CBSE NOTES CLASS 10 SCIENCE CHAPTER 9
HEREDITY AND EVOLUTION
Recognisable features like colour of hair, colour of iris, height, complexion etc. are called traits or characters.
The transmission of traits from the parents to offspring is called heredity.
The differences in the characters (or traits) among the individuals of a species is called variation. Example – attatched ear lobe vs free ear lobe, blue vs black colour of eyes, tall vs dwarf etc.
Traits, which are acquired due to interaction with the environment, are called acquired traits. Acquired traits are not inheritable. For example; if a boxer develops bulging biceps, it does not mean that his son would be born with bulging biceps.
Traits, which can be expressed in subsequent generations, are called inherited traits. Such traits bring a change in the genotype of the organism and hence become inherited.
Accumulation of variations during reproduction
- Significance of variation is shown up only if it continues to be inherited by the offspring for several generations.
- The advantage of variation is that it increases the chances of survival of species in changing environment. For example if some of the bacteria get a variation to tolerate higher temperature, they will survive even if the temperature of the earth rises.
- Asexual reproduction involves a single parent and is hence not ideal for facilitating variations. Some minor variations do occur due to inaccuracies in DNA replication. But the quantum of variations would be too little and would take too many years to show effect.
- Sexual reproduction, on the other hand, is ideal for facilitating variations because two parents are involved in it. The offspring’s genotype is contributed by two parents and hence chances of variations are very high.
- Only those individuals with variations which match the cell and the environment will survive, others will perish and the variations will not be inherited.
- Characteristics that came into existence earlier are likely to be more basic and likely to be found in larger portion of the population than characteristics that have come into existence later.
Chromosome is a thread like structure in the nucleus of a cell formed of DNA which carries the genes
Genes are units of heredity which transfer characteristics from parents to offsprings during reproduction. A gene is a unit of DNA on a chromosome, which governs the synthesis of one protein that controls a specific characteristic or trait. Genes occur in pair, one from each parent.
The gene, which decides the appearance of an organism even in the presence of an alternative gene, is known as dominant gene. Represented by capital letter e.g. tallness, with ‘T’.
The gene, which can decide the appearance of an organism only in the presence of another identical gene, is called recessive gene. Represented by small case letter e.g. dwarfness, with ‘t’.
The complete set of genes in an organism’s genome is called genotype.
The characteristic, which is visible in an organism, is called the phenotype. Phenotype is a result of genotype’s interaction with the environment. Due to this reason, many phenotypes are not inheritable.
First and Second Filial (F1, F2)
When two parents cross or breed to produce progeny or offsprings, then their progeny is called first filial or F1 generation.
When the F1 generation cross among themselves to produce second generation progeny, it is called, second filial or F2 generation.
Rules of Inheritance
Gregor Mendel conducted experiments on pea plants and proposed the rules of inheritance; based on his observations. Mendel observed that characters are often present in pairs. A pair of contrasting characters is called allele.
Reasons of choice of pea plants by mendel
- Pea is a biennial plant, i.e. two generations of a pea plant can grow in a given year. This means that Mendel could get enough time to observe a larger number of generations.
- Many easily identifiable and contrasting characters are present in pea plants.
- Cross pollination can be easily induced in pea plants.
- Pea plants grow quite fast.
A new plant (organism) resulting from cross of different varieties of a plant (organism) is known as a hybrid.
The cross in which only one pair of contrasting characters is studied is called monohybrid cross. He selected a pair of contrasting characters tallness (T) and shortness (t) for his experiment.
TT represents the genotype of tall plant and tt represents the genotype of short plants. In the F1 generation, all plants were tall but their genotype was Tt; which means they were not pure tall plants.
In the F2 generation; in which most of the plants were tall and some of the plants were short. The ratio of number of tall plants to that of short plants in F2 generation was 3:1.
Mendel's First Law or Law of Segregation
The characteristics (or traits) of an organism are determined by internal factors (genes), which occur in pairs (allele). Only one such factor can be present in a single gamete. Phenotype corresponding to only the dominant factor is visible and that due to the recessive factor is not visible.
