HEREDITY AND EVOLUTION

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HEREDITY AND EVOLUTION

 

Heredity and variations

It is commonly seen that members of a species are largely alike. An elephant resembles other elephants, a rose plant looks alike other rose plants, and children resemble their parents, even grandparents or great grandparents. This resemblance among the individuals of a species has given rise to a general truth ‘like begets like’ which implies continuity of life. It is, however, not absolutely true as the members of a species are seldom exactly alike. For instance, in human beings, the children often have some individual characters in which they differ from one another, and also from their parents. In fact, their differences are as marked as their resemblances. This is true about other species as well.

The similarities and differences among the members of a species are not coincidental. They are received by the young ones from their parents. The hereditary information, in fact, is present in the gametes (egg and sperm) which fuse to form the fertilized egg or zygote during sexual reproduction. The zygote then develops into an organism of a particular type. For instance, fertilized eggs of sparrows hatch into sparrows only and the fertilized eggs of pigeons hatch into pigeons only. Similarly, a cow gives birth to calves only. The wheat plant gives rise to seeds which, in turn, develop into wheat plants.

 

Heredity:- The transmission of characters [or traits] from one generation to another generation.

OR

The transmission of characters from the parents to their offsprings.
Variations:- The differences in the characters [or traits] among the individuals of a species are called variations. e.g. Plant height – Tall, dwarf & middle.
Ear lobe in human being:– The lowest part of our ear is called earlobe.

  •  In most of the people, the ear lobe is hanging and it is called free earlobe.
  • In some people, the earlobe is closely attached to the side of the head and it is called attached ear lobe.

Accumulation of Variations During Reproduction

Heredity involves inheritance of basic body design (similarities) as well as subtle changes (variations) in it from one generation to the next generation, i.e., from parents to the offspring. When individuals of this new generation reproduce, the offsprings of second generation will have the basic body design, the differences that they inherit from first generation as well as newly developed differences. Asexual reproduction involves single parent. When a single individual reproduces asexually, the resultant two individuals again after sometime reproduce to form four individuals. All these individuals would be similar. However, there would be only very minor differences between them. These very minor differences arise due to small inaccuracies in DNA copying.

Sexual reproduction, on the other hand, generates even greater diversity. This is so because sexual reproduction involves two parents (father and mother) and every offspring receives some characters of father and some characters of mother. Since, different offsprings receive different combination of characters of their parents (father and mother), they show distinct differences (variations) among themselves as well as from their parents. The variations accumulate and pass on to more and more individuals with each generation.

During sexual reproduction the variation caused by
(i) Chance separation of chromosomes during gamete formation (gametogenesis).
(ii) Crossing over during meiosis.
(iii) Chance coming together of chromosomes during fertilization.
(iv) Mutations, i.e., alterations in the genetic material.
All the variations in a species do not have equal chances of surviving in the environment in which they are generated. Depending upon the nature of variations, different individuals would have different kinds of advantages. For instance, bacteria that can withstand heat will survive better in a heat wave than the others. In other words, environmental factors select the variants and this selection forms the basis of evolution.

 

Gregor Johann Mendel (1822 – 1884) 

Mendel was born on 22 July 1822 at Heinzendorf  in Austria at Silesia village. Mendel was worked in Augustinian Monastry as monk at Brun city, Austria. In 1856-57, he started his historical experiments of heredity on pea(Pisum sativum) plant. His experimental work continued  on pea plant till 1865 (19th centuary). The results of his experiments were published in the science journal. “Nature For Schender Varein” in 1866. This  journal was in Germen language. Title is ‘ Verschue Uber pflangen hybridan’. This journal was published by ‘Natural History society of Bruno’. A paper of Mendel by the name of Experiment in plant Hybridization published in this journal.   Mendel were unable to got any popularity no one understand of him. he died in 1884 (due to kidney disease (Bright disease)) without getting any credit of his work. After 16 years of Mendel’s death in 1900 Mendel’s postulates was rediscovered. Mendel experiment remain hidden for 34 years.

