Evolution

Sweet potato is a modified root tuber while potato is a modified stem tuber. They have different anatomic origins but perform analogous food storage functions, exemplifying analogy through convergent evolution.

The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation.
Darwin's finches of the Galapagos Islands and Australian marsupials are examples.

On the Galapagos Islands Darwin observed many varieties of small black birds, later called Darwin's finches.
All the varieties evolved on the islands themselves from an original seed-eating stock; forms with altered beaks arose, enabling them to become insectivorous and vegetarian finches.
This process of evolution of different species in a given geographical area, starting from a point and literally radiating to other habitats, is called adaptive radiation.
Convergent evolution is the reverse case, in which unrelated groups come to resemble each other; industrial melanism is an example of natural selection in moths.
Hence Darwin's finches are a good example of adaptive radiation.

During his voyage Darwin observed small black birds on the Galapagos Islands, later called Darwin's finches.
He found many varieties of finches on the same group of islands, and concluded that all of them had evolved on the islands themselves from one original seed-eating stock.
From the seed-eating features, many other forms with altered beaks arose, enabling them to become insectivorous and vegetarian finches.
This process of evolution of different species in a given geographical area, starting from a point and literally radiating to other areas of geography (habitats), is called adaptive radiation.
Another example is the Australian marsupials.
So Darwin's finches are an excellent example of adaptive radiation.

Adaptive radiation is the process of evolution of different species in a given geographical area, starting from a point (a common ancestor) and radiating to other habitats.
The descendants of the common ancestor become adapted to different modes of life and so diverge into different species; it is also known as divergent evolution. The term was given by H.F. Osborn.
Examples are Darwin's finches of the Galapagos Islands and the Australian marsupials.
The placental mammals also show it: they became adapted for aerial (bat), aquatic (whale and dolphin), burrowing (rat), running (horse) and tree-living (monkey) modes of life.
It is not the migration of a species, and it is not the power of one individual to adapt to many environments.
So adaptive radiation refers to the evolution of different species from a common ancestor.

Flippers of penguins and dolphins perform similar functions but are anatomically different in origin – they are analogous organs. Analogous structures are a result of convergent evolution.

In 1953, S.L. Miller created electric discharge in a closed flask containing $CH_4$, $H_2$, $NH_3$ and water vapour at 800 °C, and observed the formation of amino acids.

The concept of chemical evolution was proposed by Oparin of Russia and Haldane of England.
According to them, the first form of life could have come from pre-existing non-living organic molecules such as RNA and proteins.
The formation of life was preceded by chemical evolution, i.e. the formation of diverse organic molecules from inorganic constituents.
The conditions on the primitive earth were high temperature, volcanic storms, lightning and a reducing atmosphere containing $CH_4$, $NH_3$, water vapour and $H_2$.
Under these conditions simple chemicals combined to form amino acids, sugars and nitrogenous bases, which further combined into macromolecules. Miller and Urey supported this experimentally in 1953.
So chemical evolution is based on the possible origin of life by the combination of chemicals under suitable environmental conditions.

According to Oparin and Haldane, the primitive atmosphere of the earth was a reducing atmosphere.
It contained methane ($CH_4$), ammonia ($NH_3$), hydrogen ($H_2$) and water vapour, but no free oxygen.
Energy from lightning, UV radiation and volcanic heat made these gases react to form simple organic molecules (chemical evolution). This was tested by Miller and Urey in 1953.
Free oxygen appeared much later, after cyanobacteria began oxygenic photosynthesis.
So oxygen was not present in the primitive atmosphere.

Reference: NCERT Class XII, page 121. Frequency of allele A = p = 0.1. At Hardy-Weinberg equilibrium, frequency of AA = $p^2=(0.1)^2=0.01$.

Hardy–Weinberg equilibrium is disturbed by mutation, gene flow, genetic drift, non-random mating and natural selection.
Selective mating is non-random mating, so the population will not be in equilibrium.

Hardy–Weinberg equilibrium assumes random mating. Lack of random mating, like mutation, migration, drift and selection, disturbs it.

