Genetics is a branch of biology that deals with the study of heredity and variations. Heredity is the transmission of characters from parents to offspring. Offspring or siblings are the product of sexual reproduction and commonly biparental in origin. They resemble their parents, family and species sufficiently that one can recognize them to belong to their particular group. However, the resemblance is never 100%. Child of the same parents, possess genetic variations.
Some amount of variations is produced during asexual reproduction. But the number of successful variations are maximized by the process of sexual reproduction. Mechanism by which variations are created and inherited would be dealt with in the subsequent paragraphs. The long term consequences of accommodation of variations would be dealt with under evolution.
A number of view points were put forward even before Mendel to explain the transmission of characters from parents to offspring. They are often called theories of blending inheritance as they believed that characters of the parents blended or got mixed during their transmission to the offspring.
Gregor Johann Mendel is known as father of genetics, because he was the first to demonstrate the mechanism of transmission of characters from one generation to the other. He also gave generalizations some of which were later raised to the status of principles or laws of inheritance. They constitute the foundations of genetics.
Gregor Johann Mendel was born to a poor family on 22nd July 1922 in Silisian village in Heinzendorf (Austria) now part of Czech Republic. His father was a gardener. He joined Augustinian monastry of St. Thomas in 1843 at Brunn (then in Austria; now Brno in Czech Republic), where at the age of 25, he was made a priest at monastery in 1847.
In 1951, he went to the University of Vienna to study Natural Science and Mathematics but he returned to Brunn again in 1854 without completing the courses and took teaching responsibility at the monastery. From 1856 to 1664, Mendel combined his talents, background and interests and carried a series of experiments in plant hybridization on garden pea (Pisum sativum). He observed occurrence of two types of seeds growing in his monastery. He took two years (1857-1859) for selecting different varieties of pea plant. Mendel employed laws of probability and statistical methods for the analysis of his results.
In 1865, he formulated the laws of heredity and presented it before the two meetings of Natural History Society of Brunn. In 1866, he published his research paper “Experiments in Plant Hybridization” in Fourth Volume of “The processing of Natural History Society of Brunn”. In 1884, Mendel died on 6th January due to a kidney disorder at Brno (Czech Republic).
Mendel proposed the concept of hereditary units – “Equal numbers of factors (what we now called genes) inherited from each parent determined the observable characters of the offspring. Characters are themselves not inherited but the particles, units or factors that determine or control the observable traits are transmitted from parents to offspring’s” – this is the basic theme of Mendelism. Mendel’s work remained unnoticed for 34 years.
Mendel’s Experimental Material. Mendel selected Garden pea (Edible Pea = Pisum sativum) for his experiments because (i) He discovered for the first time the occurrence of two types of seeds in Pea plants growing in the garden of his monastery. (ii) Pure varieties of Pea were available. (iii) Pea plants showed a number of easily detectable contrasting characters. (iv)The flower structure of Pea is such as to allow controlled breeding. It can be crossbreed manually. (v) Pea flower normally remains closed and undergoes self-pollination. (vi) It is an annual plant and gives results in a year time (vii) A large number of seeds are produced per plant. (viii) The plant is grown easily and does not require after-care except at the time of pollination of hybrid plants and raising of subsequent generations like F2, F3, F4.
Selection of Parents
Mendel selected 7 pairs of pure or true breeding varieties as the starting material for his experiments. Mendel employed traits of several characters for his experimentation, one from each variety. All the characters had easily distinguishable alternate traits, e.g., tallness and dwarfness, violet or red flowers and white flowers.
Hybridisation for F1 Generation. Mendel performed reciprocal crosses between plants having alternate forms of a character, tall and dwarf, red flower and white flower. The cross in which only two alternate forms of a single character are taken into consideration is called monohybrid cross. Mendel also performed between involving two characters. They are called dihybrid crosses. Trihybrid and polyhybrid crosses were performed as well.
Rediscovery of Mendelism
In 1900, hybridization experiments similar to Mendel’s work were independently performed by three botanists, Carl Correns of Germany, Eric Von Tschermack of Austria and Hugo deVries of Holland (Netherland) and reached the same conclusions, which Mendel had predicted some 34 years ago. It was in this background that they collectively agreed for naming the Laws of heredity after Mendel.
In 1901, Mendel’s original work was republished in Flora.
In 1905, Bateson, coined the term genetics. He translated Mendel’s work in Roman and extended Mendelian principles to animals and found them to be equally applicable to them.
Reasons For Mendel’s success
(i) Careful choice and selection of experimental material. Several varieties of pea plants were easily available, required little space, had shorter generation time and produced many offsprings.
(ii) Reproductive structures being enclosed by petals (characteristics of Papillionaceae). The cleistogamy (self-pollination) helped Mendel perform selfing experiment.
(iii) Pea plants, after emasculation, could also be crossed easily. Mendel actually selected 22 varieties out of 34, which showed distinct morphological differences in allelic characters. Out of 22, finally 7 characters (one is vegetative i.e. height of the plant and the other six reproductive, two each are related to flowers, seeds, and fruits).
(iv) The most important reason for the success of Mendel was that he took into consideration only one character at a time when analyzing the progeny of a cross (Mendel’s predecessors got the data intermingled because of the consideration of many characters at a time).
(v) He made statistical analysis of the offspring of his crosses.
(vi) Mendel’s mathematical background helped him in analyzing the progeny of his crosses.
(vii) He pooled the data of many similar crosses and analysed results statistically.
A hereditary unit occupying a specific site (locus) on a chromosome and which controls the expression of a character.
Alleles are a pair of alternate forms of a gene which are situated at the same loci of homologous chromosomes.
Alternate forms of a character e.g., red and white, tall and dwarf.
Out of these alternate forms of a character, only one form is wild and is abundantly present in nature, and is termed dominant. The recessive character originates through mutation of the wild gene.
Homologous and Heterologous Chromosomes
The two chromosomes of a diploid cell inherited from the two parents, which are identical in their structural and genetical characters are homologous chromosomes. Any two chromosomes of a diploid cell which are different from each other in their structural as well as genetical characters are heterologous chromosomes.
