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

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

The preserved remains, impressions, or traces of ancient organisms found in Earth's crust that provide evidence of past life and evolution are called fossils.

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

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

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

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

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.

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

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

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.

Biogeography is the study of the geographical distribution of plants and animals on Earth. It provides strong evidence for evolution by studying how continental shifts affected species survival and adaptation.

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.

Neo-Darwinism is the refined version of Darwin’s original theory, incorporating modern discoveries in genetics. It updates, rather than alters, the basic structure of Darwinism.

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 an 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.

Migration is defined as the movement of individuals from one local population (deme) to another, resulting in the transfer of genetic information (alleles) between populations and directly altering their gene pools.

Genetic drift is the change in gene or allele frequency in a population due to chance events, not because one allele is inherently better adapted than another. It becomes more significant when the population is small or reproductively isolated.

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

The founder effect is an evolutionary phenomenon that occurs when a small portion of a population migrates to a new location, causing such a drastic change in allele frequencies that the original group becomes the "founders" of a new, distinct species.

An adaptation is any advantageous structural or functional characteristic developed over time that enables a living organism to survive, reproduce, and constantly adjust to its specific changing environment.

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 the dominant allele and q is the frequency of the 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 detect the roles of natural selection and other evolutionary forces.

• Organic evolution is the gradual process by which modern complex organisms descended from earlier, simpler life forms.
• The development of life transitioned from chemical evolution on primitive Earth to biological evolution, creating diverse life forms ranging from single-celled to multicellular organisms.
• Despite vast physical diversity, all organisms display a basic unity by sharing essential life processes like energy utilisation, reproduction, and environmental adaptation.
• Palaeontology provides direct proof of this progression, utilising fossils from various geological strata to trace the historical course of life.
• The evolutionary process is further supported by evidence across multiple biological disciplines, including comparative anatomy, embryology, biochemistry, and geographical distribution.

• 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 study extinct organisms, reconstruct evolutionary history (phylogeny), and identify connecting links between species.

• Extinction is the permanent loss of a species, whereas mass extinction is the rapid, global disappearance of multiple species due to extreme environmental stress.
• These events occur over very short geological timeframes, making existing survival adaptations completely useless.
• Earth has experienced at least five major mass extinctions, including the famous Cretaceous event that eliminated the dinosaurs.
• Historical triggers include asteroid impacts and severe climate shifts, while modern extinction threats are primarily driven by human activities like habitat destruction.
• Fossils – whether preserved in rocks, amber, or asphalt – provide the key physical evidence for tracking the rise and fall of past species.

• The geological timescale 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 timescale 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, including an increase in body size, a reduction in digits, and the adaptation of teeth for grazing.
• Environmental changes, especially the shift from forests to grasslands, influenced structural modifications in evolving organisms.
• Fossils like Archaeopteryx act as missing links, proving an evolutionary connection between major groups such as reptiles and birds.

• Homologous organs have the same origin and basic structural plan but perform different functions in different organisms.
• Homology is strong anatomical evidence for evolution and indicates common ancestry among related species.
• Vertebrate forelimbs (human, horse, whale, bat, and bird) are classic examples of homologous organs with varied functions.
• Plant structures like Bougainvillaea thorns and Cucurbita tendrils are stem modifications that illustrate homology in plants.
• Homologous organs support divergent evolution, whereas analogous organs with similar functions but different origins support convergent evolution.

• Analogous organs perform the same function but have different origins, anatomy, and development.
• They arise through convergent evolution in unrelated groups adapting to similar environments.
• They may look superficially similar but differ in internal structure (e.g., wings of insect, bird, bat).
• Analogous organs do not indicate close common ancestry and have limited use in phylogeny.

• 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 (tailbone), 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.

