Revision: Genetics and Evolution >> Evidences and Theories of Biological Evolution Biology (Theory) ISC (Science) ISC Class 12 CISCE

The preserved remains, impressions, or traces of ancient organisms found in Earth's crust, which provide evidence of past life and evolution, is called a fossil.

Fossils are preserved remains or impressions of past organisms found mainly in sedimentary rocks, and their study is called palaeontology.

Homology is the phenomenon in which organs of different organisms show similarity in structure and origin due to common ancestry.

Organs that perform different functions but have the same embryonic origin, basic structural plan, and evolutionary ancestry are called homologous organs.

Organs that perform the same function and show superficial similarity but differ in origin and basic structure are called analogous organs.

Analogy is the phenomenon in which organs of different origin perform similar functions due to similar environmental pressures.

Vestigial organs are reduced and non-functional organs present in an organism that were functional in its ancestors.

Vestigial organs are those organs that have ceased to be of any use to the possessor but still persist generation after generation in a reduced form. In other words, vestigial organs are the remnants of features that served important functions in the organism's ancestors.

Atavism, also known as reversion, is the sudden reappearance of a certain ancestral but not parental structure which has either completely disappeared or greatly reduced.

Embryology, the study of the development of an organism from egg to adult, also provides evidences for the organic 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. 

The origin of new species by gradual modification is called speciation.

The fittest individuals in nature are most likely to reproduce and pass on their good qualities to their offspring. It is called natural selection.

The modern version of Darwin’s theory of natural selection, which incorporates genetics as the source of variations, is called Neo-Darwinism.

Differences between individuals of the same species that arise naturally and can be passed to offspring are called variations.

Mutation is a sudden change in one or more genes, or in the number or in the structure of chromosomes.

or

Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism.

The phenomenon by which maternal and paternal genes are reshuffled to produce new combinations of characters in sexually reproducing organisms is called recombination.

Organisms that exhibit a combination of characters derived from both parents are called recombinants.

The movement of alleles from one population to another through migration and interbreeding is called gene flow.

Divergent evolution is the evolutionary process in which homologous structures with a common ancestral origin become different in form and function due to adaptation to different environments.

According to A.E. Emerson, speciation is the evolutionary process by which new species arise due to genetic divergence and isolation.

The process of formation of a new species from the pre-existing species is called speciation.

• Hardy–Weinberg’s principle states that allele frequencies in a population remain constant from generation to generation in the absence of evolutionary forces.
• The total collection of all alleles in a population is called the gene pool.
• Genetic equilibrium means no change in allele frequencies over time.
• If p is the frequency of dominant allele and q is the frequency of recessive allele, then
  p + q = 1.
• Genotype frequencies are expressed as:
  p² (AA) + 2pq (Aa) + q² (aa) = 1.
• Any deviation from Hardy–Weinberg equilibrium indicates that evolution is occurring.
• The principle helps in detecting the role of natural selection and other evolutionary forces.

• Organic evolution states that present-day complex organisms originated from earlier, simpler life forms, supported by multiple branches of biology.
• Evidence comes from Palaeontology (fossil records), Comparative Anatomy & Morphology, Embryology, and Molecular Biology.
• Biochemical evidence (similarities in DNA, proteins) and biogeographical evidence (distribution of species across regions) further support evolution.
• Together, all these evidences strongly support that life on Earth has undergone gradual evolution from simple to complex forms over millions of years.

• Fossils are the preserved remains or impressions of organisms buried under the 
• Palaeontology is the study of ancient life using fossils, which are preserved remains of organisms found mainly in sedimentary rocks.
• Fossils provide direct and strong evidence for evolution and help in understanding the history of life on Earth.
• During fossilisation, older and more primitive organisms are found in lower layers, while more advanced forms occur in upper layers.
• Types of fossils include actual remains (e.g. mammoth in ice), moulds (impressions), casts (filled moulds), and compressions (carbon film outlines).
• Fossilisation occurs through processes like replacement and infiltration, where organic material is replaced or filled with minerals.
• Palaeontology helps in studying extinct organisms, reconstructing evolutionary history (phylogeny), and identifying connecting links between species.

