent to offspring through gametes over successive generations. So genes are the units of inheritance.
• Genes contain information required to express a particular trait in an organism.
• Genes which code for a pair of contrasting traits are known as alleles.
• Mendel also proposed that in a true breeding tall or dwarf pea variety the allelic pair of genes for height are identical or homozygous TT and tt respectively.
• TT and tt are called the genotypes of the plant while the descriptive terms tall and dwarf are the phenotypes.
• Mendel found the phenotype of heterozygote Tt to be exactly like the TT parent (tall) in appearance.
• Mendel proposed that in a pair of dissimilar factors one dominates the other and hence it is called dominant factor while the other factor is recessive.
• In monohybrid cross, between tall and dwarf T for tallnessis dominant over t for dwarfness.
• If the allelic pair of genes for a character are identical the organism is homozygous for that character and if the allelic pair of genes are dissimilar, it is heterozygous.
• When tall and dwarf plants produce gametes by meiosis, the alleles of the parental pair separate or segregate from each other and only one allele is transmitted to a gamete.
• The types of gametes produced by the parents the formation of the zygotes and the progeny can be represented in a diagram called punnett square or checker board.
• The checker board was developed by a british geneticist Reginald c. Punnett.
• Checker board indicates all possible union of gametes and the probability of all possible genotypes of offspring in a genetic cross.
• In monohybrid cross, in F2 generation the phenotypic ratio is 3: 1 and genotypic ratio is  1: 2: 1 .
Test cross is used to know the genotype of a dominant plant.
• In a test cross an organism showing a dominant phenotype is crossed with the recessive parent.
• In monohybrid test cross the phenotypic and genotypic ratios are 1: 1 only
• If the F1 hybrid is crossed with the parental type having dominant trait, it is called back cross.
• But when F1 individuals are crossed with any one of its parents or organisms that are phenotypically and genotypically similar to the parents it is generally called back cross.
• Test cross is one kind of backcross.
• In a back cross although all the plants show phenotypically dominant character, they show a genotypic ratio of 1: 1.
• Based on his observations on monohybrid crosses, Mendel proposed two general rules of inheritance or principles or laws of in heritance namely law of dominance (first law) and law of segregation (second law).
5. FIRST LAW [OR] Law of dominance
• According to law of dominance:
a. Characters are controlled by discrete units called factors
b. Factors occur in pairs.
c. In a dissimilar pair of factors pertaining to a character one member of the pair dominates (dominant) over the other (recessive)
• Law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1.
• Law of dominance is used to explain the proportion of 3: 1 obtained at the F2 of monohybrid cross
• According to law of segregation, the two alleles of gene when present in a heterozygous state do not fuse or blend in any way but remain distinct and segregate during meiosis or in the formation of gametes so that each gamete will carry only one of them.
• A homozygous parent produces all gametes that are similar while a heterozygous one produces two kinds of gametes in equal proportions each having one allele.
• Segregation of genes is a universal phenomenon in all organisms reproducing by normal sexual method.
6. Deviations from Mendelian concept of dominance
• In incomplete dominance, F1 has a genotype that does not resemble either of the two parents and is in between the two.
• The inheritance of flower colour in Antirrhinum sps (dog flower) is a good example for incomplete dominance.
• In a cross between homozygous red flowered (RR) and homozygous white flowered plants (rr) F1 plants are pink flowered (Rr).
• In incomplete dominance F2 phenotypic and genotypic ratios are exactly similar and the ratio is 1: 2: 1.
• In co-dominance, F1 generation resembles both the parents.
• Examples of co-dominance are ABO blood grouping in human beings and seed coat pattern and size in lentil plants.
• Lens culinaris ssp. culinaris (lentil) is major pulse crop in north America.
• A cross between pure breeding spotted lentils and pure breeding dotted lentils, produced heterozygotes that are both spotted and dotted.
• The phenotypic and genotypic ratios in F2 progeny of co-dominance are the same that is  1: 2: 1.
• In pleiotropy, a single gene product may produce more than one effect so that a single gene may be related to more than one character.
• In pea seeds starch synthesis and shape of the seed are controlled by a single gene.
7. Explanation of the concept of dominance
• Every gene contains the information to express a particular trait.
• In diploid organisms, there are two copies of each gene called alleles.
• One of the alleles may be different due to some changes that it has undergone which modifies the information that the particular allele contains.
• The unmodified allele which represents the original phenotype is the dominant allele and the modified allele is generally recessive allele.