• Atavism (reversion) is the sudden reappearance of an ancestral trait that had disappeared or become greatly reduced in previous generations.
• The trait shown in atavism is ancestral, not a usual visible character of the immediate parents.
• Human examples of atavism include a rudimentary tail in newborns, movable pinna, large canines, very long and dense body hair, and additional mammae.
• Atavism is considered an important evidence of evolution because it shows that present-day organisms still carry ancestral genetic information.

• Recapitulation Theory was proposed by Ernst Haeckel.
• It states that ontogeny recapitulates phylogeny.
• Early developmental stages were interpreted as showing ancestral traits.
• Frog and plant examples were historically used in support of the theory.
• Modern biology accepts embryonic similarity as evidence of common ancestry, but not a literal replay of evolution.

• 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 the 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 is the evolution of different species from a common ancestor, radiating into different habitats and ecological niches in a given geographical area.
• Mechanism: common ancestor → spreads into new habitats → different environmental pressures → natural selection favours useful variations → distinct species form.
• Darwin's finches (Galapagos): an ancestral seed-eating finch evolved into many species with different beak shapes for different food sources.
• Australian marsupials: many marsupial species evolved from a common ancestor and adapted to different ecological roles.
• Convergent evolution: unrelated placental mammals and marsupials (e.g., the placental wolf and the Tasmanian wolf) evolved similar forms under similar environments.

• Organic evolution is the process by which simple organisms gradually gave rise to complex organisms over time.
• Lamarckism: evolution by use and disuse of organs and inheritance of acquired characters (e.g., a giraffe's long neck); not accepted today.
• Darwinism: based on branching descent and natural selection - organisms with useful heritable variations survive, reproduce more, and form new species over generations.
• Microbes vs higher organisms: new forms appear in microbes within days, but in higher organisms, it takes millions of years.
• Hugo de Vries's Mutation Theory: evolution occurs through sudden, random mutations (saltation), not through gradual variation as Darwin proposed.

• Organisms evolve by acquiring traits during their lifetime to adapt to environmental changes and passing these traits to their offspring.
• New organs develop in an organism as a direct result of a continuous "new need" created by their environment.
• Continuous use of an organ strengthens and develops it, while continuous disuse leads to its degeneration.
• Lamarck supported this with examples like giraffes developing long necks from stretching and snakes losing limbs from disuse.
• The theory was rejected after Weismann's rat-tail experiments proved that acquired changes only affect body cells and cannot be inherited.

• Neo-Lamarckism is a modified version of Lamarckism which states that acquired characters are caused by environment and habit rather than the internal needs of the organism.
• It accepts the inheritance of acquired characters and strongly emphasises the environment's role in modifying these traits.
• Unlike the original theory, Neo-Lamarckism relies on direct experimental observations to support its claims.
• Tower’s potato beetle experiment demonstrated that abnormal environmental conditions, such as temperature and moisture, can induce heritable changes in offspring.
• The pneumococci experiment showed that external influences can alter characteristics, as rough bacteria transformed into smooth ones when exposed to dead smooth extracts.

• Charles Darwin developed the idea of natural selection based on extensive data collected during his voyage on the HMS Beagle.
• His views on organic evolution were heavily influenced by the essays of Thomas R. Malthus and Charles Lyell.
• Alfred Russel Wallace independently reached the exact same conclusions while studying the fauna of the East Indies, resulting in a joint presentation in 1858.
• Darwin officially detailed his theory in 1859 by publishing his famous book, Origin of Species by Natural Selection.
• He identified variations, the struggle for existence, and heredity as the three chief factors driving the complex process of evolution.

• Darwin failed to clearly explain the actual sources of biological variations and the underlying genetic mechanisms of natural selection.
• The theory effectively accounts for the "survival of the fittest" but fails to explain the origin or the "arrival of the fittest."
• Natural selection acts merely as a guiding factor on existing traits, rather than as a creative force capable of generating new heritable variations, such as mutations.
• Survival is not always determined by advantageous traits, as natural disasters can cause widespread, non-selective mortality regardless of an organism's genotype.
• Chance factors, such as the random genetic makeup of a few individuals colonising a new habitat, play a much greater role in evolution than Darwin realised.