 

Rediscovery by three scientist independently.
1. Carl Correns – Germany – (Experiment on Maize)
2. Hugo deVries (Holland) (Experiment on Evening Primerose)
3. Erich von Tschermak Seysenegg – (Austria) (Experiment on different flowering plants)

 

Character

A recognizable feature of human beings or any other organisms are called characters.
eg. (i) Height    (ii) Complexion   (iii) Shape of hair    (iv) Colour of eyes     (v) Shape of nose

Traits:- Various forms of a character are called traits.
S. No. Character                                Dominant                                                       Recessive
1. Length of Plant                                    Tall                                                                        Dwarf
2. Flower position                                  Axial                                                                  Terminal
3. Shape of pod                                  Inflated                                                               Constricted
4. Colour of pod                                    Green                                                                    Yellow
5. Shape of seed Round                Wrinkled
6. Colour of cotyledon                Yellow Green
7. Colour of flower                       Violet White

 

Mendel’s Monohybrid cross

A breeding experiment dealing with a single character is called a monohybrid cross.
Mendel first selected ‘pure line’ plants (i.e., the plants that produced similar traits generation after generation). He, then, cross pollinated such plants having the contrasting traits, considering one trait at a time. For instance, in one such cross breeding experiment, he cross bred garden pea plant having round seeds with plant having wrinkled seeds. In this monohybrid cross, the pollen grains from the flower of the desired plant raised from round seeds were transferred over the previously emasculated flower of a plant raised from wrinkled seeds or vice-versa. After the transfer of pollen grains, the cross pollinated flower was properly covered and seeds produced were allowed to mature. All the seeds of F1 generation were carefully observed. Mendel observed that all the seeds of F1 generation were of round type and there were no intermediate characteristics. He raised plants from F1 seeds and allowed the flowers to self-pollinate to produce the seeds of F2 generation. The flowers were kept covered from the beginning to avoid unwanted pollens to reach these flowers. In F2 generation, Mendel observed the appearance of both round and wrinkled seeds in approximately 3 : 1 proportion.

Mendel’s Dihybrid Cross

A cross involving two pairs of contrasting characters.or A cross in which two pair of contrasting characters are studied at a time. In one such cross, Mendel considered shape as well as colour of the seeds simultaneously. He selected pure line plants and then cross pollinated flowers raised from seeds of round shape and yellow colour with those from wrinkled seeds and green colour. Mendel observed that in F1 generation all seeds had the features of only one parental type, i.e., round shape and yellow colour. He raised plants from F1 generation seeds and allowed the flowers to self pollinate to produce the seeds of F2 generation. These flowers were kept covered from the beginning. In F2 generation, Mendel observed the appearance of four types of combinations. These included two parental types (round shaped and yellow coloured seeds, and wrinkled shaped and green coloured seeds) and two new combinations (round shape d and green coloured seeds, and wrinkled and yellow coloured seeds) in approximately same proportion.

Dihybrid cross

Some Important definitions

Dominant gene:- The gene which decided the appearance of an organism even in the presence of an alternative gene.
Recessive Gene:- The gene which can decide the appearance of an organism only in the presence of another identical gene.
Chromosomes:- A thread – like structure in the nucleus of a cell formed of DNA which carries the genes. Genotype:- The genetic constitution of an organism.or The description of genes present in an organism e.g. TT, tt, Tt.

 

Phenotype:– External and morphological appearance of an organism for a particular character.

Allele:- Alternative forms of a gene which are located on same position [loci] on the homologous chromosome.

F1 Generation:- When two parents cross or breed to produce progeny [or offsprings], then their progeny is called F1 generation or first filial generation.
or
The offspring produced by the parental generation.

F2 generation:- When the first generation progeny cross or breed among themselves to produce second progeny, then this progeny is called second filial generation
or
F2 generation.or The offspring produced by the F1 generation

Hybrid:- A new form of plant resulting from a cross or breeding of different varieties of a plant is known as a hybrid.

Pure-breeding:- Characteristics that appear unchanged generation after generation.

Dominant characters:- Any character that appears in the F1 generation offspring from a cross between parents possessing contrasting characters such as tallness & dwarfness in pea plants.

Recessive characters:- Any character present in the parental generation that does not appear in the F1 generation but reappears in the F2 generation.

Homozygous:- A condition in which the 2 members of an allelic pair are similar. e.g. TT, tt.

