Evolution of Life10th Grade - Biology

Chapter 7 - Evolution of Life

Neodarwinism - Mutation - Genetics - Experiments of Mendel on pea plants - Laws and concepts of heredity - Ancestors of man - Human phylogeny.

In the last class you have studied how complex organisms have developed from the pre-existing simple organisms over a period of time. You have also studied the theories put forth by Lamarck and Darwin to understand how evolution might have taken place

In this class, you are going to study how the theory of evolution proposed by Darwin has been modified in the light of modern evidences from genetics, molecular biology, ecology and palaeontology and other branches. It is known as Neodarwinism.

Molecular biology - study of biological molecules. Genetics -study of heredity. Variation in any change in traits llike colour of skin or eyes, the offspring exhibits when compared to parents

Mutuation occur when an organism is subjected to higher environmental temperature as it caused chemical reactions in cellsMututations also occur due to (1) chemicals such as colchicine (2) High energy radiation such a x-

rays, atomic explosion and ultra-violet rays

7.1 NeodarwinismThe theory which explains evolution in light of natural selection of inherited characteristics is called NeoDarwinism. Neodarwinism is based on evidences from a broad range of sources.

In order to accept Neodarwinism as an evolutionary theory, it is necessary to

a) establish the fact that evolution has taken place.

b) demonstrate a mechanism which results in evolution.

c) observe evolution as an ongoing process, when the hereditary variations caused by mutations are passed on to the successive generations, it results in the formation of new species. This is the basis of evolution,

In the second unit, you have studied the structure of chromosomes and how the characters are inherited by a systematic replication of D. N.A. This process is the key to, heredity.

But some times this process of replication of DNA undergoes changes to mutations. There are two types of mutations.

1. Gene mutation and

2. Chromosome mutation

The mutations of chromosomes and genes brings about changes in off springs and they differ from their parents. The mutant may survive by natural selection and pass the new characters to the next generations resulting in the formation of new species.

7.2 InheritanceNormally, organisms maintain their characters through generations. We know that, a baby elephant is born of an elephant, a chick of a fowl and a mango plant of a mango seed.

The children of a family though they resemble one another, yet are not exactly alike. Genetics, a branch of biology explains the similarities and differences among the organisms. Heredity deals with factors responsible for resemblances between the parents and their offspring, while variations deal with factors concerning the differences among organisms . The heriditary variations are of great importance in the process of evolution of a new species.

The term genetics was coined by Bateson in 1905

Although mendel published his theories on heredity in 1866 Darwin was not aware of them. Only after their deaths, their works were brought together to give an explanation about evolutionary change

Mutations may be disadvantageous or advantageous for the survival of the organism.

For Example : In Drosophila, a reduced wing caused by mutation, has become disadvantageous.

When the barks of trees in the industrial area of England turned brown due to soot and coal dust, the pepper moth which lived there had their wings turned to dark colours due to mutation. This mutation has helped the organism to escape from its predators. Thus, this is a good illustration for natural selection.

Gregor Johann Mendel is called the father of Modern Genetics. He laid a firm foundation for the study of inheritance. Though he had no information of meiosis, structure of chromosome and gene. Even then, he could formulate certain principles and laws in genetics based on the results of his experiments on pea plants. Genetists who conducted similar experiments later, arrived at the same conclusions.

Mendel was born in a poor farmer's family at Heinzendor (Czechoslovakia) on 22nd July 1822. He had his basic education but could nor continue further due to poverty he was put to job as a clergyman in a monastery in Brunn

He developed lot of interest in conducting experiment on plants in the monastery in Brunn. Work started in 1856 and continued for about 14 years. During this period he conducted class experiments on pea plants (Pisum sativum) in the back-yard Of his monastery . Based on the results of his experiments he drew some Important conclusions

He explained certain basic phenomena and laws about heredity in a journal in 1855. Unfortunately. his work went unnoticed. He died in 1884. His work was rediscovered in 1900 by three European geneticists namely Corrers Of Germany, Hugo devries Of Holland and Tchermark of Austria. They came across Mendels work when they were conducting experiments on other plants. As they drew conclusions similar to those Of Mendels they rewrote his work and published it under the title Medels ‘Principles and Laws'

7.21 Mendel's experiments

Mendel selected pea plants for his experiments because of the following advantages.

