Key Points

• The theory of evolution by natural selection was proposed by Charles Darwin.
• It stated that all life forms arise due to natural selection of small inheritable changes that increase the survivability of a species.
• Darwin also defined evolution as descent with modification, meaning we have evolved from our ancestors through inheritable variations of characteristics.
• The evidences for evolution include:
  • Paleontological Evidence - Study of fossils have revealed that different life forms existed in different geological time-periods and similarities exist between existing and pre-existing life forms.
  • Anthropogenic Evidence - Overuse of pesticides and herbicides have resulted in natural selection of resistant varieties in much lesser time.
  • Comparative Anatomy & Morphology - Homology and analogy are based on comparisons between structure and functions of organs that show evolutionary adaptations in nature. 

Important Points

• Homology means -
  • Similar structures that differ in their functions are known as homologous organs.
  • Example - Forelimbs of whale, bat, cheetah and human.
  • All of them have humerus, radius, ulna, carpels, metacarpels and phalanges.
  • Whales use forelimbs for swimming, bats for flying, cheetah for running and humans for grasping.
• Homology indicates divergent evolution -
  • The same structure evolved in different directions.
  • This shows evolutionary adaptations due to varying needs.
• It also indicates common ancestry - 
  • The functions diverged from a common ancestor.
  • Example - Thorns of Bougainvillea and tendrils of Cucurbita are both modified branches.

Additional Information

• Analogy refers to the similarity of functions of different structures.
• It indicates convergent evolution.
• Example - Sweet potato and potato are root and stem modifications respectively but both function as storage organ.

Explanation:

The concept of ''Branching descent'' was given by Charles Darwin.

• Charles Darwin made a sea voyage around the world on a ship named H.M.S. Beagle.
• When he arrived at an island called the Galapagos island, he saw some finches and observed differences in the shapes of their beaks.
• He concluded that this change is due to the different eating habits of the finches thus, he gave the concept of Adaptive radiation or Branching descent.

Important Points

• Branching descent (Adaptive radiation) and natural selection are the two key concepts of the Darwinian Theory of Evolution.
• Adaptive radiation/ Branching descent:
  • It is the evolutionary process in which different species starting from a common point in a given geographical area radiate to other geographical areas or habitats.
  • The best example of Adaptive radiation is Darwin's finches.
  • Darwin observed many varieties of finches on the Galapagos island.
  • All these varieties evolved from the original seed-eating finches. With the alternation in beaks, some become insectivorous while the others remain vegetarian.
  • This alternation in the beak's shape occurred due to the availability of certain types of food in different habitats. For eg. finches living on trees developed sharp beaks to peck the tree and eat the insects.

Additional InformationLamarck’s theory of evolution or Lamarckism - 

• According to Lamarck, the evolution of life occurred due to the use and disuse of organs.
• He explained it in his famous book ‘’Philosophie Zoologique’’ in 1809.
• This theory is no more accepted today.

Theory of chemical evolution - 

• The theory of chemical evolution was proposed by Oparin & Haldane.
• According to this theory, the first form of life could have come from pre-existing non-living organic molecules (e.g. RNA, protein, etc.) and that formation of life was preceded by chemical evolution, i.e., the formation of diverse organic molecules from inorganic constituents.

Law of Inheritance - 

• Gregor Mendel proposed the law of heredity through his work on pea plants and discovered the fundamental laws of inheritance. 

Explanation:

• Different theories were given to explain the origin of life like the theory of special creation, the theory of spontaneous generation & the theory of chemical evolution.
• The theory of chemical evolution was proposed by Oparin & Haldane.
• According to this theory, the first form of life could have come from pre-existing non-living organic molecules (e.g. RNA, protein, etc.) and that formation of life was preceded by chemical evolution, i.e., the formation of diverse organic molecules from inorganic constituents.
• The conditions on earth at that time were – high temperature, volcanic storms, reducing atmosphere containing CH₄, NH₃, etc.
• Two scientists S.L. Miller and H.C. Urey conducted an experiment in the year 1953 and proved the theory of chemical evolution. 
• Hence, the correct option is ''Evolution''.

Additional Information 

Experimental evidence of chemical evolution:

• In 1953, S.L. Miller, an American scientist created similar reducing environmental conditions on a laboratory scale.
• He created electric discharge in a closed flask containing CH₄, H₂, NH₃ and water vapour at 8000C.
• He observed the formation of amino acids. In similar experiments, others observed, formation of sugars, nitrogen bases, pigment and fats.
• Analysis of meteorite content also revealed similar compounds indicating that similar processes are occurring elsewhere in space. With this limited evidence, the first part of the conjectured story, i.e., chemical evolution was more or less accepted.

