Evolution Question Answers: NCERT Class 12 Biology

Darwinian selection theory states that individuals with favourable variations are better adapted than individuals with less favourable variation. It means that nature selects the individuals with useful variation as these individuals are better evolved to survive in the existing environment. An example of such selection is antibiotic resistance in bacteria. When bacterial population was grown on an agar plate containing antibiotic penicillin, the colonies that were sensitive to penicillin died, whereas one or few bacterial colonies that were resistant to penicillin survived. This is because these bacteria had undergone chance mutation, which resulted in the evolution of a gene that made them resistant to penicillin drug. Hence, the resistant bacteria multiplied quickly as compared to non-resistant (sensitive) bacteria, thereby increasing their number. Hence, the advantage of an individual over other helps in the struggle for existence.

The evolution of horse started with Eohippus during Eocene period. It involved the following evolutionary stages.

(i) Gradual increase in body size

(ii) Elongation of head and neck region

(iii) Increase in the length of limbs and feet

(iv) Gradual reduction of lateral digits

(v) Enlargement of third functional toe

(vi) Strengthening of the back

(vii) Development of brain and sensory organs

(viii) Increase in the complexity of teeth for feeding on grass

The evolution of horse is represented as

(i) Eohippus

It had a short head and neck. It had four functional toes and a splint of 1 and 5 on each hind limb and a splint of 1 and 3 in each forelimb. The molars were short crowned that were adapted for grinding the plant diet.

(ii) Mesohippus

It was slightly taller than Eohippus. It had three toes in each foot.

(iii) Merychippus

It had the size of approximately 100 cm. Although it still had three toes in each foot, but it could run on one toe. The side toe did not touch the ground. The molars were adapted for chewing the grass.

(iv) Pliohippus

It resembled the modern horse and was around 108 cm tall. It had a single functional toe with splint of 2nd and 4th in each limb.

(v) Equus

Pliohippus gave rise to Equus or the modern horse with one toe in each foot. They have incisors for cutting grass and molars for grinding food.

Fossils of dinosaurs have revealed the evolution of reptiles in Jurassic period. As a result of this, evolution of other animals such as birds and mammals has also been discovered. However, two unusual fossils recently unearthed in China have ignited a controversy over the evolution of birds.Confuciusornis is one such genus of primitive birds that were crow sized and lived during the Creataceous period in China.

Species can be defined as a group of organisms, which have the capability to interbreed in order to produce fertile offspring and do not breed with members of other species.

The various components of human evolution are as follows.

(i) Brain capacity

(ii) Posture

   iii.            Food / dietary preference and other important features

There are many animals other than humans, which have self consciousness. An example of an animal being self conscious is dolphins. They are highly intelligent. They have a sense of self and they also recognize others among themselves and others. They communicate with each other by whistles, tail-slapping, and other body movements. Not only dolphins, there are certain other animals such as crow, parrot, chimpanzee, gorilla, orangutan, etc., which exhibit self-consciousness.

The modern day animals and their ancient fossils are listed in the following table.

You can draw image of any animals of your wish.

Adaptive radiation is an evolutionary process that produces new species from a single, rapidly diversifying lineage. This process occurs due to natural selection. An example of adaptive radiation is Darwin finches, found in Galapagos Island. A large variety of finches is present in Galapagos Island that arose from a single species, which reached this land accidentally. As a result, many new species have evolved, diverged, and adapted to occupy new habitats. These finches have developed different eating habits and different types of beaks to suit their feeding habits. The insectivorous, blood sucking, and other species of finches with varied dietary habits have evolved from a single seed eating finch ancestor.

No, human evolution cannot be called adaptive radiation. This is because adaptive radiation is an evolutionary process that produces new species from a single, rapidly diversifying lineage, which is not the case with human evolution. Human evolution is a gradual process that took place slowly in time. It represents an example of anagenesis.

G.H. Hardy and W. Weinberg independently discovered a rule that related allelic and genotypic frequency in the Mendelian population (shares common gene pool). There is an aspect of this law which explains that the allelic frequencies at an autosomal locus in the population will not change from one generation to the next (allelic- frequency equilibrium), i.e. will remain constant. There are five factors that are responsible for the failure of Hardy- Weinberg equilibrium: gene flow, genetic drift, non-random mating or genetic recombination, natural selection and mutation.

Equation of Hardy and Weinberg;    p² + 2pq + q² = 1

Where; p is the frequency of dominant allele, q is the frequency of recessive allele and pq defines the heterozygous population.

There are some assumption that are necessary for this law, population should be panmictic and there would be no natural selection, no mutation, no migration occurs.    

1. Genetic drift: Random genetic drift occurs as the result of non- directional fluctuations in the transfer of alleles from one generation to the next in the small population purely by the chance irrespective of selective advantage or disadvantage of the gene. Genetic drift results in the increased variation within a population and an increase in the divergence between populations.  
2. Gene migration or gene flow: It is the movement of alleles from one gene pool to another. There are some different models for the gene flow like Continent-island model (it is one way migration to a separated population), Stepping-stone model (it is a migration where migrants come only from neighbouring populations).
3. Mutation: Mutation is the source of all genetic variations and is considered as random phenomenon; and occurs in all directions.  
4. Natural selection: Darwin described the process of natural selection in which the forms of organisms in a population that are best adapted to the environment increases in frequency relative to less adapted forms over a number of generations.
5. Genetic recombination: The exchange of genetic material between different organisms that leads to the production of offspring with the combination of traits is genetic recombination.