The cross, in which two pairs of contrasting characteristics are studied, is called dihybrid cross. Mendel took plants with round and green seeds and those with wrinkled and yellow seeds.
The genotype of round-green seed is shown by RRyy and that of wrinkled-yellow seed is shown by rrYY.
In the F1 generation, all plants produced round-yellow seeds, which means that wrinkled character was the recessive character and so was the green colour of seeds.
When plants of F1 generation were allowed to self pollinate; it was observed that most of the plants in F2 generation produced round-yellow : round-green : wrinkled-yellow : wrinkled-green in the ratio 9 : 3 : 3 : 1.
Mendel’s Second Law or Law of Independent Assortment
In the inheritance of more than one pair of traits in a cross simultaneously, the factors responsible for each pair of traits are distributed independently to the gametes.
In the above example; alleles of roundness of seed were assorted independently from those of seed colour.
How are Characteristics transmitted to Progeny?
The characteristics or traits of parents are transmitted to their progeny or offsprings through genes present on their chromosomes during the process of sexual reproduction. Genes work in pairs. One gene of the pair is inherited from each parent. Depending on the dominance of the genes, phenotype of offspring will be decided.
Blood group inheritance
A person has one of the four blood groups A, B, AB or O. The gene responsible for it is the I-gene in human beings and has three forms, denoted by IA, IB and IO.
IA and IB are co-dominant and IO is recessive.
IA + IA → IAIA ⇒ Blood group A
IA + IO → IAIO ⇒ Blood group A
IB + IB → IBIB ⇒ Blood group B
IB + IO → IBIO ⇒ Blood group B
IA + IB → IAIB ⇒ Blood group AB
IO + IO → IOIO ⇒ Blood group O
Sex Determination in Humans
Somatic cells in human beings contain 23 pairs of chromosomes. Out of them the 23rd pair is composed of different types of chromosomes which are named as X and Y chromosomes.
The pair is XY in a males and XX in females.
When a sperm with X chromosome fertilizes the egg, the resulting zygote would develop into a female child. When a sperm with Y chromosome fertilizes the egg, the resulting zygote would develop into a male child.
Hence it is the father who is responsible for the sex determination of the baby. Therefore mother should not be held responsible for birth of a girl or boy baby.
The sequence of gradual changes, which take place in inherited traits in biological population over millions of years, in which new species are formed, is called evolution.
Scientists have proven that life evolved in the form of simple unicellular organisms on this earth; and all the organisms which are present today have evolved from a common ancestor.
The idea of evolution is based on the premise of a common ancestry.
Example of evolution
Petrosaur is an ancient flying reptile which lived about 150 million years ago. It began life as a lizard which could just crawl on land. Over millions of years ago, small folds of skin developed between its feet, which enabled it to glide over the trees. Over many generations the folds, bones and muscles supporting them grew to form wings, which could make it fly. This is the evolution of a flying reptile from a lizard.
Process of evolution
To understand how evolution takes place, let us take some imaginary examples.
Situation 1: A group of red ants is living in a bush. As hunting birds can easily spot red ants in the green background, they enjoy feasting on ants. Because of some error in DNA replication, some blue ants come into origin. Hunting birds cannot spot blue ants against a green background. As a result, blue ants survive and red ants become extinct over a period of time. The origin of blue ants happened by chance but it gave survival benefit to the ants. Finally, blue ants could survive and proliferated in the surrounding.
Situation 2: In the same group; some blue ants came into origin. Ants of both colours were almost equal in population. One day, an elephant came and trampled the bushes. All the red ants perished in the accident; leaving only the blue ants. This resulted in extinction of red ants but blue ants could continue their race. The survival of blue ants was because of an accident and the accident was the cause of natural selection.
Situation 3: A group of red ants was living in a bush. Due to draught like conditions, availability of food became a problem for the ants. All the ants became weak and underweight. Subsequent generations comprised of smaller ants and the trend continued for a few generations. Situations changed and plenty of food became available. Ants once again developed to their normal size. In this case, the change in size was a change in phenotype and hence was not inheritable. The change in size could not produce variation and evolution in the species.