8. dihybrid cross
• When plants that differ in two characters are crossed it is called dihybrid cross.
• When pea plant that has seeds with yellow colour and round shape is crossed with a pea plant that has green and wrinkled seeds, all the seeds in F1 generation are yellow and round.
• Thus yellow seed colour was dominant over green and round shape dominant over wrinkled.
• In dihybrid cross F2 phenotypic ratio is 9: 3: 3: 1 and genotypic ratio is 1: 2: 1: 2: 4: 2:  1: 2: 1
9. Law of independent assortment
• Based on dihybrid cross Mendel proposed law of independent assortment.
• According to law of independent assortment when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.
• Although there are 16 squares in punnett square of F2 in dihybrid cross, there are 9 types of genotypes and 4 types of phenotypes.
10. Chromosomal theory of inheritance
• Even though Mendel published his work on inheritance of characters in 1865 it remained unrecognised till 1900 because – communication was not easy in those days. his concept of genes or factors was not accepted by his contemporaries, Mendel’s approach of using mathematics to explain biological phenomena was totally new and unacceptable to biologists of those days and he could not provide any physical proof for the existence of factors
• In 1900 de Vries, Correns and Von Tschermak independently rediscovered Mendel’s results on inheritance of characters.
• Walter Sutton and Theodore Boveri proposed ‘Chromosomal theory of inheritance’.
• According to Sutton & Boveri the behaviour of chromosomes was parallel to the behaviour of genes as predicted by Mendel.
• They used chromosome movement to explain Mendel’s laws.
• The chromosomes as well as genes occur in pairs
• The two alleles of a gene pair are located on homologous sites on homologous chromosomes
• According to Sutton & Boveri. the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried.
• Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance.
• Chromosomal theory of inheritance was experimentally verified by Thomas Hunt Morgan on the fruifly, Drosophila melanogaster.
• Fruit flies complete their life cycle in about two weeks and the male and female flies are easily distinguishable.
• The fruit fly has many types of hereditary variations that could be seen with low power microscope.
11. Linkage and recombination
• The term linkage was coined by Morgan.
• Linkage is the physical association of genes on a chromosome.
• When two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations was much higher than the non parental type.
• Morgan hybridized yellow bodied. white eyed females to brown bodied and red eyed males and intercrossed their F1 progeny.
• The two genes did not segregate independently of each other and the F2 ratio deviated very significantly from 9:3:3:1 ratio.
• Recombination is the generation of non – parental gene combinations.
• Morgan and his group also found that even, when genes were grouped on the same chromosome, some genes were very tightly linked while others were loosely linked.
• The genes white and yellow were very tightly linked and showed only 1.3 percent recombination while white and miniature wing showed 37.2 percent recombination.

• Alfred Sturtevant, a student of T.H Morgan used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and mapped their position on the chromosome.
• Genetic maps are now a days extensively used as a starting point in the sequencing of whole genomes.
12. Mutations
Mutation is a phenomenon which results in alteration of genes and consequently results in changes in the genotype and the phenotype of an organism.
• Mutation leads to variation in DNA.
• Mutations were first noticed by Hugo de vries in Oenothera lamarckiana (lamarck’s evening primrose).
• Loss (deletion) or gain (insertion /duplication) of a segment of DNA results in alteration in chromosomes.
• Alteration in chromosomes results in abnormalities or aberrations.
• Chromosomal aberrations are commonly observed in cancer cells.
• Mutations also occur due to change in a single base pair of DNA and this is known as point mutation
• A classical example of point mutation is sickle cell anemia
• Deletions and insertions of base pairs of DNA cause frame shift mutations.
• Mutations are induced by chemical and physical factors called mutagens.
• UV radiation is a mutagen that can cause mutations in organisms.
• Mutations generate a large amount of variability in a population from which a breeder can select the desirable types.
• Improved varieties of crop plants with several desirable characters can be obtained by mutations after careful selection and hybridization.
13. How Blood Groups are Inherited
• A person has one of the four blood groups: A, B, AB or O. This blood group system is controlled by a gene which has three different forms denoted by the symbols IA, IB and 10.
• The genes IA and IB show no dominance over each other, that is, they are codominant. However, genes IA and IB both are dominant over the gene 10. In other words, the blood gene 1° is recessive in relation to genes IA and IB.
• Although there are three gene forms (called alleles) for blood, but any one person can have only two of them. So, the blood group of a person depends on which two forms of the genes he possesses.
• If the genotype (gene combination) is IAIA, then the blood group of the person is A. And if the genotype is IA10 even then the blood group is A (because 10 is a recessive gene).