1.They can be easily grown.2.They have a short life cycle.3.They produce self pollinating flowers.4.They produce large number of seeds.5.They show contrasting heritable characters and upon induced cross pollination, they produce hybrids which are fertile.

Mendel concentrated on 7 contrasting characters. Such as,

1.stem length - tall and dwarf.2.colour of flower - red and white.3.colour of seed - yellow and green.4.colour of pod - green and yellow.5.nature of seed- round and wrinkled6.nature of pod - inflated and constricted.7.position of flower - axial and terminal

Mendelian factors are now called genes which were coined byJohannsen.

The work Of Beadle and Tatum has confirmed the theory that one gene controls one specific reaction in the biochemistry of the body.

Experiments on Monohybrid cross

A cross between two plants carrying one pair of contrasting characters, is called monhybrid cross.

Pea plants are self fertilizing plants. Mendel bred them for several generations till he obtained pure varieties. He cross pollinated pea plants having contrasting characters such as tall and dwarf, yellow-green seeds and of white and red flowers.

To cross a pure line tall plant with dwarf plant, Mendel had to ensure prevention of self pollination. If he was using the pollen of the dwarf plant he had to remove the anthers of the tall plant. He prevented undesired pollination by covering the flowers with bags.

Later, he dusted the mature pistils of tall pea plants with pollen collected from dwarf plants and effected cross pollination. He collected seeds and sowed them in the next season. He expected some plants to be intermediate. But to his surprise, he found all plants were tall. He called them as first filial or F1. generation seeds.

In the next season, he planted the F1. generation seeds. As the plants flowered he allowed them to self pollinate. When he collected and sowed the seeds, he found a mixture of tall and dwarf plants in the ratio of 3:1. He collected the seeds and called them as second filial generation seeds.

A cross between two plants carrying two pairs of contrasting characters, is called dihybrid cross. He obtained for this experiments pure varieties of round yellow coloured seeds and wrinkled green coloured seeds.

He crossed the plants with round yellow coloured seeds, with the plants with wrinkled green coloured seeds by cross pollination. When those flowered, he collected the seeds and sowed them.

He found all the plants with round and yellow seeds and he called this as first F1, generation. In the next season, he planted the F1 generation seeds. As the plants flowered, he allowed them to self pollinate. He collected the seeds and sowed them. This time he found the mixture of plants with round yellow coloured seeds, round and green coloured seeds, wrinkled and yellow coloured seeds, wrinkled green coloured seeds in the ratio of 9: 3 : 3: 1

Parental Plants - Round - Yellow x Wrinkled - green, RRYY x rryy

gametes - RY x ry.Fl hybrid - RrYy all Round and yellow

coloured seedsgametes at F1 - RY, Ry, rY, ry.

Checker board for F1 generation was first designed by Punnet. Hence it Is. also known as Punnet’s square.

Experiments on Dihybrid cross

Checker board showing the plants of F2 generationGametes RY Ry rY ry

RY RRYYRound &Yellow

RRYyRound &Yellow

RrYYRound &Yellow

RrYyRound &Yellow

Ry RRYyRound &Yellow

RryyRound &Yellow

RrYyRound &Yellow

RryyRound &Yellow

rY RrYYRound &Yellow

RrYyRound &Yellow

rrYYRound &Yellow

rrYyRound &Yellow

ry RrYyRound &Yellow

RryyRound &Yellow

rrYyRound &Yellow

RryyRound &Yellow

9 : 3 : 3 : 1Round & Round& wrinkled& wrinkled &Yellow green yellow greenseeds seeds seeds seeds

After analysing the results of his experiments, Mendel arrived at the conclusion that every trait (character) of the plant would be controlled by a pair of factors. The dominant factor for roundness and yellow colour (R,y) marked the wrinkleness and green colour (r,y) which he regarded as recessive. Crossing of these is called hybridization (RrYy). When these two factors expressed as either dominant or recessive. They are said to be pure for that trait.