Homozygous and Heterozygous Condition
When the two homologous chromosomes bear identical genes or alleles it is known as homozygous (like AA, aa) but when the two homologous chromosomes bear alternate forms of a character, it is termed heterozygous (Aa, Tt, Dd, etc.)
Phenotype and Genotype
Genotype is the genetic constitution of an organism.
Phenotype is the external appearance of an organism which is controlled by the genotype and to some extent by the environment.
The seven pairs of characters which Mendel selected are as follows :
Results of a Monohybrid cross.
Crosses in which the dominant allele is contributed by one parent and the recessive allele by the other parent and vice-versa. They give similar results.
It is a cross between F1 individual and either of the parents (dominant or recessive). It is of two types :
(i) Out cross : – a cross between F1 individual and dominant parent
(ii) Test cross :– a cross between F1 individual and recessive parent.
Results of a back cross and test cross.
Mendel’s 1st Postulate : Law of Unit Factor
Observable characters themselves are not inherited but particles, units or factors that determine or control the observable characters (traits) and exist in pairs in an individual organism are transmitted from parents to offspring. Since factors occur in pairs, three combination, TT. Tt and tt (homozygous and heterozygous tall and homozygous dwarf) can only be expected.
Mendel’s 2nd Postulate : Law of Dominance
In a cross between two organisms – pure for a particular pair of alternate forms of characters, only one form which is dominant appears in first filial generation and it masks the effect of other, which is recessive.
Mendel’s 3rd Postulate : Law of Segregation (Purity of Gametes)
When a pair of alleles is brought up in a hybrid, they do not fuse or mix-up rather remain as such and they segregate during anaphase I. This is also known as the law of purity of gametes since after segregation one gamete inherits only one allele for a character.
In order to explain Dihybrid ratio, Mendel considered two characters at a time i.e., colour of the seed and the shape of the seed in another cross in pea plant.
(1) Colour of seed – yellow and green, (2) Shape of seed – round and wrinkled
Results of a dihybrid cross.
Mendel’s 4th Postulate : Law of Independent Assortment
When parents showing two or more pairs of contrasting characters are crossed the inheritance of any one character is independent of the inheritance of any other character.
Phenotypic ratio = 9 (Round & yellow) : 3 (Round & green) : 3 (wrinkled & yellow) :1 (wrinkled & green) Genotypic ratio = 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1.
Test cross of Dihybrid cross : Test cross was employed by crossing the F1 plants with recessive parents giving a ratio of 1 : 1 : 1 : 1.
Summary of Mendelian Principle
- Each characteristic of an organism is controlled by a pair of alleles.
- An organism, derived from two parents, which differ for a pair of characters, only one (dominant) appears in 1st filial generation bringing total exclusion of the other (recessive) (Law of dominance).
- During meiosis, each pair of alleles separates (segregates) and each gamete receives one of each pair of alleles (Law of segregation).
- During gamete formation in each sex, either one of a pair of alleles may enter the same gamete cell (combine randomly) with either one of another pair (Law of independent assortment).
- Each allele is transmitted from generation to generation as a discrete unchanging unit.
- Each organism inherits one allele (for each characteristic) from each parent.
ABO Blood Groups
There are four types of blood groups in human being – A, B, AB and O which are due to presence or absence of two types of proteins called A and B on the surfaces of RBC. These blood groups are controlled by the multiple alleles of the autosomal gene 1 (iso-haemoagglutinogen) IA, IB, and IO as follows :
Blood group A – IA or IAIO
Blood group B – IBIB or IBIO
Blood group AB – IAIB
Blood group O – IOIO
The antigens (agglutingogens) produced by the blood types and the antibodies (aglutinins) produced by them are shown below :
The blood groups showing anti-factors found in plasma of blood
Blood group Anti-factor May donate May receive
Present Blood to blood from
A anti-B A, AB A, O
B anti-A B, AB B, O
AB none AB A, B, AB, O
O anti A and anti B AB, AB, O O
During blood transfusion incompability of the blood creates serious complications. The antibodies already present in the blood plasma of the recipient cause agglutination or clumping of the donor’s RBC if they contain the corresponding antigens. O negative which does not have antigens for A or B is, therefore, a universal donor. The O blood group cannot, however, receive blood from A, B or AB because it has anti-factor for both A and B. AB blood group is a universal acceptor because it does not have antifactor for A or B. The characteristics of the different blood groups are given below :
Human RBC shows another eight types of antigens called Rh factors in addition to the A and B. Since they also occur in the blood of Rhesus monkey it is called Rh factor. Those who have the factor are termed Rh-positive (Rh+) and those who lack it are called Rh- negative (Rh–). The homozygous dominant (RR) and heterozygous (Rr) individuals have the Rh protein and homozygous recessive (rr) individuals lack the Rh protein.
If the blood of Rh-negative comes in contact with that of Rh-positive individual due to transfusion of blood for the first time it may not create any problem. The second transfusion of blood if given immediately, the antibodies formed due to earlier transfusion will create problem by attacking the donors blood. Similarly the first pregnancy does not creates any problem because the Rh-positive blood of the foetus does not come in contact with that of the Rh-negative mother. The contact occurs too late in pregnancy and, therefore, enough anti-Rh factor are not produced in the mother. The first child escapes any harm. During second pregnancy the RBC of the Rh-positive foetus are damaged by the anti-Rh factors already formed in the blood of the mother due to earlier pregnancy. The breakdown of RBC and the conversion of the red haemoglobin into yellow pigment results in jaundice and anemia in the child. The child dies either before or shortly after birth.
Polygenic (Quantitative) Inheritance
In Mendelian experiments there are only two types of individuals in all the generations. The characters are either dominant or recessive. There are no intermediate types. The investigation of several examples of continuous variation by Galton and others led to the suggestion that some of these characters showing quantitative variations were controlled by two or more genes.
Characters like height, intelligence and skin colour of human-beings, height of plants, size, shape and number of seeds, fruits, milk yield in animals were found to be quantitative in nature and the genes controlling them were supposed to have the same but cumulative or additive phenotypic effect. Quantitative inheritance was later on termed polygenic or multiple factor inheritance or metric inheritance.