According to the Hardy-Weinberg principle, in a large, randomly mating population at equilibrium, $p^2 + 2pq + q^2 = 1$.
Here $p$ = frequency of allele A = 0.6 and $q$ = frequency of allele a = 0.4.
$p^2$ = frequency of AA, $q^2$ = frequency of aa and $2pq$ = frequency of the heterozygotes Aa.
$2pq = 2 \times 0.6 \times 0.4$
$2pq = 0.48$
(Check: $p^2 = 0.36$, $q^2 = 0.16$, and $0.36 + 0.48 + 0.16 = 1$.)

The Hardy–Weinberg principle states that allele (gene) frequencies in a population remain constant from generation to generation if certain conditions are met.
These conditions are: a large population, random mating, and no mutation, no migration (gene flow), no genetic drift and no natural selection.
Sexual selection, mutation and gene flow all change gene frequencies.
So the gene ratio remains constant under random mating (along with the other conditions).

As per the mutation theory given by Hugo de Vries, evolution is a discontinuous or saltatory phenomenon (saltation).

Reference: NCERT Class XII, page 125.

The cranial capacity increased steadily through human evolution.
$Homo\ habilis$: 650–800 cc; $Homo\ erectus$: about 900 cc; Neanderthal man: about 1400 cc; modern $Homo\ sapiens$: about 1350–1450 cc.
The increase in brain size and complexity led to intelligence, tool making, language and culture, which set modern man apart from his ancestors.
Upright posture, shortening of jaws and binocular vision also evolved, but they were already present in earlier hominids or primates and are less significant.
Hence the most significant trend was increasing brain capacity.

The cranial capacity increased steadily during human evolution.
$Homo\ habilis$: 650–800 cc; $Homo\ erectus$: about 900 cc; Neanderthal man: about 1400 cc; modern $Homo\ sapiens$: about 1350–1450 cc.
This remarkable increase in brain size and complexity is the most apparent change, and it led to intelligence, language, tool-making and culture.
Upright walking, shortening of jaws and loss of body hair also occurred, but they are less marked than the increase in brain size.
Hence the most apparent change is the remarkable increase in the brain size.

The oldest fossils of hominids, such as Australopithecus (Lucy), Homo habilis and early Homo erectus, have been found in East and South Africa.
Molecular (mitochondrial DNA) studies also show that modern humans, Homo sapiens, arose in Africa and then migrated to other continents ('Out of Africa' theory).
Fossils found in Java (Java man), China (Peking man) and France (Cro-Magnon man) are of later forms.
So, according to the fossils discovered, the origin and evolution of man started in Africa.

Comparison of DNA sequences, chromosomes, proteins and blood groups shows that the chimpanzee is the closest living relative of humans; about 98–99 percent of DNA is identical.
Humans and chimpanzees shared a common ancestor about 6–7 million years ago.
So the closest relative of man is the chimpanzee.

Among the given primates, the gorilla, an African great ape, is the most closely related to man (after the chimpanzee, which is not among the options).
Molecular studies show that humans, chimpanzees and gorillas form a closely related group; the orangutan and gibbon branched off earlier, and the rhesus monkey is an Old World monkey, more distant still.
So the gorilla is the one directly related to man among these.

On the Galapagos Islands, Darwin observed many species of finches, each on particular islands and with beaks adapted to different foods (seed eating, insect eating and so on).
They all evolved from a common ancestor from the South American mainland, by adaptive radiation into different habitats on the isolated islands.
This geographical distribution of related species is evidence from biogeography.
So Darwin's finches are related to evidences from biogeographical distribution.

Early Greek thinkers believed units of life called spores were transferred to different planets including Earth. This theory of panspermia is still favoured by some astronomers.

Mutation theory was given by Hugo de Vries. Mutations are large differences arising suddenly in a population; they are random and directionless, and a single-step large mutation (saltation) causes speciation.

When unrelated organisms develop similar adaptations in response to the same environmental challenge, it is called convergent evolution.

When unrelated organisms (with different genetic make-up) live in similar environments, they may evolve similar features to meet similar needs.
This is convergent evolution, which produces analogous structures, e.g. wings of birds and insects, flippers of whales and fins of fish.
Divergent evolution is the opposite: related organisms become different (homologous organs).
So similarities in organisms with different genotypes indicate convergent evolution.