Heterozygous:- A condition in which the 2 members of an allelic pair are dissimilar. e.g. Tt.

Offspring:- Organisms produced as a result of sexual reproduction.

Homologous chromosomes:– All chromosomes found in pair & chromosomes of a pair are called homologous chromosomes.

Non-homologous chromosomes:- Chromosomes of different pair are called non-homologous chromosomes.

Genes:- Unit of heredity which transfers characters from parents to their offsprings during reproduction. Gene → Protein synthesis ® Enzymes [Controls phenotype of a character]

 

Laws of Mendel/Laws of inheritance

On the basis of Mendel’s work, 3 basic laws of inheritance were proposed.
(i) Law of Dominance
(ii) Law of Segregation
(iii) Law of Independent Assortment

Law of dominance:- In crossing between organisms pure for contrasting characters of a pair, only one character of the pair appears in the F1 generation. This character is termed dominant while the one which does not express itself in F1 generation is termed recessive.

Law of segregation:- Allele or genes remain together and segregate at the time of gamete formation. This means that the alleles don’t mix in the hybrids [Non-mixing of alleles] This is also known as the Law of Purity of Gametes.

Law of Independent Assortment:- This law states that – when individuals differing in 2 or more than 2 pairs of contrasting characters are crossed, the inheritance of any one pair is not affected by the presence of the other.

e.g. The inheritance of tall character is not way related to the smooth character of the seed. Rather, the 2 characters are inherited independent of each other.

Advantage of pisum sativum
Pisum sativum was a unique plant selected by Mendel for his experimentations had following importance-

Sex Determination How is the sex of newborn individual determined ? In human beings, the sex of the individual is largely genetically determined. In other words, the genes inherited from our parents have assumed that similar gene sets are inherited from both parents. If that is the case, how can genetic inheritance determine sex?
All human beings these are 23 pairs of chromosomes are present out of these 23 pairs, are autosomes (similar in males and females) and 1 pair is called sex chromosome (different in males and females). In males sex chromosome are XY and in females sex chromosome are XX.

Sex determination

So, it is clear from the above that male is responsible for producing male child.

  • The term Genetics comes from “gene” word  – means “to grow”.
  • The term “Genetics” coined by Bateson.
  • Study of heredity and variations is called Genetics.
  • Father of Genetics – Gregor Johann Mendel.
  • Father of Modern Genetics – Bateson.
  • Father of Experimental Genetics – Thomus Hunt Morgan.
  • “Heredity” term coined by Spencer.
  • Bateson coined terms Genetics, Allele, Homozygous, Heterozygous, F1 and F2 Generation.

 

Deoxyribonucleic acid (DNA)

The expanded form of DNA is deoxyribonucleic acid. It was first isolated by the scientist Frederick Meisher from the nucleus of the pus cells in 1869. He named it as ‘nuclein’ or nucleic acid because of its acidic nature. Later, it was experimentally proved by the scientists Griffith (1928), Avery, McLeod and McCarty (1944) that DNA is the carrier of the genetic information from generation to generation. It transmits the hereditary characters in a coded language from parents to the offsprings (i.e., from one generation to another). DNA is a macromolecule or polymer. It is made of very large number of ‘nucleotide’ units and hence is termed polynucleotide. Each nucleotide unit in a DNA molecule is made up of three components

1. Deoxyribose sugar:- It is a pentose sugar.

2. Nitrogenous base:- Each nucleotide unit has a nitrogen containing base. In a DNA molecule, nitrogenous bases are of two types :
(a) Purines:- The purines in a DNA molecule are — Adenine (A) and Guanine (G).
(b) Pyrimidines:- The pyrimidines in a DNA molecule are — Cytosine (C) and Thymine (T).

3. Phosphate group:– The phosphate group contains one phosphorus atom and four specifically linked oxygen atoms. Thus, there are four types of nucleotides in a DNA molecule depending upon the kind of nitrogenous base present in each nucleotide.

Double Helical Model of DNA

J. D. Watson and F.H.C. Crick proposed the double helical model of DNA in 1953. They were awarded the Nobel Prize for this discovery in 1962. The important features of the double helical model are

(i) DNA molecule is made up of two long polynucleotide strands forming a double helical structure (double helix) just like a spiral staircase. Each helical turn of the DNA molecule is 3.4 nm in length in which ten nucleotide base pairs are present.