The first example of polygenic inheritance was obtained in kernel colour of wheat by a Swedish geneticist H.Nilsson-Ehle (1908). He found that the colour of the kernel was controlled by as many as 2 genes. Assuming two genes to be responsible for the kernel colour a cross between red and white grained plants of wheat shows a ratio of 1 : 4 : 6 : 4 : 1 in their progeny.
A cross between red and white grained wheat plants
The colour of skin of human beings was studied by C.B. Davenport (1913). It is controlled by 3 pairs of genes (quantitatively six genes). Three genes can give rise to 8 types of gametes and 64 types of zygotes. Each dominant allele is capable of producing equal amount of melanin and, therefore, on the basis of the number of dominant alleles there would be variation in the amount of melanin in the skin of human beings. The 64 types of combinations of alleles are in the ratio of 1 : 6 : 15 : 20 : 15 : 6 : 1 i.e. of 7 types.
It will be seen that the frequency of intermediate type of complexion is maximum i.e. it is 20 out of a total 64 combinations of the alleles.
It is believed that the multiple copies of the same gene must have developed as a result of duplication of chromosomes or chromosome its parts during the course of evolution.
Human skin colour inheritance.
Expression of the Traits
Cellular DNA is the information source for making proteins in the cell. A section of the DNA that provides information of one protein is called gene for that protein. Proteins control the characteristics that we have earlier discussed. For example tallness is controlled by hormones that can trigger growth. Plant height thus depends on the amount of a particular plant hormone. The amount of the plant hormone made will depend on the efficiency of the process for making it. An enzyme may be important for this process. If this enzyme works efficiently a lot of hormone will be made and the plant will be tall. If the gene for that enzyme has an alteration that makes that enzyme less efficient the amount of hormone will be less, and this plant will be short. Thus, genes control characteristics or traits.
During sexual reproduction both parents contribute equally to the DNA of the progeny. Both parents together determine the trait in the progeny by contributing a copy of the same gene. Thus each pea plant has two sets of all genes, one inherited from each parent. Each germ cell produced by meiosis has one gene set only.
However, the progeny plants do not unherit all the genes as a single whole gene set from each parent. For example, the two characteristics ‘R’ and ‘Y’ are not necessarily linked to each other and are independently inherited. Each gene set is present, not as a single long thread of DNA but as separate independent pieces, each called a chromosome. Thus, each cell has two copies of each chromosome. One each from the male and female parents. Each germ cell will take one chromosome from each pair and these may be either maternal or paternal in origin. When two germs cells combine, they will restore the normal number of chromosomes in the progeny, ensuring the stability of the DNA of the species. Such a mechanism of inheritance explains the results of Mendel’s experiments and is used by all sexually reproducing organisms.
Establishment of male and female individuals or male and female organs of an individual is called sex determination It is of two types – environmental and chromosomal.
- Environmental or Non-genetic Determination of Sex.
- Marine mollusc Crepidula becomes female if reared alone. In company of a female, it is develops into male (Coe, 1943).
- Marine worm Bonellia develops into 3 cm long female if its larva settles down in an isolated place. It grows into small (0.3 cm long) parasitic male if it comes closer to an already established female (Baltzer, 1935). The male enters the body of the female and stays there as a parasite.
- Ophryortocha is male in the young state and female later on.
- In Crocodiles and some lizards high temperature induces maleness and low temperature femaleness. In turtles, males are predominant below 28ºC, females above 33º C, and equal number of the two sexes between 28–33ºC.
- Chromosomal Determination of Sex
Stevens (1905) put forward chromosome theory of sex and named the X- and Y- bodies as sex chromosomes, X and Y. Chromosomal or allosomic determination of sex is based on heterogamesis or occurrence of two types of gametes in one of the two sexes.
In human the females possess two homomorphic (=isomorphic) sex chromosomes, named XX. The males contain two heteromorphic sex chromosomes, i.e. XY. The Y-chromosome is often shorter and heterochromatic. The XY chromosomes are homologous and synapse during zygotene. Homologous regions of the two help in pairing. They carry same genes which may have different alleles. The genes present on both X and Y chromosomes are XY-linked genes.
Human beings have 22 pairs of autosomes and one pair of sex chromosomes. All the ova formed by female are similar in their chromosome type (22 + X). Therefore females are homogametic. The male gametes of sperms produced by human males are of two types, (22 + X) and (22 + Y). Human males are therefore, heterogametic (male digamety).
Sex of the offspring is determined at the time of fertilization. It cannot be changed later on. It is also not dependent on any characteristic of the female parent because the latter is homogametic and produces only one type of eggs
(22 + X). The male gametes are of two types, androsperms (22 + Y) and gynosperms (22 + X). They are produced in equal proportion. Fertilization of the egg (22 + X) with a gymnosperm (22 + X) will produce a female child (44 + XX) while fertilization with an androsperm (22 + Y) gives rise to male child (44 + XY). As the two types of sperms are produced in equal proportions, there are equal chances of getting a male or female child in a particular mating. As Y- chromosome determines the male sex of the individual, it is also called androsome.
Sex determination in humans
Origin of Life
As far as known life occurs only on earth though there is possibility of its presence elsewhere as well. There are several theories about the origin of life like theory of special creation, theory of eternity, theory of catastrophism, cosmozoic theory, theory of panspermia, theory of spontaneous generation and Oparin and Haldane theory.
According to Oparin and Haldane
- Spontaneous generation of life under present conditions is not possible.
- Earth’s surface and atmosphere during the first billion years of its existence were radically different from today’s condition.
- Earth’s initial atmosphere was reducing.
- The first life arise from a collection of chemicals through a progressive series of chemical reactions in which atoms combined into inorganic molecules, inorganic molecules into simple organic compounds, and simple organic compounds into complex organic compounds, which finally got organized into the living matter. This abiogenesis occurred about 3.7 billion years ago.
- Solar radiation, heat radiated by earth and lightening provided energy for evolution of molecules.
The surface temperature of earth (50-60ºC), UV radiations, cosmic rays and lightening, dry heat of volcanic eruptions, all provided the necessary energy which helped methane, ammonia, water and other molecules to form a whole array of simple organic substances like aldehydes, alcohols, organic acids, amino acids, purines, pyrimidines and other sugars, fatty acids, glycerol etc. There was little degradation due to absence of enzymes, life and oxygen. As a result organic molecules accumulated. Water bodies changed into a broth. The experimental evidence for this was provided by Miller and Urey.