Reference: NCERT Class XII, page 120.

Reference: NCERT Class XII, pages 117, 118.

Reference: NCERT Class XII, pages 114, 115.

Forelimbs of different animals show a common pattern in the arrangement of bones, but their functions differ (divergent evolution); such organs are homologous. The wings of birds and flippers of whales are modified forelimbs.

Divergent evolution occurs in the same structure, e.g. forelimbs, heart and brain of vertebrates, which have developed along different directions due to adaptation to different needs. The eyes of octopus, bat and man are examples of analogous organs showing convergent evolution.

In different vertebrates the bones of the forelimbs are similar, but the forelimbs are adapted in different ways as per their needs; hence this is an example of homology.

Artificial selection to obtain cows yielding higher milk output shifts the peak in one direction, hence it is an example of directional selection. In stabilising selection, organisms with the mean value of the trait are selected. In disruptive selection, both extremes are selected.

Analogous structures result from convergent evolution: organs with different structure and origin but similar functions, e.g. the eyes of octopus and mammals.

Random change in allele frequencies by chance, especially in small populations, is called genetic drift.

Homo habilis: 650–800 cc; Homo erectus: about 900 cc; Homo neanderthalensis: about 1400 cc; Homo sapiens: about 1450 cc.
So Homo habilis had the smallest brain capacity.

Neanderthal man, with a brain size of about 1400 cc, lived in near east and central Asia between 1,00,000 and 40,000 years ago.
They had the features described, used hides to protect their bodies and buried their dead.

The most significant trend in human evolution is the increase in brain size (cranial capacity).
It rose from about 650–800 cc in Homo habilis to about 900 cc in Homo erectus, 1400 cc in Neanderthal man and about 1450 cc in Homo sapiens.

These forelimbs have the same basic bone structure but perform different functions, so they are homologous organs (divergent evolution).

The eyes of octopus and cat differ in structure and origin but do the same job, so they are analogous organs.
Analogous organs arise by convergent evolution.

Homologous organs have the same basic structure and origin but different functions, such as the forelimbs of the whale, bat, cheetah and man.
They show that these organisms have descended from a common ancestor and have diverged (divergent evolution), so they are strong evidence for organic evolution.
Vestigial organs are reduced, functionless organs that were fully developed and functional in the ancestors, such as the vermiform appendix and the nictitating membrane in man.
Their presence can be explained only if the organism has evolved from ancestors in which these organs were useful, so they too are evidence for evolution.
Analogous organs have different origins and only a similar function; they result from convergent evolution and do not indicate common ancestry.
So the important evidence is the occurrence of homologous and vestigial organs.

Before industrialisation (1850), the tree trunks in England were covered with white lichens; white-winged moths were camouflaged on them and escaped predators, while dark-winged moths were picked out by birds.
After industrialisation (1920), smoke and soot killed the lichens and darkened the tree trunks. Now the white moths were conspicuous and were eaten, while the dark (melanic) moths were camouflaged and survived.
So the proportion of dark moths increased: the predators selected against the conspicuous form. Both forms already existed in the population; the colour was not acquired from the environment.
This is industrial melanism, an example of natural selection.
Hence it is natural selection whereby the darker forms were selected.

According to Darwin, variations exist in every population; the individuals whose variations make them better fitted to the environment survive and reproduce more, so their traits increase in the population over generations.
When a pesticide is used, most insects are killed, but a few that happen to carry genes for resistance survive.
These resistant insects reproduce, and their offspring inherit the resistance. With continued use of the pesticide, the resistant forms become more and more common.
So the prevalence of pesticide resistant insects is a clear example of natural selection; resistance of bacteria to antibiotics is a similar example.
Cloning (Dolly), organ development from stem cells and transgenic animals are artificial techniques of biotechnology, not natural selection.
So the phenomenon that supports natural selection is the prevalence of pesticide resistant insects.