(ii) Deoxyribose sugar and phosphate molecules are joined alternately to form the backbone of each polynucleotide strand. The nitrogenous base of each nucleotide is attached to the sugar molecule and projected towards the interior of the double helix.

(iii) In the interior of double helix, the nitrogenous bases of two polynucleotide strands form a pair with the help of hydrogen bonds. Adenine (A) always pairs with thymine (T) and guanine (G) always pairs with cytosine (C).

Thus, the two polynucleotide strands of the DNA molecule are joined by hydrogen bonds between specific nitrogenous bases. Such a specific pairing of the bases of the opposite strands of the DNA molecule is called complementary pairing. Adenine (A) and thymine (T) are complementary to each other. Similarly, guanine (G) and cytosine (C) are complementary to each other. The hydrogen bonding between the specific nitrogenous bases keeps the two strands to hold together. Therefore, all the base pairs remain stacked between the two strands.

Blood groups
Four different types of blood groups in human beings are : A, B, O, AB
O → universal donor
AB → universal recepient

Evolution
The term evolution has been derived from the Latin word ‘evolvere’ means unroll. Evolution can be defined as sequence of gradual development of complex form of life from simple form of life over the course of geological time ” Descent with modification.” Evolution is of two types –
(i) Chemical evolution
(ii) Organic evolution.

 

Organic evolution:
After origin of a living cell the next questions that arose was how did so many different species of complex life form come into existence? Here are various view points.

Carolus linnaeous:– Said that no species is new i.e. each species originates from pre existing species.

Jeans baptist lamarck:– Explained in his book namely philosoophie zoologique, the theory of inheritance of acquired characters or lamarkism. This theory states that use and disuse of an organ leads to change in the organ. Which is inherited by the offsprings. The favourable variations which remain for longer period of time results in evolution of new species. This theory was discarded by August Weismann as he experimentally proved that even after cutting tails of mice for 21 generation tailless mice was never born.

Darwinism
Charles Robert Darwin was born on 12 Feb, 1809 in England. He travelled by HMS Beagle ship along with Dr. Henslow. He visited many islands of south America, South Africa, Australia and Galapagos Islands. Darwin was influenced by two books. “Principal of population” of malthus. “Principal of Geology of charls leyell.

In his book origin of species he answered this questions. The theory presented by him is called theory of natural selection or Darwinism.

Alfred Russel wallace:– He travelled south eastern Asia and south America. The idea of natural selection striked in his mind wallace wrote an essay and sent it to Darwin. On the tendency of varieties to depart indefinitely from original type. There is striking similarity between the views of Darwin and Wallace.

Charles Darwin explained the mechanism of origin of new species by natural selection. But he failed to explain the mechanism of source of heritable variations. This was explained by Hugo de Vries a dutch botanist. According to him, heritable variations arise when there is a change in genes of the germplasm
(protoplasm of germcell). He called it mutation.

 

Origin of Life on Earth (Chemical Evolution of Life)

Life originated on earth through chemosynthesis or formation and coming together of biochemicals. It is called naturalistic theory or theory of chemical origin of life. The theory was given by Russian scientist Oparin (1924) and British turned Indian scientist J.B.S. Haldane (1929). (i) At the time of formation of earth lighter elements present  on the surface were in their atomic state, viz, hydrogen, carbon, nitrogen and oxygen. (ii) With the slight lowering of surface temperature of earth, the lighter elements interacted and formed water (H2O), methane (CH4), ammonia (NH3), molecular hydrogen (H2) and carbon dioxide (CO2). They formed a reducing hot atmosphere of earth. It was exposed to ultraviolet radiations, cosmic rays, lightning and hot lava from volcanoes. Methane, ammonia, hydrogen and other molecules interacted and formed sugars, amino acids, alcohols, fatty acids, nucleotides and other biochemicals. With further interactions and polymerisation, complex organic molecules were formed. Their aggregation formed coacervates or colloidal complexes that developed covering membrane and nucleic acids to produce protocells or primitive cells.