Miller and Urey’s Experiment :
That simple organic compounds could be formed in nature was experimentally demonstrated in 1953 by Stanley Miller and Harold C. Urey. They designed a glass apparatus comprising a gas flask, a condenser, and a liquid flask interconnected with tubes and fitted with sources of energy. The apparatus simulated the conditions on the primitive earth, including a reducing atmosphere and an “ocean”. They circulated in this apparatus a mixture of methane (CH4), ammonia (NH3) and hydrogen (H2) in the ratio of 2 : 2 : 1, and water vapour (H2O). They provided energy for the interaction of the gases present in the mixture in the form of electric sparks of 75, 000 volts from electrodes in the gas flask. The electric sparks simulated lightening. Then the gases were condensed in a narrow tube and passed through a liquid flask. Here, energy was provided as heat with an electric heater. The latter simulated volcano. Thus, they recreated in their apparatus the probable conditions of the primitive earth. They kept their experiment working continuously for one week. A mixture of small organic molecules was formed in the gas flask (atmosphere) and was carried by condensation (rain) to the liquid flask (ocean). They then analysed the chemical composition of the product of the chemical reactions in the mixture by chromatographic and calorimetric methods. They found many simple organic compounds which included amino acids, such as glycine, alanine, aspartic acid; adenine and simple sugars such as ribose.
The smaller organic molecules thus, formed larger organic molecules which in due course of time formed first living aggregates called protocells or coacervates. Protocells developed into first living cells, which were prokaryotes.
It is the formation of newer types of organisms from the pre-existing ones through modification. With passage of time there are changes in the properties of population of organisms or groups of such population. The changes pile up and give rise to new forms. The new forms possess a common ancestor.
Illustration of Evolution
Consider a group of twelve red beetles. They live in some bushes with green leaves. Their population will grow by sexual reproduction and therefore can generate variations. Also, assume that crows eat these beetles. The more beetles the crows eat, the fewer beetles are available to reproduce. Now, let us assume two variations to occur in beetle population.
A colour variation arises during reproduction so that resultant progeny is green instead of red. This beetle can pass the colour on to its progeny. Crows cannot see green colour beetles on green leaves of bushes and therefore cannot eat them. Thus, progeny of red beetles is eaten and not the green beetles. As a result, there are more and more green beetles than red ones in beetle population.
In second situation another colour variation arises during reproduction, but results in a beetle that is blue in colour instead of red.
This beetle can also pass the colour on to its progeny. So that all its progeny, beetles are blue. Crows can see blue coloured beetles on green leaves and red ones also. So, they can eat both. In the population, as it expands, there are a few blue beetles, but most one red. But, at this point an elephant comes by and stamps on bushes where beetles live. This kills most of the beetles. By chance, the few beetles that have survived are mostly blue. The beetle population slowly expands again, but now, the beetles in population are mostly blue.
Thus, in both situations, what started out as a rare variation came to be a common characteristic in the population. In other words, the frequency of an inherited trait changed over generations.
Acquired and inherited Traits
l Acquired Traits : Characters are acquired by organism to meet the new needs. Thus, the changes brought about by use and disuse of organs and by the influence of environmental factors in an individual during its life time are called acquired characters. Since, these changes do not affect the germ cells, they are not inherited e.g. Piercing of earlobes in women.
l Inherited Traits : Certain new trait arise by mutation in the DNA of an individual. If the new trait is useful for the animal, it is retained and passed on to its progeny. Such traits are called inherited traits. e.g. Melanism in peppered moth.
Theories of Organic Evolution
l On the basis of various evidences, it is now established that all the existing (or extinct) diverse forms of life on earth have evolved from earlier simpler ancestral forms by gradual modifications through generations i.e., via evolutionary process.
l The question as to how the process of evolution has took place? Has been explained by many scientists e.g., (i) Theory of Natural selection – Darwinism, (ii) Theory of Inheritance of acquired characters – Lamarckism
l Charles Darwin (1809 – 1882) proposed the theory of natural selection. Variations if selected were suggested to be the cause of speciation. Darwin as leader of the expedition of a team on the ship named ‘H.M.S. Beagle’ made global survey for studying flora and fauna.
l During his study lasting five years, he collected variety of flora and fauna from different parts of globe. After systematic studies of various facts & phenomena he put forward his theory of evolution of diverse forms of life. He was also one of the believers of the theory of special creation.
l Wallace, surveyed and studied the diversities of life around the world (mainly S. America and Southern Oriental region). He sent his paper entitled Tendency of organism to depart indefinitely from original type to Alfred Russell Wallace for his comments. By coincidence Wallace had also written similar article. They later agreed to publish a joint paper entitled Origin of Species (1858)
l Darwin published (1859) Origin of new species by Natural Selection in which he stated his own theory of evolution in detail.
l Darwin in his other book entitled Descent of Man, Darwin stated that monkeys are the ancestors of man.
l Darwin’s theory was influenced by the work of Malthus (1798) (malthusian theory of population), Lyell (1832) (principles of geology), and Spencer (1856) (survival of fittest).
Salient features of Darwin’s theory of natural selection
l The Darwinian principles of natural selection and process of evolution states that “natural selection is differential success in reproduction and its product in adaptation of organisms to their environment.” Thus, natural selection occurs through an interaction between the environment and the variation(s) inherited in the population.
l The success of survival and reproduction depends on the characteristics of the individual. If the traits that enable an organism to survive and reproduce, are heritable, then such an offspring will be fit to cope up with various environmental factors and live successfully.
l The favourable variations accumulate over the generations to ultimately form individual different from the original ones. This results in the formation of new species.
- Enormous capacity to reproduce and rapid rate of multiplication: Every animal has enormous capacity of reproduction. The number of members of a natural population increases geometrically like 6 ® 18 ® 54 ……but, all the offsprings do not survive hence the system of natural control operates to keep their number limited.