Hugo de Vries, on the basis of his work on the evening primrose ($Oenothera\ lamarckiana$), brought forth the idea of mutations.
He described mutations as large differences arising suddenly in a population and believed that it is mutation, and not the minor variations emphasised by Darwin, that causes evolution; he called this single-step large mutation saltation.
Darwin proposed natural selection acting on small, directional variations.
Mendel proposed the laws of inheritance from his work on the garden pea, and Hardy and Weinberg gave the principle of constant allele frequencies in a population.
Hence the idea of mutations was brought forth by Hugo de Vries, who worked on evening primrose.

Homologous organs are organs that have the same basic structure and origin but perform different functions.
The thorn of Bougainvillea and the tendril of Cucurbita both arise in the axillary position and both are modifications of the stem (axillary bud).
The thorn is used for protection, while the tendril is used for climbing.
Since their origin is the same but their functions are different, they are homologous organs.
Homology indicates common ancestry and is the result of divergent evolution.
Analogous organs, in contrast, have different origins but perform similar functions, for example the wings of a butterfly and of a bird.

August Weismann proposed the theory of continuity of germplasm. According to it, the body has two kinds of protoplasm, the germplasm (in the germ cells) and the somatoplasm (in the body cells).
Only the changes in the germplasm are transmitted to the next generation, and the germplasm passes continuously from one generation to the next. This disproved the inheritance of acquired characters. So this pair is correctly matched.
Hugo de Vries proposed the mutation theory; the theory of natural selection was given by Charles Darwin.
The theory of pangenesis was given by Darwin; Mendel gave the laws of inheritance.
The inheritance of acquired characters was proposed by Lamarck; Louis Pasteur disproved spontaneous generation and gave the germ theory of disease.
So the correct match is Weismann - Theory of continuity of germplasm.

Ontogeny is the developmental history of an individual, and phylogeny is the evolutionary history of its race.
Ernst Haeckel proposed the biogenetic law or recapitulation theory, which states that ontogeny repeats (recapitulates) phylogeny, i.e. an organism passes through its ancestral stages during its embryonic development. This is the correct statement among the four.
The embryos of all vertebrates, including mammals, develop a row of vestigial gill slits just behind the head, so there is evidence of gills during the embryogenesis of mammals.
All plant cells with a nucleus are potentially totipotent, but most animal cells are not.
Stem cells are undifferentiated, unspecialised cells that can divide and give rise to specialised cells.
So the correct statement is that ontogeny repeats phylogeny.

During his voyage on H.M.S. Beagle, Darwin visited the Galapagos Islands and observed many varieties of small black birds, later called Darwin's finches.
All the varieties had evolved on the islands themselves from a common seed-eating ancestor that came from the South American mainland.
On the different islands, which were geographically isolated from each other and offered different kinds of food, the finches developed different beaks and feeding habits (seed-eating, insect-eating, cactus-eating and so on).
This is evolution linked with geographical distribution and isolation, i.e. an example of biogeographical evidence of evolution, and of adaptive radiation.
It was not explained by mutation, it is progressive and not retrogressive, and it goes against the theory of special creation.
So the finches of the Galapagos islands provide evidence in favour of biogeographical evolution.

When unrelated organisms, i.e. species of different ancestry (genealogy), live in similar habitats, they face the same environmental demands.
They develop similar adaptive features and so come to resemble each other. This is called convergent evolution.
It gives rise to analogous structures, which have different origins but perform similar functions, such as the wings of a butterfly and of a bird, the eyes of the octopus and of mammals, and the flippers of penguins and dolphins.
Divergent evolution is the opposite: related organisms become different as they adapt to different needs, giving homologous structures.
Co-evolution is the evolution of two species in response to each other, and microevolution is a small change in gene frequencies within a population.
So the phenomenon is convergent evolution.

Industrial melanism was observed in the peppered moth (Biston betularia) in England.
Before industrialisation, tree trunks were covered with pale lichens, and the light (white) moths were camouflaged, while dark (melanic) moths were easily picked off by birds.
After industrialisation, soot killed the lichens and darkened the tree trunks. The dark moths now matched the background, escaped predation, and became common, while light moths became rare.
So the moths that resembled their surroundings were protected from predators and selected. It is not a darkening of the skin by smoke.
So industrial melanism is an example of protective resemblance with the surroundings (natural selection).