• The geological time-scale represents the entire history of the Earth and helps correlate geological and biological events in proper sequence.
• The Earth’s crust is made of layered rocks, and radioactive dating techniques estimate the age of the Earth to be about 4.5 billion years.
• Earth’s history is divided into five major eras: Archaeozoic, Proterozoic, Palaeozoic, Mesozoic, and Cenozoic.
• Each era is further divided into periods and epochs, marked by changes in climate, plant life, and animal life.
• The time-scale shows the gradual evolution of life, from simple unicellular organisms to complex plants, animals, and humans.
• Major events such as the origin of life, the rise and extinction of dinosaurs, and the evolution of mammals and humans are recorded in the geological time scale.
• Scientists believe Earth is currently entering a sixth mass extinction (Anthropocene), mainly due to human activities causing rapid loss of biodiversity.

• Fossil records provide a geological history of organisms, showing that evolution occurs through gradual and successive stages.
• The evolution of the horse demonstrates progressive changes such as increase in body size, reduction of digits, and adaptation of teeth to grazing.
• Environmental changes, especially the shift from forests to grasslands, influenced structural modifications in evolving organisms.
• Fossils like Archaeopteryx act as missing links, proving evolutionary connection between major groups such as reptiles and birds.

• Vestigial organs are degenerate or underdeveloped structures that have lost their original function in certain organisms.
• These organs persist in a degenerate form due to inheritance from ancestral species, despite having little or no current use.
• Examples in animals include splint bones in horse, rudimentary wings in ostrich, vestigial limbs in python, and functionless eyes in burrowing animals.
• Humans possess many vestigial organs such as the appendix, coccyx (tail bone), wisdom teeth, ear muscles, and nipples in males.
• Vestigial organs provide strong evidence for evolution, showing gradual changes in structure and function over time.

• Connecting links are organisms that show features of two different groups, indicating evolutionary relationships.
• Examples include lungfish (link between fishes and amphibians), duck-billed platypus (link between reptiles and mammals), and Peripatus (link between annelids and arthropods).
• These organisms show transitional features, combining traits of both ancestral and descendant groups.
• Connecting links provide strong evidence for evolution, proving continuity and gradual change between major groups of organisms.
   

• Cell biology evidence shows that basic cell structures and organelles are similar in most organisms, indicating a common ancestral origin.
• Biochemical molecules such as DNA, RNA, proteins, and ATP are universal, supporting the idea of unity of life.
• Molecular homology, seen in similarities of DNA and protein sequences (e.g., cytochrome-c), reflects the degree of evolutionary relatedness among organisms.
• Differences in biochemical compounds like phosphagens and blood pigments help distinguish major evolutionary groups while supporting their common descent.
• Metabolic processes such as protein synthesis, respiration, and ATP usage are fundamentally similar in all living organisms.
• Similarities in traits like nitrogenous waste excretion and blood groups further support evolutionary relationships, especially between humans and apes.

• Biogeography studies the geographical distribution of plants and animals and provides evidence for evolution through patterns of similarity and difference across regions.
• According to continental drift theory, all continents were once united as Pangaea, and their separation led to the formation of distinct biogeographical realms.
• Isolation of continents by seas and barriers caused independent evolution of flora and fauna in different regions.
• The dominance of marsupials in Australia shows evolution in isolation due to early separation of the continent from others.
• Adaptive radiation occurs when organisms diversify into different forms in isolated environments, as seen in island ecosystems.
• Darwin’s finches of the Galapagos Islands provide strong evidence of evolution, showing diversification from a common ancestor due to adaptation to different ecological niches.

• Adaptive Radiation - Process of evolution of different species starting from one point and radiating to other habitats in a given geographical area.
• Darwin's Finches - Best example of adaptive radiation; small birds observed by Darwin in the Galapagos Islands; originally migrated from the American mainland.
• How it Occurred - Original seed-eating birds adapted to different environmental conditions of different islands; evolved into many forms with altered beaks (e.g. insectivorous features).
• Australian Marsupials - Another example: many marsupial mammals in Australia evolved from a common ancestor through adaptive radiation.
• Significance - Provides strong evidence of evolution; shows how natural selection drives species diversification and the creation of new species from a single ancestor.

• Jean-Baptiste Lamarck proposed that morphological changes in organisms occur due to their activities or laziness.
• He introduced the principle of use or disuse of organs, leading to development or degeneration of body parts.
• He explained evolution through acquired characters, such as the long neck of giraffes or strong shoulders of blacksmiths.
• These acquired characters, according to Lamarck, are passed on to the next generation.
• Lamarck’s theory was later disproved, as acquired characters are not inherited genetically.