Survival of the fittest means to those that survives are called fit. Darwin fitness describes how successfully an organism has been passing onto genes from one to another and this explains the evolutionary phase. The more individuals survive more they reproduce, higher the fitness of the individuals. Fitness is the ability to survive and reproduce and it is the outcome of the struggle. These traits could give the members a better chances of survival and which is known as ‘Survival of the fittest’. This brings the variations according to the environmental conditions which forms the basis for origin of different types of adaptations.     

Mendelian population consists of individuals that shares a common gene pool when crossed with each other and are characterized by equal survival of all genotypes. Three most important characteristics criteria for designating a Mendelian population are as follows:

1. Population should be in the large extent. A small population is characterized by free genetic exchange whereas genetic exchange is restricted up to an extent in the large population.
2. The intra-specific Mendelian population maintain more or less distinct gene pools this is as a rule, separated in space. Two Mendelian population may or may not share similar gene pools. There should be free flow of genetic materials that will be ensured through the sexual selection.
3. There should be no migration in the Mendelian population.                                                                                                        

Migration is the movement of organisms among the subpopulations and this includes both immigration and emigration. Migration results in homogenization of allele frequencies among all the population in the absence of selection process. It enhances the gene pool of those alleles which have been selected and blur the effects of selection through the replenishment of alleles that were selected against by the nature.

The selection can amplify the effects of migration. If the selection is stronger than migration, then the differences among the population will be maintained. And, if the migration is stronger than selection then the differences among the population will be reduced. The diversity among the population of intermediate age reduces as the results of migration.

Ramapithecus was more man- like while Dryopithecus was ape- like. Dryopithecus is considered to be the ancestors of both apes (chimpanzee, gorilla) and humans. They are closely related to the chimpanzee but had arms and legs of approximately equal length, heels it its feet indicates the semi-erect posture. The human DNA is approximate 98.4 % similar to that of chimpanzees. On the basis of genomic studies, we have found that humans and chimpanzee ancestors diverged and then converged. Ramapithecus lived on the open grassland, have thickened tooth enamel, shorter canines, robust jaws and hence, showed a hominid status.  

The first hominid was known ‘Homo habilis’. It was the first human, or the first recognizable human species which is designated as the Handyman or also known as the ‘tool maker’ which has been called because rudimentary tools have been found with the fossils of this species. They had a brain larger than Australopithecus and also had some similarities with the Australopithecus africanus.     

Among all three of them, Ramapithecus was the one who never ate meat. Ramapithecus lived on the open grassland, have thickened tooth enamel, shorter canines, robust jaws and hence, showed a hominid status. They couldn’t ate meat cause of their dental structure and ate hard nuts and seeds like modern man.  Molars show the wear and tear method that indicates Ramapithecus were the “tough Herbivores” and their diet includes variety of nuts, seeds and grasses. 

When two or more related species become more or more dissimilar and the similarity only occurs due to the same origin or same ancestors is known as divergent evolution; and this shows the homology. As the bones of forelimbs of whale, bat, cheetah and man are similar in structure and have the common ancestors (mammals) but performs different function like whales use their forelimbs as the flippers by which they control their speed while swimming; bat’s forelimb is modified for flying; cheetah uses forelimbs for capturing prey or running; and man use their forelimbs to perform various tasks, so they are known as homologous organs. Another good example is Darwin’s finches, which has now 80 species varieties which all are diverged from the single original species of finch. The driving force behind the divergent evolution is adaptations to the new habitat or environment.   

By the natural selection, industrial melanism phenomenon used to describe the evolutionary process in which light coloured peppered moth population become dark. This phenomenon affected over 70 species of moths in England, best studied in peppered moth ‘Biston betularia’. In 1800 or prior, the typical moth species had a light coloured pattern and dark coloured or melanic moths were rare. During the industrial revolution, industrial waste leads to darkened tree trunks and which killed off lichens. Then light coloured peppered moths rely on camouflage (also called cryptic colouration and is a defence or tactic that is used to disguise their appearance usually to blend in with their surroundings colour) to avoid predation and this sudden change in the environment, made them high vulnerable to predators. Over the time, in 1886, light coloured moth became rare and dark one became abundant.

This leads to better chances of survival of dark winged moths and this shows reversible evolution. When the industries have removed, it will help in bringing down population level. And, this will help in to bring the lichens population again flourish on the tree trunks and then tree trunks will gain a whitish layer of lichens which will result in surviving the more population of white- winged moths. 