• Neo-Darwinism integrates modern genetics with Darwin's original theory, establishing the population - rather than the individual - as the fundamental unit of evolution.
• Evolutionary change is defined as the alteration of allele frequencies within a population in response to local environmental conditions.
• Genetic variation among individuals is primarily driven by the accumulation of mutations and genetic recombination during sexual reproduction.
• Organisms inherit genes, not physical traits; the final observable characteristic (phenotype) results from interactions between these genes and the environment.
• Natural selection is recognised as one of several contributing evolutionary forces, rather than the sole driver of adaptation.

• Evolution is driven by mutations, which are sudden, large, and inheritable genetic variations (saltations), rather than minor, gradual changes.
• These mutations emerge spontaneously and randomly in any direction within naturally breeding populations.
• Mutant traits are clearly distinct from parental characteristics and are directly inherited by offspring, facilitating the origin of new species.
• Natural selection operates on these directionless mutations by eliminating unfavourable variants and preserving beneficial ones.
• While mutations provide the essential raw material for evolution, it is highly unlikely they alone account for complex, specialised adaptations.

• The Modern Synthetic Theory integrates Darwin's natural selection with principles from population genetics, palaeontology, and biogeography.
• It establishes the population, rather than the individual, as the fundamental unit that undergoes gradual evolutionary change.
• Genetic variation within a population is primarily generated by gene mutations, chromosomal alterations, genetic recombination, and hybridisation.
• Evolutionary processes are actively directed by the forces of natural selection and reproductive isolation.
• While migration introduces new genetic variations, mutations, genetic recombination, and natural selection remain the most critical drivers of evolution.

• Variation refers to the differences among closely related organisms and serves as the fundamental basis for evolution only when the traits are heritable.
• Environmental conditions induce non-heritable, somatogenic variations that affect only the individual's body and do not contribute to evolutionary processes.
• Evolutionary change strictly depends on heritable variations resulting from alterations in the inherited gene combinations.
• Sexual reproduction constantly generates genetic diversity through recombination mechanisms such as crossing over, random chromosomal distribution, and random fertilisation.
• Mutations act as sudden, inheritable genetic changes - classified as point, chromosomal, or genomic - serving as the ultimate source of novel genetic material in a population.

• A mutation is a sudden heritable change in DNA sequences that leads to changes in the genotype and phenotype of an organism.
• Loss of DNA segment = deletion; gain of DNA segment = insertion/duplication; both cause chromosomal aberrations, commonly seen in cancer cells.
• Frame-shift mutation - caused by loss or gain of a DNA segment; Point mutation - change in a single base pair (e.g., sickle cell anaemia).
• Physical mutagens that cause mutation include UV radiation, X-rays, alpha, beta and gamma rays; Chemical mutagens include mustard gas, phenol and formalin.
• Mutation is an important source of genetic variation in organisms, alongside recombination.

• Chromosome mutations have variable effects, ranging from preventing an organism's development to creating highly beneficial traits.
• Mutations can bring beneficial genes closer together, which protects them from being separated during reproduction.
• Gene mutations introduce new alleles, directly increasing the genetic diversity and the overall gene pool of a population.
• While normal gene mixing causes temporary changes, specific mutations create major, lasting variations that drive evolution.
• Most gene mutations are recessive and remain hidden in a population for generations until inherited from both parents.

• Recombination is the biological process of mixing maternal and paternal characteristics to produce offspring with entirely unique genetic combinations.
• Genetic variation in sexually reproducing species stems from three primary sources: the independent assortment of chromosomes, crossing over during meiosis, and random fertilisation.
• Unlike a mutation, sexual reproduction does not generate new genetic information; instead, it shuffles existing genes from both parents to create a genetically distinct individual.
• While recombination does not alter the overall allele frequencies within a population, it can produce highly advantageous trait combinations that significantly increase an organism's chances of survival and reproductive success.
• Gene conversion is a specific form of recombination involved in DNA repair, utilising similar genomic regions as templates to correct damage and homogenise sequences over time.