Examples : Oyster lays about a million eggs and a Salmon about 2.5 crore eggs at a time. One pair of sparrow will give rise to 275 billion offsprings in 10 years. One pair of elephant, the slowest breeder, can breed 2 crores of elephant in 750 years if all the offsprings remain alive. But this does not happen in nature.
l Struggle for existence: The enormous increase in the population size of a particular species results in the struggle between the members of the same species and other species as well for space, food, etc. This struggle is of the following types:
l Interspecific struggle or internecine struggle: Struggle between two different species of animals (organism) for food, shelter or space takes place. Example: Host and parasite, prey and predators for food; similarly struggle for shelter among animals of habit and habitat occurs. All this leads to fierce competition that ultimately cause reduction in their population size.
l Intraspecific struggle or intrarecine struggle: Struggle within the same species is very common for food, shelter and reproduction. There is always fight among males to establish reproductive superiority culminating in mating with the fertile female.
l Environmental resistance: Various types of natural calamities like flood, famine, cyclone, earthquake and volcanic eruptions take heavy toll of life. This also contributes in determining the size of the population and also the intensity of inter- and intra-specific competitions.
Survival of fittest: Spencer (1856) used the term survival of the fittest. According to him, only those organisms who achieve suitable changes or variations (traits) are able to survive or continue through the struggle while the others which are weak will be gradually eliminated and thus become extinct. The role played by nature in selecting such organisms to continue while rejecting the other to be eliminated from the population. Variation is a usual process by which the offsprings of same parent differ from each other and the offsprings whose phenotypic traits are more compatible to the environment are selected by nature to survive and proliferate.
Origin of species by natural selection: Nature selects only those organisms which possess well defined characters best suited to the environment. Such organisms with higher rate of adaptation and more suitability to the changed condition will be regarded as future life stocks. These organisms are capable of reproduction to produce offsprings and these are fully supported by the nature.
Examples of natural selection
(i) DDT resistance in mosquito
l Both DDT-resistant and DDT-sensitive population of mosquito were present before the use of DDT. But, DDT-resistant variety had no advantage over DDT sensitive.
l After the use of DDT, the DDT-sensitive population was affected. While the DDT-resistant variety enjoyed better time and their population grew larger.
(ii) Sickle-cell anemia
l The continuation of this trait in heterozygous individual has been found to be advantageous against the effect of malarial parasite because its life cycle cannot proceed in the sickle shaped RBC.
l Therefore, this trait is mostly found in the people of area where malaria is most prevalent. Natural selection maintains this abnormal trait along with normal trait.
This was the first scientific theory about the mechanism of organic evolution propounded by the French Naturalist, Jean Baptist de Lamarck (1744-1836). He published his ideas in a book entitled “Philosophie Zoologique” (1809)
Central theme of this theory was in the form of following 3 points
l Continuous change in environment and direct effect of environment: The environment plays vital role in the determination of overall characters of living organisms. The living organisms adapt themselves according to the condition prevailing in that particular environment or habitat.
l Use and disuse of organs: The organ or structure which is in frequent use will develop more stronger and all those structures which are in less use gradually become non-functional and behave like rudimentary or vestigial structures.
l Inheritance of acquired characters: These changes of characteristics acquired by the organism in their life time, passed on to their offsprings. By accumulating these many changes their progeny become different from their ancestors and they are now regarded as a new species.
- Example for use of organs
Long neck and fore limb of giraffe: The ancestors of present day giraffe were short grazing animal like common deer. Grassland changed into desert. Thus, there was no option but to switch over the feeding on some bushes and tall trees left around. To reach the leaves of these bushes and trees these ancestors of giraffe had to make continuous effort by stretching its neck and forelimb. As a result of this the neck and fore limbs developed more, which finally gave rise to present day giraffe. This acquired trait was transmitted to their offsprings.
- Example for disuse of organs
Ancestors of snakes were the limbed-lizard like animals which lived in bushes or thick forests. So, they had to pass through narrow passages. Crawling on the belly was more suitable mode of locomotion in such a habitat. Their limbs became obstacles for such movement. Being of no use, they started degenerating and finally disappeared. The present day snakes evolved from such lizards which have lost their limbs. They acquire slender and cylindrical body which was best suited for fossorial (burrowing) habit.
Criticism of Lamarckism
l Lamarck’s theory was criticised vehemently by contemporary biologists.
l Darlington stated that “Larmarcks’s theory is an evergreen superstition”.
Examples that invalidate Lamarckism
l Circumcison of foreskin of penis in jews and muslims, piercing of ear-lobe in Indian women, shaving of beard in man for many generations brought no change in them which should have been as per the theory of Lamarck.
l Muscles of arms and shoulders in blacksmiths did not change even after many generations.
l Castle and Philips transplanted the ovary of black guinea pig into the body of white guinea pig and found no change in their progeny. This showed that environment does not cause change as stated by Lamarck.
l August Weismann (1812 – 56) disproved the concept of Lamarckism by performing experiments in which the tail of mice was cut continuously for 70 generations. He showed that the tail appeared even after such an operation in succeeding generations of mice.
l Weismann propounded the theory called as Theory of Germplasm (1892) which entails that every organism consists of two components: (1) Somatoplasm, and (2) Germplasm. Changes which appear only in the germplasm are inheritable, but not of somatoplasm.
Theory of Pangenesis
l Darwin (1868) gave another theory to rectify the criticisms and questions raised against his theory of Natural selection. He gave the idea of pangene or gemmule as smaller unit representing each part of the body. Such pangenes are circulated in the blood. Pangenes reach to gonads through blood and get incorporated into the gametes. This is how these pangenes or factors are passed to the offsprings who exhibit variations.