On the basis of these results Mendel formulated two basic laws.1. Law of segregation.2. Law of independent assortment.

But for convenience and clear analysis of Mendel's work the following concepts were subsequently formulated.1. Concept of unit character : The characters or units by themselves are inherited from generation to generation. These factors (now called genes) are responsible for the expression of characters.2. Concept of Dominance: For any given character there are two forms of factors. One factor is dominant and the other factor is recessive.3. Law of Segregation: The pair of factors for a given character seperates in equal ratio at the time of gamete formation during meiosis. It means that a gamete can carry a factor responsible for only one form of a character.4. Law of independent assortment: When two organisms possess more than one set of traits, each pair of trait is inherited quite independently of the inheritance of other pairs of factors.

7.3 Evolution of man

Man like other animals, is a product of organic evolution. He holds the unique distinction of being the most recent and at the same time the most intelligent vertebrate on this planet. The supremacy of man is mainly due to the evolution of his hand and his brain.

Recent discoveries support the theory of human origin from apes. The course of human evolution has been followed mainly by means of the fossil record which is incomplete. However, the fragmentary fossils recovered, enabled palaeontologists to present a phylogeny of primates.

The early stages of human evolution are studied by means of comparative study of fossils. Later stages of human evolution are studied using the archeological investigations.

The existence of facts or objects made by man such as stone tools, pottery and fire hearths provide us with an insight in to the way in which modern humans have developed culturally as well as biologically.

7.31 Phylogeny of manHuman beings belong to a group of mammals called primates which also includes tarsiers, lemurs, monkeys and apes. In the year 1871 Charles Darwin published the book 'The descent of man' in which he noted the similarities between man and apes. and suggested that they had a common ancestor.

Humans share many characters with primates. Within the order of primates man belongs to the order Hominidae..

About 25-30 million years-ago ape and human stalk diverged from that of monkeys and subsequent seperation of apes and human ancestors occured between 5-10 million years ago.

Particular significance in the evolution of human was the development o an upright posture (bipedism) and increase in brain size.

The transition from walking on four legs to walking on two legs involved the changes, in skeleton and muscles it is now believed that acquisition of upright posture and changes in the nervous system allowed the subsequent enlargement of the cerebral hemispheres.

This common ancestor of humans and apes are likely to have used all four limbs for movement. But Ramapithecus spent more time in an upright posture. By about four million years ago our hominidae ancestors were bipedal and fully errect.

Freedom for forelimbs from locomotion enabled, fore-limbs holding objects. He is able to use his hands for various activities such as picking up an object, for carrying it. The thumb is opposable to all other digits.This unique feature has enabled man to develop both power grip and precision grip. This advantage of man is called dexterity.

1 Ramaaitheas2 Australopithigus3.1-arno-arm us4 Neanderthal man 5. Mortemman

Table Showing stages of human ancestery

Genus Age of appearance – Million years ago

Skull Brain capacity/cm3

Teeth Diet Posture Significance

Dryopithecus 25 Large - Large canines, incisors, molars

Soft fruit leaves

Knuckle walker

Earliest fossil age until 10 million years ago

Ramapithicus 15 Deeper Jaw

- Small canines, flattened mollas, thick enamel

Seeds and nuts

Partially upright

Earliest hominid ground dwelling in savanna

Australo pithicus

4.0 Larger jaws

450 Small canines, small incisors

Herbivorous

Fully errect

Still at home in trees but savanna dwellers

Homohabilus 2.0 Lighter jaws

700 Small carnivorous

Carnivorous

Fully errect

Earliest stone tools, began hunting, increase in brain

Table Showing stages of human ancestery

Genus Age of appearance – Million years ago

Skull Brain capacity/cm3

Teeth Diet Posture Significance

Homo erectus 1.5 Thick low forehead - browfridges

880 Small carnivorous

omnivorous

5-6 feet tall

Beginning of cultural evolution stone tool hunting in bands , rudimentary language , used fire