• Migration involves the movement of individuals between populations, directly altering the genetic makeup of both the origin and destination groups.
• The continuous transfer of individuals shifts allele frequencies and gene pools, particularly when rare genetic information is introduced.
• Even minimal levels of ongoing migration are sufficient to drive evolutionary change by exchanging alleles between populations.
• Restricted gene flow and severe inbreeding drastically reduce genetic diversity, endangering captive animal populations and modern domesticated crops.
• Gene banks are established to preserve the wild relatives of crops, securing essential genetic variety against future agricultural and environmental threats.

• Genetic drift is the random fluctuation of allele frequencies due to chance events rather than adaptive advantage.
• It exerts a profound impact on small or isolated populations, frequently causing the random loss of low-frequency alleles and reducing overall genetic variation.
• This reduction in genetic diversity directly leads to a decline in heterozygosity and a subsequent increase in homozygosity.
• Isolated populations subjected to genetic drift may diverge significantly from their parental populations, actively contributing to the formation of new species.
• The random elimination of essential alleles severely restricts genetic diversity, thereby increasing the risk of extinction for small populations.

• Gene flow is the transfer of alleles from one population to another, driven by interbreeding between their members.
• This continuous exchange alters the allele frequencies of both the donor and recipient populations, directly increasing genetic variation.
• Despite introducing variation, gene flow exerts a conservative evolutionary effect by distributing alleles and reducing genetic differences between populations.
• The complete interruption of gene flow between distinct populations is a fundamental prerequisite for the formation of new species.
• The frequency of allele exchange depends heavily on the geographical proximity and the physical mobility of organisms or their gametes.

• Selection favours the survival and reproduction of organisms that are physically, physiologically, and behaviorally best adapted to their environment.
• Competition for limited environmental resources generates selection pressure, driving differential mortality and reproductive success within a population.
• The process actively dictates which alleles are transmitted to subsequent generations based on the phenotypic advantages they confer.
• Evolutionary change occurs as selection pressure alters the frequency of specific alleles within the population's overall gene pool.
• Dominant mutant alleles undergo immediate selection, whereas recessive alleles remain unselected until they are expressed in a homozygous condition.

• According to Darwin’s natural selection theory, only the most viable and adaptable species successfully survive the struggle for existence.
• The continuous adjustment of a species to its new surroundings serves as the primary factor driving the formation and conservation of species.
• Organisms that are unable to adjust to shifting environmental conditions will gradually diminish in the course of evolution and eventually face extermination.
• These constant structural and functional adjustments directly match living forms to their changing environment, ensuring their survival within specific ecological limits.

• Mutation, selection, and genetic drift function as the three elementary forces that collectively drive evolutionary change.
• These forces operate simultaneously within a population to disrupt genetic equilibrium, thereby inducing micro-evolutionary changes.
• Selection enhances environmental interaction by refining the gene pool and favouring specific gene combinations that arise through mutation or recombination.
• Genetic drift contributes to adaptation by fixing neutral or non-adaptive gene recombinations within a population.
• The interaction of these primary forces, often supported by additional factors, produces population changes across generations and facilitates the formation of new populations.

• Speciation is the evolutionary process responsible for the formation of new, distinct species.
• A species is defined as an interbreeding population that shares a specific gene pool and is reproductively isolated from other distinct populations.
• While members within a species exhibit continuous variation, there is a complete discontinuity of variation between different species.
• A species can be divided into geographical races with unique gene frequencies due to environmental differences, yet these races can still interbreed successfully.
• Reproductive isolation, which is the inability of individuals from different populations to interbreed, constitutes the most definitive boundary between different species.