Evidences of Evolution
l The term evolution has come from the word to revolve which means rolling. Life is always in the process of moving around as a result of which we see such a vast diversity of life.
l The term was first used by Herbert Spencer (1856).
l Evolution is a slow, continuous, irreversible and natural process of change as a result of which simple forms of life gradually change into complex form.
|· All the diverse organisms have evolved as a result of gradual change from pre-existing organism. To support this assertion there are available evidences from:
· Connecting links,
· Morphology and comparative anatomy,
· Physiology and biochemistry,
· Cytology and genetics,
|Phenological tree of life|
- Evidences from taxonomy
l The classification of more than 15 lakh species of animals on the basis of their natural similarities and dissimilarities prove that organic evolution is an established process.
l Members of different groups show common feature, which establish their geneological relationship and common ancestory.
l A geneological tree called tree of life was first drawn by Lamarck (1809).
l Animal of similar origin are kept in one group. This is how their kinship, quality or relationship with other animals is defined.
l This is observed at all levels of classification i.e. Phylum Class Order Genus Species.
l The system of binomial nomenclature by Linnaeus was adopted to denote the name of genus and species.
l All similar species are grouped together in one genus.
|II. Evidences from connecting link
Connecting link (s) represent the species which have characteristics of two phyla and thus provide evidence for the evolution of one phylum from the another.
l Neopilina– has annelidean and molluscan traits.
l Peripatus– exhibits feature of both Annelida and Arthropoda.
l Latimeria– is a connecting link between fishes and amphibians, as it displays features of Crossopterygians and dipnoi (lung fishes).
l Archaeopteryx– it is a connecting link between reptiles and aves as this Jurassic fossil exhibits characters of both.
l Prototherian (Monotremes) – manifest both reptilian and mammalian features.
III. Evidences from morphology and comparative anatomy
l The evolutionary relationship can be traced out on the basis of structural features of the organisms. Evidence exists based on:
(iii) Vestigial organ
(iv) Atavism (reversal of characters)
l Richard Owen (1804 – 1892) introduced the term homologous which means organs having the same origin, and structure but their function may be same or different. This is also called as divergent evolution.
Examples of homologous structures
l Heart of different vertebrates (from fish to mammal) have same basic structural plan but have changed in the course of evolution. Similar is the case of brain structure of vertebrates.
l Hand of man, wing of bat, wing of birds, flipper of whale & forelimb of cow are similar to tetrapods, but have different functions. They also appear to be different.
l Leg of insects e.g. cockroach, grasshopper, honey bee, cricket, aquatic beetle are different
in their form but their origin is the same i.e., all consist of five segments; coxa, trochanter, femur, tibia and tarsus.
l Mouth parts of different insects are homologous but are adapted to their specific feeding habits. The thorns of Bougainvillae and the tendril of Cucurbita are homologus but functionally different.
(ii) Analogous organs
l Analogous organs have different origin but similar structure and function. This is also called as convergent evolution
Examples: Wings of insect, bat, and bird are of different origin but have similar function.
(iii) Vestigial Organ
l The vestigial organs are those which were functional in the ancestral form but became non-functional or degenerate in subsequent evolved forms.
l The remnant, therefore, indicates the origin of such animal from the stock where it was functional.
Examples: Vestiges of hind limb and pelvic girdle occur in python. Splint bones are found in horses.
|l Humans have more than hundred vestigial organs. Some of the vestigial organs or structures of human are as follow:
(i) Ear (pinna) muscle,
(ii) Wisdom teeth,
(iii) Vermiform appendix,
(v) Plica semilunaris (nictitating membrane of eye),
(vi) Body hair
|Vestigial organs of man|
(iv) Atavism (Reversal)
l Sudden reappearance of ancestral character in any individual is called atavism
Examples: Longer canines of humans, tail in human baby.
- Evidences from physiology and biochemistry
(i) Biochemical similarity
l In the cell of all organisms the basic molecules are the same, e.g. carbohydrates, amino
acids, lipids, enzymes, etc.
l Other macromolecules like the content of tissue matrix (protein) are similar for all the vertebrates.
l The genetic code and the mechanism of protein synthesis are also similar in all organisms.
l Trypsin (the oldest known enzyme) is found in all animals for digestion of protein
(ii) Serological similarities
l Amongst vertebrates the constitution of blood is similar, cell types are the same e.g. RBC and WBC.
l Plasma contains the same types of protein and other substances.
l Respiratory pigment is haemoglobin.
- Evidences from embryology
l Haeckle’s (1866) Biogenetic law or Recapitulation theory or theory of Paleogenesis states “Ontogeny recapitulates phylogeny” which means the development process of an individual repeats the story of development of its race.
l The seedling of Acacia initially develop simple leaves which later transforms into compound leaves. Modern day oak of southern USA retain foliage through out the year while of northern USA shed leaves in winter. The southern species is considered to be more primitive then the northern species.
- Evidences from cytology and genetics
l The organization of cells in prokaryotes and eukaryotes show definite relationship.
l Structure and function of plasma membrane and other cellular processes are similar.
l The process and patterns of cell division are same for all eukaryotic cell, e.g., mitosis and meiosis
l Basic principles and laws of genetics apply similarly to all organisms except some individual differences.
VII. Evidences from palaeontology (study of fossils)
l Various fossil examples indicate the pattern of evolution of many animals and the related groups.
l Archaeopteryx and other related fossils give clear impression of birds having reptilian origin.
l Likewise, the fossil records of camel, elephant, horse and man are sufficient to suggest about their evolution.
l Fossils of dinosaurs show the process of their evolution; their rule on earth and then extinction in mesozoic era (golden age of reptiles).
Formation of fossils
l It involves the replacement of organic molecule substance by inorganic molecule in the dead body. The shape and form of the body is, therefore, preserved as such.
Conditions for fossilization
l The dead body of organism shouldn’t get destroyed or decayed.
l It must be burried as such under sand, soil, snow, volcanic lava, oil or tar, resin or amber secreted by plants.
Types of fossil and their formation
The process of fossilization took place in the following ways:
l Petrifaction: Occured by the total replacement of organic molecules by minerals like iron pyrites, silica, calcium carbonate, etc. It was the main and most common way of fossil formation
l The hard part like bone, teeth, shell, (mollusca), cuticle and tree trunks got fossilized more readily than soft parts as these already contain minerals.
l Coal is the similar form of fossils of plants forming under the process of petrifaction which resulted in the formation of coal which is regarded as fossil of plants.
l The other processes for formation of fossils and their types are:
l Preservation: When a dead body or living animal gets suddenly embedded within a liquid medium which soon becomes solid, the body remains intact within it without any change at tissue level. Such liquids as resin, amber, gum or other plant secretions act as preservatives. It also occurs in ice e.g., wooly mammoth of Siberia.