• If the genotype is IBIB, then the blood group of the person is B. And if the genotype is IB 10 even then the blood group is B (because B is a recessive gene).
• If the genotype is IAIB, then the blood group of the person is AB.
• If the genotype is I0I0, then the blood group of the person is O.
14. Sex Determination
• The process by which the sex of a person is determined is called sex determination.
• Genetics is involved in the determination of the sex of a person.
• The chromosomes which determine the sex of a person are called sex chromosomes.
• There are two types of sex chromosomes, one is called X chromosome and the other is called Y chromosome.
• A male has one X chromosome and one Y chromosome.
• A female has two X chromosomes but no Y chromosomes.
• The sex of a child depends on what happens at fertilisation.
• If a sperm carrying X chromosome fertilises an ovum (or egg) which carries X chromosome, then the child born will be a girl (or female). This is because the child will have XX combination of sex chromosomes.
• If a sperm carrying Y chromosome fertilises an ovum which carries X chromosome, then the child born will be a boy. This is because the child will have XY combination of sex chromosomes.
• Please note that it is the sperm which determines the sex of the child.
• This is because half of the sperms have X chromosomes and the other half have Y chromosomes. Thus, there is a 50 per cent chance of a boy and a 50 per cent chance of a girl being born to the parents. This is why the human population is roughly half males and half females.
15. Acquired and Inherited Traits
• A trait (or characteristic) of an organism which is ‘not inherited’ but develops in response to the environment is called an acquired trait.
• Suppose the tail of a mouse gets cut. The ‘cut tail’ of this mouse is also an acquired trait which has been brought about by some agent in its environment.
• The acquired traits of organisms cannot be passed on to their future generations.
• The changes in the non-reproductive body cells of an organism cannot be inherited by its offsprings. This will become clear from the following examples.
• A trait (or characteristic) of an organism which is caused by a change in its genes (or DNA) is called an inherited trait.
• Inherited traits can be passed on to the progeny of the organism because they have produced changes in the genes (or DNA) of the organism.
• Inherited traits actually mean the characteristics which we receive from our parents.
• The children in the family inherit some characteristics from each of their parents.
16. Evolution
Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of years in which new species are produced.
• Since the evolution is of the living organisms, so it is also called ‘organic evolution’.
• All the plants and animals (or organisms) which we see today around us have evolved from some or the other ancestors that lived on this earth long, long ago.

• The development of ‘pterosaur’ (an ancient flying reptile) from a big lizard is an example of evolution.
• The process of evolution will become clear from the following example of ‘pterosaur’.
• Pterosaur is an ancient flying reptile which lived on the earth about 150 million years ago.
• The development of pterosaur is an example of evolution. It began life as a big lizard which could just crawl on land Over millions of years, small folds of skin developed between its feet which enabled it to glide from tree to tree.
• Over many, many generations, spread over millions of years, the folds of skin, and the bones and muscles supporting them grew to form wings which could make it fly [see Figure 26(c)].
• In this way, an animal which crawled on ground evolved into a flying animal. This evolution led to the formation of a new species (of a flying reptile).
17. Evidences For Evolution
Homologous Organs Provide Evidence for Evolution
• There are many organs in different groups of animals or plants which all seem to be built from the same basic design but are used for many different purposes. These are called homologous organs.
• For example, the forelimbs of a man, a lizard (reptile), a frog (amphibian), a bird and a bat (mammal) seem to be built from the same basic design of bones. but they perform different functions.
• The forelimbs of a human (man), a lizard, a frog, a bird and a bat have the same basic design of bones.
• The presence of homologous forelimbs in humans (man), a lizard, a frog, a bird and a bat indicate that all these forelimbs have evolved from a common ancestral animal which had a ‘basic design’ limb.
• Please note that the wings of a butterfly (which is an insect) and the wings of a bat cannot be considered to be homologous organs because they have different basic designs (though they are used for the same purpose of flying).
Analogous Organs Provide Evidence for Evolution
• The organs which have different basic structure (or different basic design) but have similar appearance and perform similar functions are called analogous organs.
• The analogous organs provide the evidence for evolution.
• For example, the wings of an insect and a bird have different structures (the insects have a fold of membranes as wings which are associated with a few muscles whereas a skeleton, flesh and feathers support bird’s wings) but they perform the same function of flying.
• Since the wings of insects and birds have different structures (or different designs) but perform similar functions, they are analogous organs.
• The analogous organs have different basic design, so they do not indicate a common ancestor for the organism.