                    Diagrammatic representation of Miller’s experiment

Concept-

• In a given population one can find out the frequency of occurrence of alleles of a gene or a locus.
• This frequency is supposed to remain fixed and even remain the same through generations.
• Hardy-Weinberg's principle stated it using algebraic equations.
• This principle says that allele frequencies in a population are stable and are constant from generation to generation.
• The gene pool (total genes and their alleles in a population) remains constant. This is called genetic equilibrium. 
• Disturbance in genetic equilibrium, or Hardy- Weinberg equilibrium, i.e., change of frequency of alleles in a population would then be interpreted as resulting in evolution.
• Five factors are known to affect Hardy-Weinberg equilibrium. These are gene migration or gene flow, genetic drift, mutation, genetic recombination and natural selection.

Explanation-

• When the migration of a section of the population to another place and population occurs, gene frequencies change in the original as well as in the new population.
• New genes/alleles are added to the new population and these are lost from the old population. 
• There would be a gene flow if this gene migration, happens multiple times. If the same change occurs by chance, it is called genetic drift.
• Genetic drift is expected to be of significance only in small populations.

Therefore chance elimination of alleles from the small population is genetic drift.

Additional InformationFounder's effect

• Sometimes the change in allele frequency is so different in the new sample of the population that they become a different species.
• The original drifted population becomes founders and the effect is called the founder effect

Adaptation

•  The possible variations in different parts of the plants and animals according to their environment are called adaptation.
• For example, adaptions to various habitats, for protection, climbing, storage, etc.

Speciation

• Speciation is the process of forming a new species from the existing species. 
• Speciation takes place through continuous mutation of genes.

Key Points

• Geological time refers to that period of Earth's history which can be studied geologically from the rocks.
• Geological time scale is how that timeframe is sub-divided like a calendar for our understanding.
• The fossils that are found in the rock strata helps in establishing the timeline for appearance or disappearance of species.
• It also helps us in studying the timelines of biological evolution of different species.
• It is divided into eons, eras, periods and epochs.

Important Points

• Angiosperms are the flowering plants that evolved relatively recent on the geological time scale.
• Among the angiosperms, the dicots originated first and then the monocots, which have more advanced evolutionary features.
• The fossil records show that the earliest appearance of the angiosperms happen during the Cretaceous period, which is about 140-65 million years ago and comes in the Mesozoic era.
• But the angiosperms developed and diversified during the Coenozoic era.
• The earliest angiosperms were more herbaceous similar to the present-day families like Ranunculaceae.
• Later on these gave way to the woody plants that led to the formation of forests and grasslands.
• The development of flowers in angiosperms required pollination to take place in order to avoid self-pollination and fertilization.
• This led to the co-evolution of insects and other organisms that would help in pollinating the angiosperms.

Concept:

• The theory of natural selection was proposed by Charles Darwin as a mechanism of evolution.
• It states that individuals who fit better in the changing environment leave more progeny, which survive better and thus get selected by nature,
• Here, fitness is based on the characters that can be inherited.
• Natural selection is based on some factual observations:
  • Natural resources are limited.
  • Populations are stable in size, except for seasonal fluctuations.
  • Members of a population vary in characteristics, even when they look superficially similar.
  • Most of the variations are inherited.
• Darwin had stated that inheritable variations enhance resource utilization for few individuals, enabling them to reproduce better.
• This leads to a change in the characteristics of the population, giving rise to new forms.

Key Points

Natural selection can lead to 3 possibilities:

1. Stabilizing - More individuals acquire the mean character value, getting a higher and narrower peak in the distribution curve.
2. Directional - More individuals acquire a value other than the mean character value, causing a shift in the distribution curve.
3. Disruptive - More individuals acquire peripheral character value at both ends of the distribution curve.

Additional Information

• One such example of directional selection was industrial melanism.
• A change was observed in the population of peppermoths of England after industrialization.
• Before industrialization, white-winged moths were more prevalent on the trees that were covered with whitish lichen, which grew in the absence of pollution.
• Post-industrialization, the trees became covered with soot and smoke, exposing the white-winged moths to predators..
• Here, the dark-winged or melanized moths survived better and hence became prevalent.
• In rural areas, however, the white-winged moths prevailed.
• This shows a directional selection in favour of the dark-winged moths in industrial areas.

Concept:

• Charles Darwin made a sea voyage around the world in a ship named H.M.S. Beagle.
• When he arrived at an island called the Galapagos island, he saw some finches and observed differences in the shapes of their beaks.
• He concluded that this change is due to the different eating habits of the finches thus, he gave the concept of Adaptive radiation.

Explanation:

• Adaptive radiation: It is the evolutionary process in which different species starting from a common point in a given geographical area radiate to other geographical areas or habitats.
• The best example of Adaptive radiation is Darwin's finches.
• Darwin observed many varieties of finches on the Galapagos island.
• All these varieties evolved from the original seed-eating finches. With the alternation in beaks, some become insectivorous while the others remain vegetarian.
• This alternation in the beak's shape occurred due to the availability of certain types of food in different habitats. For eg. finches living on trees developed sharp beaks to peck the tree and eat the insects.