Homosapiens*Swancombe *Neanderthal

0.25 Heavy jaw, long face, brwofridges, enlarged nasal cavity

1200-1500

Small carnivorous wisdom teeth

omnivorous

5-6 feet Cave dweller, buried their dead, flint flake tools

Cromagnon 0.03 Vaultedcranium shorter skull, reduced jaws

1400 Teeth closes together, wisdom teeth

omnivorous

Giving rise to geological races

Genus Age Tool Material usedHomo habillus 2 million years

agoChopping toolsStone hammersPercussion flakes

Lava or quartz

Homo erectus 1.5 million years ago

Hand help axes with two cutting edges leading to point

Lava, Quartz

Cromagnon 35000 years ago Scraper, chiseldrill, arrow head

Flint bone, wood ivory

The upright posture gave the hominids increased height and wider range of vision. Along with the bipedism an increase in brain size became an added advantage. The brain size of hominidae was only 450cm3.But in human being it is about 1400 cm'. Along with increase in volume of brain, the complex infolding of the outer cortical tissue increased the surface area providing greater capacity for abstract thought.

Thus, the highly developed brain resulted in development of skills such as making of tools, hunting and communicating with each other. Along with these transitions in physical features there were transitions in social behaviour resulting in hominisation .Hominisation was very much influenced by the following:1. The development of skills and speech2. Changes in sexual behaviour along with pair bonding and increased

parental care.3. Establishment of community and social responsibility.

The intelligence of animals can be studied looking at the ratio between the weight of the brain and the weight of the spinal cord

Cat – 4:1 ; Monkey – 8:1 ; Man – 55:1---------------------------------The process of pre-human becoming a human is called hominisation

By his inventive nature he has conquered air and most infectuous diseases with vaccinations and antibiotics. He has harnessed nuclear energy and explored the potentials of biotechnology.

Man has developed spoken and written languages which are used for communicating information not only about physical world but also about philosophy and religion. The cultural changes were transmitted from person to person by communication rather than inheritance.

Social behaviour is developed to a greater extent in man than in any other species. In addition to the pair formation and family life, it has lead to the establishment of communities at the levels of bands, tribes, chiefdoms and nations.

Thus, modern man is the product of biological, cultural and social evolutions.

Culture is a store of information and set of behavioural patterns, transmitted by not genetical inheritance but by learning, imitation, instruction and by example as said by Stephen Tomkins (1984)

The study of stone tools gives useful insights into early human activities. The rate of progress in the making and use of hand tools from pebbles of 2.5 million years ago to hand-axes of 0.2 million years ago seems slow, when compared to human technological achivements of the last 100 years .

Man has been able to dominate the earth as the most powerful animal.The biological and social changes were accompanied by changes in culture

Exercise 1.Answer the following1. What is Neodarwinism?2. What is mutation? Name the two types of mutation.3. What is genetics?4. Give an instance by which it can be proved that mutation has harmful

effect5. Give any four reasons why Mendel chose pea plants for his experiments,6. How did Mendel obtain pure varieties of pea plants?7. Describe the experiment of Mendel regarding monohybrid cross.8. Draw a checker board to show the ratio in F2 generation indicating two

sets of characters.9. State the Laws of heredity.10. Explain the concept of dominance.

After studying the unit you are able to....1. understand the importance of Neodarwinism in explaining organic evolution.2. explain the significance of hereditary, variations and mutations in bringing about organic evolution.3. explain the advantages and disadvantages of mutations.4. explain the role played by mutations and natural selection in the formation of a new species.5. appreciate the perseverance of Mendel who conducted experiments for 14 long years.6. understand different stages of human evolution from the group primates.7. understand how early man evolved into modern man by the combination of biological, social and

cultural evolutions.