• The analogous organs provide evidence for the evolution in another way. The presence of analogous organs indicates that even the organisms having organs with different structures can adapt to perform similar functions for their survival under hostile environmental conditions. Thus, the presence of analogous organs in different animals provide evidence for evolution by telling us that though they are not derived from common ancestors, they can still evolve to perform similar functions to survive, flourish and keep on evolving in the prevailing environment.
• The analogous organs actually provide a mechanism for evolution.
Fossils Provide Evidence for Evolution
• The remains (or impressions) of dead animals or plants that lived in the remote past are known as fossils.
• The fossils provide evidence for evolution. For example, a fossil bird called Archaeopteryx looks like a bird but it has many other features which are found in reptiles. This is because Archaeopteryx has feathered wings like those of birds but teeth and tail like those of reptiles.
Archaeopteryx is, therefore, a connecting link between the reptiles and birds, and hence suggests that the birds have evolved from the reptiles. Thus, fossils provide the evidence that the present animals (and plants) have originated from the previously existing ones through the process of continuous evolution.
• We will now describe how fossils are formed. Usually, when organisms (plants or animals) die, their bodies will decompose by the action of micro-organisms in the presence of oxygen, moisture, etc. Sometimes, however, the conditions in the environment are such (like absence of oxygen or moisture, etc), which do not let the body of the organism to decompose completely.
• It is such body (or body part) of an organism which we get as fossil on digging the earth.
• In many cases the soft parts of the organisms get decomposed and what we get as a fossil is a skeleton of hard parts (like bones, etc).
• Even the soft parts of the plants and animals (which usually decompose quickly) are sometimes preserved as fossils in the form of their impressions inside the rocks. For example, if a dead leaf gets caught in mud, it will not decompose quickly.
• The mud around the leaf will set around it as a mould, gradually harden to form a rock and retain the impression of the whole leaf. This forms a leaf fossil which can be dug out from the earth a long time later.
• The fossil of a dead insect caught in mud is also formed in a similar way to leaf fossil. All such preserved impressions of the body parts of the once living organisms are also called fossils.
Fossils are obtained by digging into the earth
• The age of fossils can be estimated in two ways: by the relative method, and by the carbon dating method.
• The relative method works like this: When we dig into the earth,we find fossils at different depths. The fossils which we find in layers closer to the surface of the earth are more recent; the fossils which are found in deeper layers are older; whereas the fossils found in the deepest layers of earth are the oldest ones.
• Fossils which we find today were once living objects. All the living objects contain some carbon-14 atoms which are radioactive.
• When a living object dies and forms fossil, its carbon-14 radioactivity goes on decreasing gradually. In the carbon dating method, the age of fossils is found by comparing the carbon-14 radioactivity left in fossils with the carbon-14 radioactivity present in living objects today.
There are various kinds of fossils. Some of the important fossils which have been studied are those of ammonite, trilobite and dinosaur.
• Ammonites were the invertebrate animals (molluscs) with a flat, coiled, spiral shell which lived in the sea [Figure (a)].
• The estimation of the age of ammonite fossils have told us that they are about 180 million years old. This means that ammonites lived in the sea about 180 million years ago. Another invertebrate animal fossil which has been studied is that of trilobite [Figure (b)].
• Trilobites were marine arthropods which were common between 400 to 600 million years ago. Dinosaurs are extinct carnivorous or herbivorous reptiles
• The estimation of the age of dinosaur fossils [Figure (c)] have told us that they first appeared on earth about 250 million years ago and became extinct about 65 million years ago.
• It is clear from the above discussion that we can even study about those species which are extinct (no longer exist), by studying their fossils which are found during the digging of earth.

18. Darwin’s Theory of Evolution
• Charles Robert Darwin gave the theory of evolution in his famous book ‘The Origin of Species’.
• The theory of evolution proposed by Darwin is known as ‘The Theory of Natural Selection’. This theory is called the theory of natural selection because it suggests that the best adapted organisms are selected by nature to pass on their characteristics (or traits) to the next generation.
• Darwin’s theory of evolution applies to plants as well as animals.
The main features of theory of Natural Selection are as follows –
(1) Over Production
All Organisms have capability of produce enormous number of offspring, organisms multiply in geometric ratio.
e.g.- Plants produce thousands of seeds.
– Insects lay hundreds of egg.
– One pair elephant gives rise to about six offspring and if all survived in 750 years a single pair would produce about 19 million elephants. Thus some organisms produce more offspring and other produce fewer offspring This is called differential reproduction.