VIII. Evidences of evolution of Organs
Evolution of eye in animals : Eye is a very popular adaptation. Insects have them, so does an octopus and vertebrates. It shows complexity in structure and function from lower to higher animals.
Feathers started as providing insulation in cold weather. But, later they become useful for flight. In fact, some dinosaurs had feathers. Birds seem to have adapted the feathers to flight. This, of course, means that birds one very closely related to reptiles, since dinosaurs were reptiles.
Molecular phylogeny, also known as molecular systematic, is the use of the structure of molecules to gain information on an organism’s evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a so-called phylogenetic tree. Molecular phylogeny employs nucleotide sequences (nucleic acids) from several organisms to compute the phylogenetic tree.
Genus Homo is the Latin name for man. A number of species belonging to genus Homo have been recognized from the fossil records.
- Homo habilis was named by famous anthropologist Louis Leakey. It existed in Africa about 2 million years ago and was supposed to have larger brain. It used tools and was bipedal.
- Homo erectus. The earliest fossils (skulls) are known to be about 1.7 million years old. The famous fossils of Java man and Peking man of China date back to about 500, 000 years and are supposed to belong the same species Homo erectus, the erect man.
– They are believed to have migrated to Asia and Europe.
– They used fire and progressed with time, using refined tools.
– They stood upright and were taller, about 5 ft. high.
- Neanderthal man. Neanderthal man is supposed to be a primitive from of Homo sapiens and hence is named as Homo sapiens neanderthalensis.
– Neanderthal man lived about 70,000 to 40,000 years ago and extended from Europe to Asia.
– He resembled modern man but was relatively short, stocky and heavily built.
– He made tools and used animal hides as clothing.
– His brain was almost as large as that of modern man.
– Neanderthal man lived in caves or built hut like structures for dwelling.
– He made use of fire, stone tools and buried his dead.
About 30,000 years ago Neanderthal man disappeared. Though the reason is not well known, it is suggested that either they vanished due to the cold weather of the last glacial period or evolved into modern man.
Cro-Magnon man. It is an early form of Homo sapiens. It appeared about 34000 years ago. The fossils of this period and onwards are identical to that of modern human skeleton.
– It resembled modern man in size and appearance.
– Cro–magnon man was a great hunter and used stone tools, weapons, spears and arrows.
– He inhibited the caves of Europe and left behind beautiful picture of the animals he hunted on the cave walls in France and Spain.
CroMagnon man was superior in intelligence and lived at the same time as Neanderthal man and may have been responsible for its extinction.
- Modern man. The modern man. Homo sapiens sapiens as present today, began its expansion about 10, 000 years ago after the last glacial period. It spread all over the globe and became a dominating species. It has changed from cave-dwelling hunting life to the crop-raising life. It has cultivated plant, domesticated animals and brought about cultural revolution with its superior intelligence.
A species may be defined as a group of organization capable of interbreeding and producing fertile offsprings. The process of formation of new species from existing one by evolutionary means is known as speciation.
Speciation can be classified into 2 categories:-
- Allopatric speciation : When original population is divided into two sub-populations due to the development of a geographical barriers like river, mountain, desert, glacier etc., the two subgroups accumulate variations independently and after generation of natural selection can form two different species. Such species are called allopatric species. E.g. Finches of Galapagos Islands and birds of Hawaiian Islands.
- Sympatric speciation : When speciation occurs without the development of geographical barrier i.e. in the same geographical area. Here, a common interbreeding population changes into two populations which become reproductively isolated. Such speciation many occur by mutations, hybridization or polyploidy (specially in plants). E.g. Evolution of giant Panda (2n = 42) from ancestor with bear (2n = 74) by chromosomal fusion, evolution of flightless grasshoppers.
Very Short Answer Questions (1 Mark)
- What do you mean by term ‘Heredity’?
- Define evolution name one variation in human connected with ears.
- What constitutes the link between one generation and next?
- What are the sex chromosomes for a male and female?
- What are genes?
- What is meant by dominant and recessive genes?
- What is the genotype when the human having a blood group A?
- What do you mean by dihybrid cross?
- What is the genotype ratio of F2 generation in hybrid cross?
- In a human, how many chromosome are present in a sperm in the testes.
Short Answer Questions (2 Mark)
- Why a Mendel’s choose a pea plant only for their experiments?
- Determine the sex of the progeny which has inherited X chromosome from the father.
- Explain the term homologous and analogous organs giving suitable examples.
- Define the terms : (i) Homozygous (ii) Heterozygous
- How is the equal genetic contribution of male and female parents ensured in the progeny?
- What is the evolutionary significance of the fossils?
- Define the law of Dominance.
- Show a cross between long height plant and Dwarf height plant in F1 generation and in F2 generation.
Long Answer Questions (4 Mark)
- How do Mendel’s experiments show that traits are inherited independently?
- Give the contrasting traits of the following characters in pea plant and mention which is dominant and which is recessive :
(i) yellow seed (ii) round seed
- Describe the seven contrasting characters in pea plant as both dominant and recessive traits.
- In a human, how many chromosomes are present in :
(i) a brain cell?
(ii) an egg which has just been produced by the ovary?
(iii) a fertilized egg?
- Describe the dihybrid cross with the help of punett square?
- With the help of cross, how can you determine the sex in human being.
- How many blood groups are inherited in human beings, explain by showing cross between the blood group A and B.
Multiple Choice Questions
- An example of homologous organ is
(a) our arm and a dog’s fore-limb (b) our teeth and an elephant’s tusks
(c) potato and runner’s of grass (d) All of the above
- In evolutionary forms, we hare more in common with
(a) a chinese school boy (b) a chimpanzee
(c) a spider (d) a bacterium
- Normally, the garden pea plant reveals
(a) self pollination (b) cross pollination
(c) both (a) and (b) (d) neither (a) and (b)
- Which of the following gases was absent from the atmosphere of primitive earth?
(a) methane (b) carbondioxide
(c) oxygen (d) ammonia
- Who provided experimental evidence to support theory of origin of life from inanimate matter?