Geographical isolation is the separation of a population into two or more groups by a physical barrier such as a mountain, a river or the sea.
The barrier stops the flow of genes between the separated populations.
Each population then accumulates different mutations and is subjected to different selection pressures and genetic drift, so the populations diverge genetically.
In course of time they become so different that they can no longer interbreed even if they come together again; that is, they become reproductively isolated.
Reproductive isolation marks the formation of new species. This is called allopatric speciation.
So geographical isolation promotes speciation; it does not prevent it, and the isolated fauna does change in a non-random way through selection.
So the important consequence is speciation through reproductive isolation.

The Mesozoic era (about 250 to 65 million years ago), with the Triassic, Jurassic and Cretaceous periods, is called the Age of Reptiles.
During this era, dinosaurs and other reptiles were the dominant animals on land, in the sea and in the air.
The Cenozoic era is the Age of Mammals; the Paleozoic saw the rise of fishes and amphibians.
So reptiles were dominant in the Mesozoic era.

Phylogeny is the evolutionary history of an organism or a group of organisms, showing its origin and its relationships with other groups.
It is usually represented as a branching diagram called a phylogenetic tree.
Ontogeny is the developmental history (life history) of an individual, from the zygote to the adult.
Haeckel's biogenetic law relates the two: ontogeny recapitulates phylogeny.
Palaeontology is the study of fossils.
Ancestry is only the line of descent of an individual.
So the evolutionary history of an organism is known as phylogeny.

The theory of spontaneous generation stated that life could arise from decaying and rotting matter.
In experiments of the type performed by Spallanzani and later by Louis Pasteur, meat broth was heated (to kill the microbes already present) and kept sealed.
No microbes appeared and the meat did not spoil, while broth exposed to air soon became full of microbes.
This showed that microbes arise only from pre-existing microbes that enter from outside, and not spontaneously from the non-living matter.
The appearance of larvae or microbes in decaying or unsterilised matter would be consistent with spontaneous generation, so those observations do not disprove it.
So the experiment that disproves spontaneous generation is: meat was not spoiled when heated and kept sealed in a vessel.

Hugo de Vries proposed the mutation theory of evolution in 1901.
He worked on the evening primrose, Oenothera lamarckiana.
He observed that some plants suddenly showed new, large and discontinuous variations that bred true; he called these sudden changes mutations.
He suggested that evolution takes place by such large mutations (saltation) and not by the small continuous variations emphasised by Darwin.
Drosophila was used by Morgan for genetic studies, and Pisum sativum by Mendel.
So de Vries worked on Oenothera lamarckiana.

The great variety of plant life (algae, bryophytes, pteridophytes, gymnosperms and angiosperms) did not appear at one time.
It developed gradually over hundreds of millions of years, from simple aquatic forms to complex land plants, through the accumulation of variations and natural selection.
Single abrupt mutations or seed dispersal alone cannot produce such diversity, and plants did not appear suddenly on earth.
So diversification in plant life appeared due to long periods of evolutionary changes.

The tadpole larva of a frog lives in water, breathes through gills and has a tail and a two-chambered fish-like heart, like a fish.
During metamorphosis it loses the gills and develops lungs.
According to the recapitulation theory (biogenetic law of Haeckel), the development of an individual briefly repeats stages of its evolutionary history. So the gilled tadpole stage indicates that frogs (amphibians) evolved from gilled, fish-like ancestors.
It does not mean that fishes evolved from frogs or that frogs will have gills in future.
So the presence of gills in the tadpole indicates that frogs evolved from gilled ancestors.

Convergent evolution is the development of similar features (analogous structures) in unrelated organisms because they live in similar environments and face similar needs.
Dogfish (a cartilaginous fish) and whale (a mammal) are not closely related, but both live in water and have evolved a similar streamlined body and paddle-like fins/flippers for swimming.
Rat and dog are both mammals with common ancestry (their similarities are homologous). Bacterium and protozoan, and starfish and cuttlefish, do not show such similar adaptations.
So convergent evolution is illustrated by dogfish and whale.