Evolution and Classification
The modern system of classification is based on evolutionary relationship. Due to this, it is also known as phylogenetic classification.
Members of a species have a higher number of common characters, than members of a kingdom.
For example; all human beings belong to the species Homo sapiens. Human beings can interbreed; irrespective of their race or skin colour.
All human beings come under the class mammalia; as do the monkeys, elephants and cows. Apparently, each species of the class mammalia is quite different yet they have certain common characters; like hairs on the body and mammary glands in females.
Similarly, all animals are eukaryotes and cell wall is absent in their cells. The degree of similarity or dissimilarity shows that all animals have evolved from a common ancestor.
EVIDENCE FOR EVOLUTION
1. Homologous Organs
Organs which have common basic design but serve different functions in different animals are called homologous organs.
For example - The forelimbs of frogs, lizards, birds, bat and human being have same basic designs. Yet, the forelimbs of frogs are adapted to a jumping movement, those of birds are used for flying and those of humans are used for handling tools.
- This shows that frogs, birds and humans have evolved from a common ancestor.
2. Analogous Organs
Organs which have different basic design but serve a common function in different animals are called analogous organs.
Wings of birds and wings of bat are examples of a pair of analogous organs.
Wings of birds are composed of all the bones of forelimb and are covered with feathers. Wings of bats are mainly composed of the bones and a thin membrane covering the structure.
Yet wings in both the organisms are used for flying.
- Hence birds and bats do not have common ancestor, but they can evolve to perform different designs and survive and flourish in the prevailing environment.
The remains (or impressions) of dead animals or plants or other organisms from the distant past are called fossils. Scientists usually take 10,000 years as the minimum age for the remains to be categorized as fossil. These fossils tell us about many extinct animals and also give insights into how the evolution could have taken place. They provide evidences of evolution by revealing the characteristics of the past organism and the changes that have occurred in these organisms to give rise to the present organisms.
Fossils provide following information
Relative age of the fossil
The fossils found deeper are older than those found closer to the surface.
Exact age of the fossils can be found by finding the ratios of different isotopes of carbon isotopes. This is called carbon dating.
How are fossils formed layer by layer?
a) Around 100 million years ago, some invertebrates on the sea bed died and were buried in the sand. More sand accumulates and sandstone is formed under pressure.
b) Millions of years later dinosaurs living in the area die and their bodies to are buried in the mud. The mud is compressed into rocks, above the previous layer.
(b) Millions of years later, bodies of horse-like creatures dying in the area are buried in the rocks.
As we dig, the sequence of fossils found will be horse, dinosaurs, invertebrates etc.
Evolution by Stages
Evolution of complex organs and thus of complex organisms has happened in stages.
Example 1: Evolution of Eyes
Eye is a complex organ and its evolution could not have taken place due to a single variation. The evolution of eye has happened in stages as follows,
Planaria is the first animal which shows ‘eye’ like structure. The dark spots on planaria are light sensitive spots but a planaria cannot distinguish between two different objects.
Eyes of insects are compound eyes which are made up of thousands of optical surfaces.
Eyes of higher animals are simple eyes which are composed of a single lens.
Most of the animals cannot differentiate among colours.
Depth perception is also weak in many animals.
Human eyes are the most advanced; because humans can recognize colours and have very good depth perception.
Example 2: Evolution of Bipedal Walking - Most of the tetrapods have to use all the four limbs for locomotion. Some apes can walk up to smaller distances by using just the hind limbs. Humans have finally evolved the bipedal walking.
Example 3: Evolution of Feathers - Sometimes an evolutionary change produced in an organism for one purpose later on becomes more useful for an entirely different function.
Feathers, started out as providing insulation in cold weather. But later, they became useful for flight. Some dinosaurs had feathers, although they could not fly using the feathers. Birds seem to have later adapted the feathers to flight. This, of course, means that birds are very closely related to reptiles (dinosaurs).
Darwin’s Theory of Evolution
Charles Darwin wrote his famous book ‘Origin of Species’. Some salient points of Darwin’s theory are as follows:
(i) More individuals are produced each generation that can survive. This ensures that at least some of them will survive.