Charles Robert Darwin provide evidences for the scientific theory that all species have evolved over time from one or few common ancestors by the process of natural selection and this forms the basis of modern evolutionary theory. He publicized a book ‘The Origin of Species’ in which he presented the idea of the natural selection is the mechanism of evolution. Mainly Darwin theory had a combination of five theories: Evolution, Common descent, Multiplication of species (this explains the organic diversity in which species multiply by splitting into daughter species or by budding), Gradualism (evolutionary changes take place gradually not sudden which presents new type of organisms), Natural selection. Mainly there are two key concepts in evolutionary theory of Darwin:

Phylogenetic tree and natural selection:

Phylogenetic tree: it is the most convenient way of presenting evolutionary relationship among the group of organisms through illustrations called phylogenetic trees. It is a two dimensional graph shows evolutionary relationship between organisms and shows by branches and nodes. The branching pattern of the trees is known as tree topology. This contains internal node
(represents last common ancestor of two lineages) and external nodes (represents tip of the trees).

Natural selection: the process by which organisms with favourable conditions or variation will survive and reproduces at higher rate. An inherited variations which increases the chances of organism’s survival in the particular environment is known as adaptation and who will not adapt the variations among the environment will die. Darwin describe the survival of the fittest. Fitness is describe as the ability to survive and reproduce or outcome of the struggle in a specific manner. In the changing environmental conditions which could have the ability to adapt, have the better chances of survival and reproduction.      

This phenomenon indicated in the question is convergent evolution where the unrelated animals converging on the same structure because they are adaptive in the same environment, this type of evolution is known as convergent evolution and this produces analogous structures. In the simple way we can say that analogous organs are those which have different origin or ancestors but performs similar functions.

Example:

1.  Streamlined shape of sharks and dolphins (their body shape, fins and flippers are same but they belongs to different lineages; shark is an elasmobranch fish belongs to Pisces and dolphin is an aquatic mammal). But both of them depends on swift movement through the water, so a streamlined shape is essential. Thus, it is a similar habitat that resulted in selection of similar adapted features in different groups of organisms, but towards the same function.
2. Spines (modified leaves) and thorns (modified stem); both looks similar and provide protection to the plant, the plants to which they belong are distantly related.

Yes, humans are also evolve. Fossils are there to give the evidences that evolution is a continuous phenomenon. Fossils are the preserved remains or traces of organisms from the distant past. Fossils record shows that there have been great changes occurs in the kinds of organisms that dominated life on the earth at different points in time. Human evolution occurs at different stages like firstly Dryopithecus, then Ramapithecus, then Australopithecus, then Homo- habilis, then Homo- erectus, then Neanderthal man and then finally Homo sapiens evolves as modern man.

Some scientist suggests that evolution is indeed taking place in modern humans and some of the observations are as follows:

Human’s reproductive period has been changed and prolonged than it was 10k years ago. Earlier the human’s life was half than in today’s time. In adulthood, all other mammals develops lactose intolerance because young ones are reared away from mother’s milk after a certain age, unlike humans other mammals do not have  the facility to consume other mammals or animals milk but humans do. They consume milk throughout the life from the domesticated animals. Human beings have developed many resistant vaccine against the infectious diseases, so this is another sign of evolution.

However, evolution happens through the geographical location because modern means of transport has blurred geographical boundaries, but it can also occurs through the genetic drift.   

There is a widespread belief that Darwin could have carried out a deeper theoretical study if he had more information about Mendelism and laws of heredity.

The work of Mendel did not offer insights. He explained that the crossing of pea plants with different characteristics does not result in mixture of traits in the offspring, rather traits behave in a discrete manner, not mixing with each other.

This version of the unfortunate mismatch between Darwin and Mendel can be found in a variety of sources. Darwin published (the origin of species) Gregor Mendel wrote a ground breaking paper of inheritance in pea plant. In that paper, Mendel proposed a particular model of inheritance, which states that organisms transmit discrete heritable units (now called genes) to their offspring. Although Darwin learned about genes.  

Yes, I agree that gene flow occurs through generations. The human migratory patterns can be predicted by studying the allelic frequencies in prehistory and history. We can trace the evolutionary history and migratory patterns by the Human Genographics project. This project is a genetic anthropological study that was launched on April 13, 2005 by the National Geographic Society which aims to map history of human migration patterns by collecting and analysing DNA samples.

Note*: Shared gene pools gives the species its own identity. Reduced gene flow play important role in the speciation. Various modes have been presented whereby a single evolutionary descendent splits into two or more genetically independent lineages due to reduced gene flow.     

Race: It is a permanent breed within the species and this is considered equivalent or subordinate to the subspecies. The races different in height, pigmentation, shape of hands, hair, eyes, nose etc. They freely interbreed as they have geographical or physiological barrier for it; in this members from different races can interbreed.

Breed: It is specific group of domesticated animals. They are homogenous in appearance (uniform structure and shape) and also homogenous in behaviour and have different distinguishable characters which make them unique from other members of the species.

Cultivar: This is the basic classification category of the cultivated plants. Cultivar is also known as cultivated variety which is an assemblage of the plants selected for desirable characteristics by the process of selective breeding (is an artificial selection used to develop new organisms with desirable characteristics).

Variety: Members of the variety do not breed with members of other subspecies or variety but they can fertile offspring under the natural conditions. It is a breeding group of species having specific features which make them separate and unique from other members of the species.