(2) Struggle for existence
Every individuals competes with others of the same and other species for basic necessities like. Space, Shelter and food. It is called struggle for existence and it continues for the whole life from zygote stage to its naturl death.
(i) Intra-specific struggle: It is competition between the individuals of same species for same needs like food, shelter and breeding (most aqute type of struggle).
(ii) Inter-specific struggle: It is the struggle between the individuals of different species for food and shelter.
(iii) Environmental struggle: This struggle is between organism and their environment. All organism struggle with cold, heat, wind, rains, draught and flood etc.
(3) Variations and heredity
Except the identical twins no two individuals are similar and their requirements are also not same. It means there are differences among the individuals. These difference are called variations. Due to variation some individuals would be better adjusted towards the surroundings then the others. According to Darwin the variations are continuous and those which are helpful in the adaptions of an organism towards its surroundings would be passed on to the next generation, while the others disappear.
(4) Survival of the fittest or natural selection
The original ideal of survival of fittest was proposed by Herbert Spencer.
According to Darwin most suitable and fit individuals are successful in struggle for existence. The individual with almost fovourable adaptations are able to lead lmost successful life and are able to win over their matting partners. Darwin called it sexual Selection.
In the struggle for existence only those members survive which posseses useful variations means nature selects fit individuals. This was called Natural Selection.
(5) Origin of New Species
Drawin explained that variations appearing due to environmental changes are transmitted to the next generation. So offspring become different from ancestors. In next generation process of Natural selection repeats so after many generation a new species is formed.
Criticisim of Darwinism
1. Darwin does not explain development of vestigial organs.
2. This theory has no satisfactory explanation for the cause, origin and inheritance of variation.
3. Darwin is unable to explain why in a population only a few individuals develop useful variation and others have harmful variations.
4. Criticism of Darwinism was based on sexual selection. Why only female selects the male for mating why not vice versa.
5. Darwin was unable to differentiate somatic and germinal variations.
6. This theory was unable to explain over-specialization of some organs like tusk of elephants, antlers of deer.
7. This theory only explain the survival of fittest but unable to explain arrival of fittest.
8. The main drawback of Darwinism was lack of the knowledge of heredity
19. Speciation
A species is a population of organisms consisting of similar individuals which can breed together and produce fertile offspring.
The process by which new species develop from the existing species is known as speciation. In simple words, the formation of new species is called speciation.
Important factors which could lead speciation are as follows:
• Geographical isolation of a population caused by various types of barriers (such as mountain ranges, rivers and sea).
• The geographical isolation leads to reproductive isolation due to which there is no flow of genes between separated groups of population.
• Genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.
• Variations caused in individuals due to natural selection.
20. Evolution of Eyes
• First of all the rudimentary eye (basic eye) like that of a flatworm (Planaria) was formed.
• The eyes of flatworm are very simple that are actually just ‘eye-spots’ which can detect light.
• Even these rudimentary eyes provide a survival advantage to flatworm. Starting from this basic design, more and more complex eyes were then evolved in various organisms.
• Most of the animals have eyes. For example, the insects, octopus and invertebrates, all have eyes.
• The structure of eyes in each of these organisms is, however, different which suggests their separate evolutionary origins.
• The evolution of simple eyes called rudimentary eyes, eye is an example of evolution by stages.
21. Evolution of Feathers
• Sometimes an evolutionary change produced in an organism for one purpose later on becomes more useful for an entirely different function. For example, birds evolved feathers as a means of providing insulation to their bodies in cold weather but later on these feathers became more useful for the purpose of flying.
• Even some dinosaurs had feathers though they could not fly by using these feathers. Birds, however, adapted feathers for flying.
• The presence of feathers on birds tells us that the birds are very closely related to reptiles because dinosaurs (which had feathers) were reptiles
22. Evolution by Artificial Selection
• The wild cabbage plant is a good example to prove that entirely different looking organisms can evolve from the same organism by the process of evolution.
• The only difference is that here we are using artificial selection for evolution in place of natural selection. This will become clear from the following discussion.
• The farmers have been cultivating wild cabbage as a food plant for over two thousand years and have produced (or evolved) entirely different looking vegetables like cabbage, broccoli, cauliflower, kohlrabi and kale from it by artificial selection.
• Some farmers wanted to have very short distances between the leaves of wild cabbage and produced the common variety of ‘cabbage’.
• When farmers opted for the arrested flower development of wild cabbage plant, it led to the production of another variety of cabbage called ‘broccoli’.