Experimental Proof: Stanley Lloyd Miller and Harold C. Urey (1953) assembled an apparatus to provide conditions similar to those of early earth. They took a mixture of ammonia, methane and hydrogen (2 : 2 : 1) and water, and exposed it to electric sparks, heating and cooling to resemble lightning and provide temperature from 800ºC to just below 100ºC. After one week, 15% of carbon from methane had been converted into organic compounds of sugars, organic acids, amino acids, purines and pyrimidines. The first primitive organism would arise from further chemical synthesis.

Variations
Variations are the structural, functional or behavioral changes from the normal characters developed in living organisms. There is an inbuild tendency to variation during reproduction. Both because of errors in DNA copying as a result of sexual reproduction. Variations provide raw materials for evolution. These may be inheritable or non inheritable, only inheritable variation participate in evolution.

Type of variation:– On the basis of nature of cells where variations occur variations are of two types.
(i) Somatogenic variations or Acquired traits                   (ii) Germinal (Blastogenic) variations.

An Illustration:– Consider the following example.
1. A group of twelve red beetles living in bushes with green leaves.
2. Beetles in the population can generate variations because these are reproducing sexually.
3. Crow can  eat the beetles. The more beetles the crow eat, the fewer beetles are left for reproduction.

 Variation in a population inherited and otherwise’

 

Now consider the following situations:–

 

Heritable variation

The reason why organisms resemble their parents lie in the precise copying of their genes. Which carry hereditary characters from one generation to the next. On the other hand no two off springs have exactly the same genes. This is because offspring of sexually reproducing organisms receive varying combination of genetic material from both parents such variation result from mutations (errors in DNA copying). Variations also result from genetic recombination during sexual reproduction.

 

Genetic drift :

The random changes in the gene frequency occurring by chance alone. The effect of genetic drift is very small in large population and large in small populations.

 

Species and speciation

Biological species concept : A species is a sexually interbreeding group of individuals separated from other species by the absence of genetic exchange. Members of species are capable of breeding with one another and produce living, fertile off spring but are unable to breed with members of other species normally.

Gene pool : Sum of all the genes of all the members of a species. Speciation occurs when the gene pool of a population is some how reproductively isolated from other sister population of the parent species and gene flow no longer occurs between them. Then a population splits into independent species. Which become reproductively isolated from each other. On basis of period taken in speciation there are two types of mechanism of speciation.

Speciation

The process of formation of one or more new species from an existing species is called speciation. Let us try to understand this concept by taking the example of beetles.

Above example of speciation shows

  • Large population of beetles occur on a mountain range.
  • Few beetles started feeding in neighborhood.
  • Gene flow continued in two places.
  • They may get isolated at larger distance because of existence of river.
  • Gene flow decreases and finally stops.
  • Two sub populations change with time because of genetic drifts and natural selection.
  • Later they became reproductively isolated.
  • Two new species came up.
  • This can occur as a result of change in chromosome number.
  • Micro evolution is very important this mean that the changes may be small but significant.

Speciation due to inbreeding, genetic drift and natural selection will be applicable to all sexually reproducing animals geographical isolation does not play any role in the speciation of a sexually reproducing animals and self pollinating plants.

Evolution and classification

Classification : Classification is the system of arrangement of organism in certain groups or subgroups on the basis of hierarchies of certain characteristics. The characteristics are the details of appearance from structure, function and behaviour Phylogenetic taxonomy is branch of classification on the basis of evolutionary relationship on the basis of common ancestry.

 

Flow chart of basic characters shared by most of the organisms

Tracing evolutionary relationship 

Studies on similarities in structure of different organisms suggests that present forms have evolved through a process of slow and gradual change called evolution. They include the following :

 

1. Homologous organs: 
Homologous organs are those structures which are different in appearance and perform different functions but have similar basic structure and developmental origin. This relationship is called homology.’

 

FORELIMB IN VERTEBRATES

 

Seal                                            Bird                                Bat                                  Horse                                           Man
Appearance                      Flippers                        Wings                            Patagia                                     ElongatedSeal
Thumb opposability  Swimming flying   Support, flying               Running                               Grasping function

 

2. Analogous:  Those organs which have different origin and structural plan but appear similar and perform similar functions are called analogous organs. While this relationship is called convergent evolution or analogy. eg : Wing of an insect and a bird, Hand of man & Trunk of elephant.