Additional Information

• Mutation: It is defined as the change of gene sequence in the organism’s genome by deletion, addition, or translocation such that it alters the genetic message carried by that gene.
• The concept of use and disuse of organs was given by Lamarck. According to him the evolution of life occurred due to the use and disuse of organs.
• Convergent evolution is the process in which organisms that are not closely related evolve independently having similar characters because of having to adapt to similar environments or ecological niches (habitats).

Key Points

• Thomas Robert Malthus was an English economist who proposed the famous Malthusian theory of population growth.
• According to this theory, a population grows exponentially while the resources to sustain that population remains linear, resulting in a calamity that reduces back the population.
• Darwin wrote the book 'On the Origin of Species' in which he explained the theory of evolution by natural selection as 'descent with modification'.
• The Malthusian theory influenced Darwin’s theory of evolution where he stated that there exists a ‘struggle for existence’ in nature and ‘survival of the fittest’ takes place.
• ‘Struggle for existence’ refers to the competition for available resources required for survival.

Additional Information

• A. R. Wallace -
  • Alfred Russel Wallace was also a naturalist, biologist and geographer.
  • He worked with Charles Darwin on the theory of evolution.
  • His contributions are not just in science but also in socialism and spiritualism.
• H. Spencer –
  • Herbert Spencer was a philosopher, anthropologist, biologist who is known for his hypothesis on social Darwinism.
  • He stated that social evolution takes place much like biological evolution where the same principles of natural selection and ‘survival of the fittest’ can be applied.
• H. de Vries – 
  • Hugo de Vries was a botanist and geneticist, known for his mutation theory of evolution.
  • He suggested that speciation (evolution of species) occurs due to sudden large changes in traits rather than small gradual changes, as was previously suggested by Darwin.

Concept:

• Evolution of man took place through millions of years.
• The early primates diversified into man-like and ape-like groups, which eventually gave rise to the present-day man and apes respectively.
• The line of descent in human evolution is supported by fossil evidences found all around the world.

Key Points

• Ramapithecus -
  • They were primates that existed 15 million years ago (mya).
  • They were hairy and walked like present-day apes such as gorillas and chimpanzees.
  • They existed alongside Dryopithecus, which were ape-like primates.
  • Ramapithecus were more man-like than Dryopithecus.
• Homo habilis -
  • They are the first known hominid (human-like) species that existed about 2 mya.
  • They had a brain capacity of 650-800 cc.
  • They did not eat meat.
  • Their bones were discovered among that of Australopithecines.
• Homo erectus -
  • They existed around 1.5 mya.
  • They had a large brain size of 900cc.
  • They used to eat meat.
  • Their fossils were discovered in Java in 1891.
  • As their name suggests, they were the first hominid species to have an upright posture.
• Homo sapiens -
  • They arose in Africa and spread to other continents.
  • Modern man (Homo sapiens sapiens) arose during the ice age between 75,000 - 10,000 years ago.

Therefore, we can see that the line of descent in human evolution is:

Ramapithecus → Homo habilis → Homo erectus → Homo sapiens

Alternate Method

You may also solve this question by eliminating other options:

• Australopithecines → Ramapithecus → Homo sapiens → Homo habilis
  • Homo sapiens is modern man, so it should come last in descent.
  • Therefore, this option is INCORRECT.
• Homo erectus → Homo habilis → Homo sapiens
  • ​Homo erectus always comes after Homo habilis because upright posture is a more evolved feature.
  • Therefore this option is also INCORRECT.
• Australopithecines → Ramapithecus → Homo erectus → Homo habilis → Homo sapiens
  • ​Homo erectus always comes after Homo habilis because upright posture is a more evolved feature.
  • Therefore this option is also INCORRECT.

Therefore, only option 3 can be CORRECT.

Key Points

• Jean-Baptiste Lamarck was a French biologist who was one of the first to propose biological evolution.
• His theory is called Lamarckism and is based on two basic principles:
  • Inheritance of acquired characters
  • Use and disuse of organs during the lifetime of an organism.
• He proposed that changes in environment necessitates new needs in organism.
• This leads to new uses of pre-existing features or disuse of pre-existing features, causing a physical change in the features.
• He also stated that these changes were then acquired by the offspring in the next generation.

Important Points

• The theory was explained with the example of giraffe.
• It was stated the giraffe ancestors stretched their necks to reach out to the leaves on higher branches of a tree.
• This ones with longer neck were more successful in getting food and produced offspring with longer necks.
• This theory formed the early basis of biological evolution and also influenced Darwin's theory.
• It was rejected later in the mid-twentieth century because of lack of experimental proof.