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.

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

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

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

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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.

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

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

• 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.

• Fossils are the preserved remains or impressions of organisms buried under the Earth due to natural disasters.
• The study of fossils helps understand the organisms that lived millions of years ago and their evolutionary history.
• The carbon dating method measures the radioactivity of C-14 to estimate the age of fossils and ancient remains.
• Willard Libby developed the carbon dating method and received the Nobel Prize in 1960 for this invention.
• Fossil records suggest that vertebrates have gradually evolved from invertebrates over geological eras.
• Fossils occur in various forms such as petrification, compression, impressions, moulds and casts, coal balls, and unaltered fossils.

• 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, rise and extinction of dinosaurs, and 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.

• 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.

• Natural selection, proposed by Charles Darwin, explains evolution as a process where organisms with favourable variations survive and reproduce.
• Organisms produce more offspring than can survive, leading to a struggle for existence.
• Individuals with useful variations are better adapted to the environment and are selected by nature (“survival of the fittest”).
• These favourable traits are inherited by successive generations, resulting in gradual evolution and formation of new species.
• Darwin presented this theory in his book On the Origin of Species (1859), which became a landmark in evolutionary biology.

• 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.