(ii) Within the population, there is natural variation. Some individuals have more favourable variation than others.
(iii) Phenotypic variation exists among individuals and the variation is heritable. Those individuals with heritable traits better suited to the environment will survive
(iv) Natural selection - The struggle for survival within the population eliminates unfit individuals. Fit individuals survive and reproduce.
(v) These variations when accumulated over a long period of time and reproductive isolation will occur, giving rise to new species.
For example; a frog lays thousands of eggs at one go. They are released in water and left to fend for themselves. Most of the eggs are either washed away or are eaten by predators. However, some portion of eggs from the spawn develops into tadpoles. Once again, many tadpoles are eaten up by predators; leaving a few which develop into adults. A large number of eggs are therefore, needed to ensure that at least some of them develop into adults.
Evolution Vs Progress
Evolution does not mean progress in every case. This can be proved by example of bacteria. Bacteria are the simplest and one of the oldest organisms on the earth. Their simple body design does not make them weak from any angle. Bacteria are known to survive some of the harshest climates; like craters of volcanoes and sulfur springs.
On the other hand many animals have certain features which hamper even their routine activities.
For example; the branch-like horns of antelope are a handicap for them. When an antelope runs for its life; there are times when its horns get entangled in branches or bushes. This results in the death of the antelope.
Colourful feathers of a male peacock are very helpful when it comes to attract a female. But because of its conspicuous feathers, it can be easily spotted by a predator. Because of its bulky feather it cannot fly away to safety.
A species is a population of organisms consisting of similar individuals which can breed together and produce fertile offsprings.
The process by which new species develop from the existing species is known as speciation.
Important factors which could lead to formation of new species are,
(i) Geographical isolation: It is a major factor in speciation since it interrupts with gene flow. Geographical isolation is caused by various types of barriers such as mountain ranges, rivers and seas. It leads to reproductive isolation due to which there is no flow of genes between separated groups of population which ultimately results in speciation.
(ii) Natural selection: All populations respond to changes in their environment. Individuals will respond in different ways depending on their genes. Those individuals whose genes are best suited to the environment are more likely to survive and pass on their genes to the next generation. This is natural selection. Gradually, favourable genes will start to predominate in the population and less favourable genes will decline.
(iii) Genetic drift: This genetic variation arises from random mutations in the DNA sequence or drastic changes in the frequencies of particular genes by chance alone. Genetic drift with changes in the gene flow imposed by isolation mechanism acts as an agent of speciation which ultimately results in evolution.
Selectively breeding of plants and animals for desirable traits is called artificial selection. It allows a lot of control in the breeding process but can also lead to unintended mutations within a population of organisms.
Example: Development of Cabbage, Broccoli, Cauliflower, Kohlrabi and Kale from wild cabbage, which is a form of wild mustard.
This proves Darwin’s theory that completely different looking species can be produced from a single ancestor.
Molecular Origin of Life
Stanley L. Miller and Harold C. Urey conducted the Miller-Urey experiment in 1953 to demonstrate how the life would have originated on the earth. They created an environment in laboratory which mimicked the environment of earth as it was during the time of origin of life. Water, methane, ammonia and hydrogen were used in that experiment. The liquid was heated to initiate evaporation and electrodes were used to create electric discharge. At the end of two weeks, some organic molecules were formed in the setup. Some amino acids and sugar were also formed. This proved the hypothesis of J. B. S. Haldane that life originated from inorganic raw materials.
The modern humans are called Homo sapiens. Many scientific investigations have shown that the modern humans evolved in Africa. They migrated towards north; in due course of time and settled near what is known as the Mediterranean Sea. When the ice age ended, melting of ice resulted in the increase of water level. The humans migrated in different directions from that area. One branch went to the western Asia, then to the Indian Peninsula and finally to Australia. From the Indian Peninsula, one branch migrated towards China and subsequently to the North America. From North America, they migrated to the South America. From the Mediterranean Sea, the second branch migrated towards Europe where they are believed to replace the Neanderthals.