Analogy in these organs is due to similar adaptations to perform similar functions rather than their common ancestry.

3. Vestigial organs:  Those organs which no longer have a function are called vestigial organs. These organs have reduced structurally as well as functionally. It appears that these organs were once well developed and functional in ancestors and later on due to their less use they became reduced.
eg : Vermiform appendix, ear muscles, third eyelid in man are reduced and function less.

 

Fig.11

Common ancestry and inter-relationship: Various organisms are inter connected their resemblance suggest a common ancestry.

Ex.

Connecting links:  Some living organism have characteristics of two groups. They are known as connecting links.
Eg: Lung fish – show connection between fishes and amphibians.

 

Various connecting links

(i) Virus → Between living and non living
(ii) Euglena → Between plants and animals
(iii) Proterospongia → Between protozoa and porifera
(iv) Neopilina → Between annelida and mollusca
(v) Peripatus → Between annelida and arthropoda
(vi) Archaeopteryx → Between reptiles and birds
(vii) Balanoglossus → Between non chordates and chordates
(viii) Chimera → Between cartilaginous fish and bony fish
(ix) Lung fish → Between fishes and amphibia
(x) Platypus → Between reptiles and mammals
(xi) Echidina → Between reptiles and mammals Jurassic period is known as golden age of reptiles.

Dinosaur of dromaesaur family had feather on body and fore limb. Huxley called birds as glorified reptiles. Carnivorous dinosaur called velociraptor had a wish bone like birds.

 

Evidences from embryology :
A comparative study of the stages of embryonic development of animals reveals that in their early stages they were very similar. These embryonic stages reflect thus ancestry. The embryological stages of an organism give us an idea about the stages of its         evolution. For example when we study the human embryo, we find that at a certain stage it has gills. This suggests that fish is one of the earliest ancestors in the evolution of mammals including human beings.

 

Evidences from palaeontology

1. Fossils :

Fossil is an organic relic of a long dead life form.
Or
Fossils are the petrified remains and for impressions of the hard parts of the extinct organism preserved in the sedimentary rock or other media.

 

Palaeontology: Study of fossils is known as palaeontology.

 

How do fossils form layer by layer?

Let us start 100 million years ago. Some invertebrates on the seabed die, and are buried in the sand. More sand accumulates, and sandstone forms under pressure. Millions of years later, dinosaurs living in the area die, and their bodies, too. are buried in mud. This mud is also compressed into rock, above the rock containing the earlier invertebrate fossils.

Again millions of years later, the bodies of horse-like creatures dying in the area are fossilized in rocks above these earlier rocks.

Much later. erosion by, sun, water flow, wears away some of the rock and exposes the horselike fossils. As we dig deeper, we will find older and older fossils.

Living fossils: The animals which underwent little change during long geological periods.

 

Important living fossils

1. Peripatus, Limulus (Arthropoda)
2. Nautilus, Neopilina (Mollusca)
3. Lingula (Brachiopoda)
4. Latimeria (Coelacanth fish)
5. Sphenodon (Reptilia)

Determination of the age of fossil: There are three ways of determining age of the fossils.

1. Relative method: If we dig into the earth and starts finding fossils it is reasonable to suppose that the fossils we find closer to the surface are more recent than the fossils we find in deeper layer.

2. Using Radioactive Elements: The age of a fossil is determined with the help of certain radioactive elements such as uranium present in the rock.

3. Carbon dating: Carbon dioxide of air contains a small proportion of radioactive carbon (C14). CO2 is used during photosynthesis and there is equal proportion of C14 among carbon atoms of all organisms.
The radioactivity of C14 is lost at a precise rate half life of C14 is about 5,600 years. If a fossil shows radioactivity one fourth of that found in the living organisms the organisms died about 11,200 years ago. (Two half lives.)

Evolution by stages 

Evolutionary changes are fundamental characteristics of living organisms such changes may be convergent. It means that the similar looking structures may have different ancestral designs. This can be explained by example of eye.