(a) Oparin and Haldane (b) Miller and Urey
(c) Watson and Crick (d) Mendel and Darwin
Very Short Answer Questions (1 Mark)
- Name the scientist who disproved spontaneous generation theory?
- Name any two vertebrates body parts that are homologous to human forelimbs.
- Why are the wings of a butterfly and of a bat called analogous?
- Mention the type of evolution that has brought the similarity as seen in potato tuber and sweet potato.
- Are the thorns of Bougainvillea and tendrils of cucurbita homologous or analogous?
- Who proposed the theory of chemical evolution?
- Name the theory by which earth is said to originate.
- Who proposed that life comes only from pre-existing life?
- When did first cellular form of life appear?
- What is gene migration?
Short Answer Questions (2 Mark)
- What is oparin – Haldane theory? Can life be originated abiotically inside the laboratory today?
- What was the composition of earth’s atmosphere about 3000 million years ago.
- If abiotic origin of life is in progress on a planet other than earth, what should be the condition there? Explain.
- How do you consider tendrils of cucurbita and thorn of Bougainvillae as homologous structure?
- Give one example of analogy and homology in plants?
- What is the significance of Archaeopteryx in the study of organic evolution?
- What is differential reproduction?
- What is artificial selection in terms of evolution?
- What are referring to when we say ‘simple organisms’ or complex organisms?
- What was Lamarck’s theory of evolution? Explain the theory by quoting an example.
Long Answer Questions (4 Mark)
- Whose theory was put to test by Miller and Urey and what was the theory? How did their experiment give due to abiotic origin of life on earth?
- (a) What is adaptive radiation?
(b) Explain with the help of a suitable example where adaptive radiation has occurred to represent convergent evolution.
- Branching descent and natural selection are the two key concepts of Darwinian theory of evolution. Explain each concept with the help of a suitable example.
- Explain the salient features of Hugo de Vries theory of mutation. How is Darwin’s theory of natural selection different from it? Explain.
- (a) Name the primates that lived about 15 million years ago. List their characteristic features.
(b) (i) Where was the first man-like animal found?
(ii) Write the order in which Neanderhals, Homo habilis and Homo erectus appeared on earth. State the brain capacity of each one of them.
(iii) When did modern Homo Sapiens appear on this planet?
- Give an account of the factors that affect Hardy-Weinberg equilibrium.
- Describe the evidence of evolution from comparative anatomy and morphology?
- Write a note on a brief account of evolution?
- Describe some examples of natural selection?
- Describe the various theories of origin.
Multiple Choice Questions
- Abiogenesis is
(a) Origin of life from non-living organisms
(b) Origin of microbes from living organisms
(c) Spontaneous generation
(d) Origin of microbes and viruses
- Which of the following has been basic material for the origin of life
(a) Carbohydrates (b) Proteins
(c) Nucleic acids (d) Nucleoproteins
- Which compound has very important role in prebiotic evolution?
(a) SO2 (b) NO
(c) CH4 (d) SO3
- Possible early source of energy was
(a) Chlorophyll (b) CO2
(c) Radiations and lightning (d) Green plans
- Pasteur is famous for
(a) Cell theory (b) Germplasm theory
(c) Recapitulation theory (d) Germ theory of disease
- Which one of the following is not a vestigial organs?
(a) Epiglottis (b) Muscles of ear pinna
(c) Vermiform appendix (d) Coccyx
- Which animal has become extinct recently.
(a) Draco (b) Dinosaurs
(c) Mammoth (d) Pteridosperms
- Occurrence of higher number of endemic species in South America and Australia is due to
(a) Retrogressive evolution (b) Continental separation
(c) These species have become extinct from other regions
(d) Absence of terrestrial links between these places
- Which one is palaeontological evidence of evolution?
(a) Biston betularia (b) Archaeopteryx
(c) Darwin’s finches (d) Duck billed platypus
- Ontogeny recapitulates phylogeny is
(a) Hardy Weinberg law (b) Pauling law
(c) Biogenetic law (d) Thomas law
WORKSHEET – 1
Very Short Answer Questions (1 Mark)
- Who is known as the father of genetics?
- Define genetics?
- Name the scientist who first isolated DNA from the nucleus of the pus cells?
- Name two purines and pyrimidines nitrogenous bases present in DNA molecule.
- How many autosomes and sex chromosomes are present in normal human diploid cell?
- What do the letter P, F1and F2 represent in heredity.
- Give two examples where sex is de termined by XX-XY mechanism.
- Who gave theory of natural selection?
- Name two common restigial organ present in human beings.
- What factors could lead to the rise of a new spread?
Short Answer Questions (2 Mark)
- What are fossils? What do they tell us about the process of evolution?
- Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not?
- Why Mendel selected garden pea plant for his experiments?
- Write the names of the componens of DNA?
- Where are genes located? What is the chemical nature of gene?
- How can you explain the existence of analogous organs?
- Why is artificial selection beneficial?
- Why are required characters not inheritable?
- What are the different ways in which individuals witha particulars trait may incrase in a population.
- How do Mendel’s experiments show that traits are inherited independently?
Long Answer Questions (4 Mark)
- How is sex determinant in human beings?
- How are the areas of study-evolution and classification interlinked?
- Explain the term analogous and homologous organs with examples.
- How do Mendel experiments show that traits may be dominant or recessive?
- How does creation of variations in a species ensure survival?
- If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
- How do embryological studies provide evidences for evolution?
- What evidence do we have for the origin of life from inanimate matter?
- Only variations that can for an advantage to an individual organism will survive in a population. Do you agree with this statement? Why or why not?
- How is the equal genetic contribution of male and female ensured in the progeny?
Long Answer Questions (6 Mark)
- Explain the importance of fossils in deciding evolutionary relationships.
- Explain how sexual reproduction gives rise to more viable this effects the evolution of those organisms that reproduces sexually?
- Explain, why are human beings who look to different from each other in terms of size, colours and looks said to belong to the same species?
- A man with blood group A marries a woman with blood group O and h eir daughter has blood group O. In this information enough to tell you which of the traits blood group A or O is dominant? Why or why not?
- Explain with the help of diagram Mendel’s monohybrid cross and dihybrid cross.