Genetic drift is the random change in allele frequencies due to chance (sampling error) from one generation to the next.
In a large population, chance effects average out. In a small, isolated population, only a few individuals breed, so chance can greatly change allele frequencies; some alleles may be lost or fixed.
Examples are the founder effect and the bottleneck effect.
So genetic drift operates in a small isolated population.

Homologous organs have the same basic structure and embryonic origin (anatomical similarity), but they may perform different functions in different animals.
Example: the forelimbs of whale (flipper), bat (wing), cheetah (running) and human (grasping) all have the same set of bones (humerus, radius-ulna, carpals, metacarpals, phalanges). In plants, thorns of Bougainvillea and tendrils of Cucurbita are homologous.
They show divergent evolution from a common ancestor.
Organs that differ in structure but do the same function are analogous; organs without function now are vestigial.
So homologous structures are organs with anatomical similarities but performing different functions.

Homologous organs have the same basic structure and embryonic origin but may perform different functions.
The wings of a bird and the pectoral fins of a fish are both vertebrate forelimbs (paired anterior appendages) with the same basic plan of bones and the same origin. So they are homologous.
The wings of birds and insects (locust, butterfly), wings of bat and butterfly, and legs of frog and cockroach perform similar functions but have completely different structure and origin, so they are analogous organs.
So the homologous organs are the wings of birds (sparrow) and the pectoral fins of fish.

In a large bacterial population, a few cells already carry random (spontaneous) mutations that make them resistant to streptomycin. These mutations are pre-existing; the antibiotic does not cause them (Lederberg's replica plating experiment).
When the bacteria are grown on streptomycin medium, the sensitive cells die and only the resistant ones survive and multiply.
This is natural selection, with the antibiotic acting as the selecting agent.
So the bacteria grow in streptomycin medium due to natural selection.

A new species forms when a population can no longer interbreed with its parent population.
Reproductive isolation (by any barrier, prezygotic or postzygotic) prevents gene flow between the populations, so they evolve independently and become separate species.
Seasonal and behavioural isolation are particular types of reproductive isolation.
So reproductive isolation is important for speciation.

Water is essential for all known life. It is the medium in which biochemical reactions take place, it forms most of the body of organisms, and life itself is believed to have originated in water.
The moon has no liquid water and no atmosphere (so no protection from extreme temperatures and radiation).
It does receive light and has temperatures (though extreme), and some organisms (anaerobes) can live without oxygen.
So, as per the answer key, there is no life on the moon mainly due to the absence of water.

Carbon-14 is a radioactive isotope of carbon that decays by $\beta$-emission to nitrogen-14.
Its half life is about 5730 years (often rounded to about 5000 years).
It is used in radiocarbon dating of fossils and archaeological remains up to about 50,000 years old.
So the half life of $C^{14}$ is about 5000 years.

The great diversity of living organisms arose gradually over a very long time (about 3.5 billion years) through the accumulation of variations and natural selection.
Mutation is only one source of variation; single short-term changes cannot produce such large diversity.
So the reason for diversity in living beings is long term evolutionary change.

Endemic species are found only in a particular region.
South America and Australia separated from the other continents long ago (break-up of Gondwanaland). Their animals and plants were isolated and evolved independently.
For example, Australian marsupials radiated into many forms by adaptive radiation, without competition from placental mammals.
So the endemic species of South America and Australia are due to continental separation.

Vestigial organs are reduced, functionless organs that were useful in ancestors.
In humans, the wisdom teeth (third molars), coccyx (tail bone), segmental abdominal muscles, nictitating membrane, vermiform appendix and ear muscles are vestigial.
Nails are functional (they protect the tips of fingers and toes and help in picking things up), so they are not vestigial.
So nails are not a vestigial organ.

The Pleistocene epoch (about 2.6 million to 11,700 years ago) of the Quaternary period is the 'age of man'.
During this epoch, the genus Homo evolved, and modern humans (Homo sapiens) appeared in Africa about 200,000–300,000 years ago.
The Paleocene, Oligocene and Miocene are earlier epochs of the Tertiary period, when early mammals and apes evolved.
So Homo sapiens evolved in the Pleistocene.