• Giraffe’s neck: Continuous stretching to reach tall trees led to elongation of the neck, which Lamarck believed was inherited by successive generations.
• Evolution of horse’s feet: Change from soft forest floors to hard grasslands caused modifications like stronger teeth, longer legs, and reduction of digits due to changed habits.
• Loss of limbs in snakes: Continuous creeping made limbs useless, leading to their gradual degeneration and disappearance.
• Other examples: Reduced eyes in moles, flatfish eyes on one side, webbed feet in aquatic birds, callosities in humans, and vestigial organs were considered results of use and disuse.

1. Darwinism - Proposed by Charles Darwin after extensive travels; published in the book "On the Origin of Species by Means of Natural Selection"; the essence of the theory is natural selection.

2. Galapagos Islands - Darwin observed variations in tortoises and finches across the islands of the Archipelago, which became key evidence for his theory.

3. Natural Selection - Nature selects only those organisms that are fit to survive; the rest perish — this is the concept of "Survival of the Fittest".

4. Competition & Modification - All organisms reproduce prolifically and compete for survival; only those with useful modifications win the competition and sustain.

5. Evidences for Darwinism -

• Long-necked Giraffe - Giraffes developed long necks to reach leaves; this adaptation became fixed and was passed to offspring.
• Black Peppered Moths - Gradually became more common due to industrial melanism.
• DDT Resistance in Mosquitoes - Some mosquitoes developed resistance to DDT, survived, and produced resistant offspring.

• Mutations are sudden, discontinuous, and heritable changes that appear spontaneously in organisms, as observed by De Vries in Oenothera lamarckiana.
• These sudden variations, called saltations, produce new traits that can be passed on to offspring, leading to the formation of new species.
• Mutations are different from Darwin’s small, gradual variations and may occur in any direction.
• Mutations provide the raw material for evolution, while natural selection eliminates unfavourable mutants and preserves useful ones.

• The modern theory of evolution (modern synthesis) integrates Darwin’s natural selection with genetics, palaeontology, and biogeography, emphasizing evolution at the population level.
• Genetic variation arises through gene mutations, chromosomal changes, genetic recombination, migration, and hybridization.
• Natural selection and reproductive isolation direct these variations, leading to adaptation and formation of new species.

• Selection is the oldest plant breeding method and involves choosing plants with desirable characters and eliminating undesirable ones.
• Domestication led to continuous improvement of crops, making cultivated plants very different from their wild ancestors.
• Human selection favored useful traits such as non-shattering ears in wheat and tightly embedded seeds in maize.
• Repeated selection over generations stabilized desirable characters while unwanted traits were gradually lost.
• Limitation of conventional breeding includes being time-consuming and restricted by natural crossing barriers, leading to the development of modern methods like mutation breeding, tissue culture, and genetic manipulation.

• Natural selection is the process where better adapted individuals survive and reproduce more, leading to evolution (“survival of the fittest”).
• It acts on genetic variations in a population and causes changes in gene frequency over generations.
• According to Darwin, natural selection is the main driving force of evolution and leads to the formation of new species over time.
• Example: Industrial melanism in Biston betularia, where black moths increased in polluted areas due to better camouflage, while white moths decreased.
• Types of natural selection include stabilising (favours the average), directional (favours one extreme), and disruptive (favours both extremes).
• Natural selection removes harmful traits and favours useful adaptations, helping organisms survive in changing environments.

• Adaptation is any morphological, physiological, or behavioural feature that helps an organism survive and reproduce in its habitat.
• Desert plant adaptations → Thick cuticle, sunken stomata, and reduced leaves (spines in Opuntia) help to reduce water loss.
• CAM pathway → Special photosynthesis in desert plants where stomata remain closed during the day to minimise transpiration.
• Behavioural adaptation → Desert lizards regulate body temperature by basking in sun or moving to shade/burrowing.
• Cold region adaptations (Allen’s Rule) → Animals have shorter ears and limbs to reduce heat loss; aquatic mammals have a thick fat layer (blubber) for insulation.

• Speciation - Process of formation of a new species from pre-existing species; a species is a group of similar organisms that can interbreed and produce fertile offspring.
• Intraspecific Speciation - Formation of new species within the same population; divided into allopatric and sympatric speciation.
• Allopatric Speciation - New species formed due to geographical isolation (mountains, glaciers, migration); mode of evolution is adaptive radiation; e.g. 14 species of finches in the Galapagos Islands, marsupials in Australia.
• Sympatric Speciation - New species formed within a single population without geographical isolation; occurs due to reproductive isolation and mutations; e.g. Cichlid fishes in Lake Victoria.
• Interspecific Speciation - New species formed by hybridisation of two different species; e.g. Triticum aestivum (hexaploid) and Gossypium herbaceum (tetraploid).