Eye:  Eyes of insects, octopus and vertebrates have similar looks but different structures and must have separate evolutionary origin or different  ancestral designs. Rudimentary eye can be useful to some extent.

Feathers: The function of feathers was insulation in cold weather later this feature might have proved to be useful for the purpose of flight.

Dinosaurs is example depicting presence of feathers in them but these were not used for flying later birds seem to have adaptation of flying using feathers.  This shows close relationship of birds to reptiles and proves that characters appearing as an variation can be useful later to perform different functions.

Cabbage: Humans have developed different types of vegetables from the wild cabbage by artificial selection. S.No. Vegetable evolved Edible part

1. Cabbage Selection for fleshy terminal buds and short distance between the leaves.
2. Brussel’s sprouts Selection for fleshy lateral buds.
3. Kohl rabi Selection for fleshy stem.
4. Kale Selection for large sized leaves so it is a leafy vegetable.
5. Broccoli Selection for leafy sized and arrested flower development.
6. Cauliflower Selection for fleshy sterile flower.

 Evolution of wild cabbage [Divergent evolution]

 

Molecular phylogeny

Ancestors of different organism including humans can be traced by studying the change in their DNA. A change in DNA means a change in its protein sequences. The ancestry or phylogeny determined by comparative study of DNA sequences is called molecular phylogeny. Studies in molecular phylogeny help in the classification of organisms.
Haeckel propounded ‘The theory of recapitulation or ‘Biogenetic law’. Which states that an individual organism in its development (ontogeny) tends to repeat the stages passed through by its ancestors (phylogeny means ontogeny repeats phylogeny.

 

Evolution should not be equated with progress

Though organic evolution involves descent with modification in which there is a progressive trend of emergence of more or more complex body designs from earlier similar body designs by gradual changes but evolution should not be equated with progress because of following reasons.

1. In evolution older species are not eliminated during formation of new species and most of older and simple species still survive.
eg: Earliest organisms like bacteria are found even in many hospitable habitats like hot springs, deep-sea, thermal vents, Antarctic ice. etc.

The evolved species are not always better than the parental species evolution depends upon natural selection and genetic drift which is together result in population which is reproductively isolated from the parental species.

 

Human evolution

Evolutionary history of man has been built from the palaeontology (fossil studies) and molecular biology (especially DNA changes).

For example:- It is not true that human beings have evolved from chimpanzees. Rather both human beings and chimpanzees have common ancestors a long time ago. That common ancestors is likely to have been neither human or chimpanzee. The two resultant species have probably evolved in their separate ways to give rise to the current forms.

Anthropology:  the scientific study of tracing of human evolution is called anthropology. Scientists involved in studying human evolution are called anthropologists.

Studies have revealed that human evolution started in Africa and earliest human type was Australopithecus  Africans. African ape man fossil was discovered by Prof. Raymond Dart fossil of skull of 5-6 years old baby from old Pliocene rock of Tuang region (S.Africa). He name it Tuang baby. It had many ape like characters but had a bipedal locomotion like man. The cradle of human evolution is East Africa where genetic foot prints of earliest members of human spec ies Homosapiens can be traced. A couple of hundred thousand years ago some of own ancestors left across the planet from Africa.

Human evolution

The first human types, evolved into modern man Homosapiens through a number of intermediate human types.

Homo erectus erectus → Java man

Homo erectus pekinesis → Peking man

Homo sapiens neaderthalensis → Neanderthal man

Homo sapiens fossils → Cro-magnon man

In the course of their evolution these migrant human types went forward and backwards and moved in and out of south Africa. Modern man evolved from Cro-Magnon man about 25000 years ago and spread all over the world about 10,000 years.

 

Modern man is divided into four ethnic groups:

Negroid: African Pygmies and bushman
Caucasian: Italian English
Eastern: Chinese Japanese Eskimos
Mangolid: These ethnic group differ from one another in their skin colouration lips and hair pattern but all of these belong to same species because these are not reproductively isolated from one another. All human races have same chromosomes number and similar grass morphology of chromosomes.

Man of future:
Homo sapiens futuralis.
(A prediction by American anthropologist Dr. Sapiro.)


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IBPS Clerk 2017 Video Lecturesx