• Some farmers went in for sterile flowers of wild cabbage and developed another variety of cabbage called ‘cauliflower’.
• When farmers opted for the swollen parts of wild cabbage, it led to the evolution of a yet another variety of cabbage called ‘kohlrabi’.
• And finally, the farmers wanted to grow large leaves of wild cabbage and ended up producing a leafy vegetable called ‘kale’ which is also a variety of wild cabbage.
• Now, wild cabbage is the ancestor and cabbage, broccoli, cauliflower, kohlrabi and kale are all its varieties which have been obtained by evolution ‘induced artificially’ by the farmers.
• The ordinary cabbage, broccoli, cauliflower, kohlrabi and kale look so different from their ancestor wild cabbage that if people had not seen it being done with their own eyes, they would never have believed that vegetables having such different structures can be evolved from the same ancestral vegetable plant.
23. Evolution Should Not be Equated With Progress
• There is no real progress in the concept of evolution.
• Evolution is just the production of diversity of life forms and shaping of this diversity by the environmental selection.
• The only progress in evolution appears to be that more and more complex body designs of organisms have emerged over the ages. This will become clear from the following examples.
• When a new species is formed, it is not necessary that the old species will disappear (or get eliminated) from earth. It will all depend on the environment. Also it is not as if the newly formed species are in any way better than the older ones.
• It is simply that genetic drift and natural selection processes have combined to form a population having different body design which cannot interbreed with the older population.
• It is a common belief that chimpanzees are the ancestors of human beings.
• It is, however, not true that human beings have evolved from chimpanzees. Actually, both chimpanzees and human beings had a common ancestor long time ago.
• The two offsprings of that ancestor evolved in their own separate ways to form the modern day chimpanzees and human beings.
• Again, it is not as if the body designs of older organisms were inefficient. This is because many of the older and simpler forms of organisms still survive on earth. For example, one of the simplest and primitive life forms called ‘bacteria’ still inhabit some of the most inhospitable (or unfavourable) habitats such as hot springs, deep-sea thermal vents and the ice in Antarctica. Most other organisms cannot survive in such harsh environments.
24. Human Evolution
• Human evolution has been studied by using the various tools of tracing evolutionary relationships like excavating (digging earth), carbon-dating, studying fossils and determining DNA sequences.
• There is so much diversity of human body and features on the earth that for a long time people used to talk about different ‘races’ of human beings.
• The human races were even identified on the basis of their skin colour and named as white, black, yellow or brown. It is now known that the so called human races have not evolved differently.
• In fact, there is no biological basis for dividing human beings into different ‘races’.
• All human beings (whether, white, black, yellow or brown) are a single species (called Homo sapiens).
• It has now been established by research that the earliest members of the human species {Homo sapiens) came from Africa. So, irrespective of where we have lived for the past few thousand years, we all come from Africa.
• In other words, our genetic footprints tell us that we have African roots. About hundred thousand years ago, some of our ancestors left Africa while others stayed back.
• Those who left Africa slowly spread across the whole earth.
• Mendel’s experiments tell us the mode of inheritance of traits from one generation to the next and Darwin’s theory of evolution tells us how organisms develop from simple to more complex forms. But neither tells us anything about how life originated on earth (or began on earth). We will now discuss the origin of life on earth briefly.
25. Origin of Life on Earth
• A British scientist J.B.S. Haldane suggested in 1929 that life must have developed from the simple inorganic molecules (such as methane, ammonia, hydrogen sulphide, etc.) which were present on the earth soon after it was formed.
• He said that the conditions on earth at that time (including frequent lightning) could have converted simple inorganic molecules into complex organic molecules which were necessary for life.
• These complex organic molecules must have joined together to form first primitive living organisms.
• Haldane also suggested from theoretical considerations that life (or living organisms) originated in the sea water.
• The theory of origin of life on earth proposed by Haldane was confirmed by experiments conducted by Stanley L. Miller and Harold C. Urey in 1953.
• They assembled an apparatus to create an early earth atmosphere which was supposed to of gases like methane, ammonia and hydrogen sulphide, etc., (but no oxygen), over water.
• This was maintained at a temperature just below 100°C and electric sparks were then passed through the mixture of gases (to simulate lightning) for about one week.
• At the end of one week, it was found that about 15 per cent of carbon (from methane) had been converted into simple compounds of carbon including ‘amino acids’ which make up protein molecules found in living organisms.
• This experiment provides the evidence that the life originated from inanimate matter (or lifeless matter) like inorganic molecules.

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