Assertion (A): Natural selection is the mechanism of evolution proposed by Charles Darwin.
Reason (R): Natural selection acts on the heritable variation present in population, and individuals with favourable variations survive and reproduce more successfully.
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Charles Darwin (1809-1882): published "On the Origin of Species by Means of Natural Selection" (1859). Alfred Russel Wallace co-developed the theory independently (1858). Darwin's observations during Beagle voyage (1831-1836): especially Galapagos finches (14 species from one ancestor, different beak shapes for different food sources). Darwin's arguments for natural selection: (1) Overproduction: all species produce more offspring than can survive. (2) Struggle for existence: competition for limited resources. (3) Variation: individuals within a population vary in heritable traits. (4) Survival of the fittest: individuals with favourable variations survive more often (differential survival). (5) Inheritance of favourable traits: survivors pass on their advantages. (6) Gradual accumulation of differences → new species. What Darwin did NOT know: mechanism of inheritance (genes), source of new variation (mutations), molecular basis. Neo-Darwinism/Modern Synthesis (1930s-1950s) integrated Darwin's theory with Mendelian genetics and population genetics.

Fossil evidence: preserved remains/impressions of past organisms. Geological strata → relative dating. Carbon-14 (up to 50,000 yr), uranium-lead (millions of years) → absolute dating. Transitional fossils: Archaeopteryx (reptile-bird), Tiktaalik (fish-tetrapod), Pakicetus (land mammal-whale). Comparative anatomy: Homologous organs: same structure, different function. Forelimb of human, whale, bat, horse (all have same bones: humerus, radius, ulna, carpals, phalanges) → common ancestry. Analogous organs: different structure, same function. Wings of bird and butterfly → convergent evolution. Vestigial organs: reduced, non-functional structures from ancestors. Human: appendix, ear muscles, coccyx, wisdom teeth. Whale: pelvic bones. Embryological evidence: embryos of vertebrates are very similar in early stages → shared ancestry. Gill slits and tails in human embryos (Haeckel's recapitulation theory — "ontogeny recapitulates phylogeny" is an oversimplification but embryological similarities are real evidence).

Directional selection: shifts the population mean in one direction. Example: peppered moth (Biston betularia) — industrial melanism. Before industrialisation: light-coloured moths survived on lichen-covered trees. After: soot covered trees → dark moths survived → dark allele increased. Reverse after pollution reduction. Stabilising selection: favours intermediate phenotypes, reduces variation. Example: human birth weight (too small or too large → higher mortality; intermediate weight most survive). Most common type in stable environments. Disruptive (diversifying) selection: favours both extremes, eliminates intermediate. Rare. Example: Coho salmon (two male morphs: large aggressive vs small "sneaker" males). Can lead to sympatric speciation. Sexual selection: mates choose based on certain traits → evolution of secondary sex characters. Example: peacock tail (reduces survival but increases mating success), stag antlers, bird of paradise plumage. Can oppose natural selection.

Modern Synthesis integrated: Darwin's natural selection + Mendelian genetics + population genetics (Hardy-Weinberg equilibrium) + mutation theory (de Vries) + geographical isolation (speciation). Main components: (1) Mutation: ultimate source of new alleles. (2) Recombination: reshuffles existing alleles → new genotype combinations. (3) Natural selection: changes allele frequencies. (4) Genetic drift: random changes in allele frequencies (important in small populations). (5) Gene flow: movement of alleles between populations (reduces differentiation). (6) Isolation: geographic, reproductive → prevents gene flow → speciation. Hardy-Weinberg law: in a large random-mating population without selection, mutation, migration, or drift: allele frequencies remain constant (equilibrium). Deviations from HW = evolution is occurring.

Speciation: formation of new species from existing species. Allopatric speciation (geographic isolation): populations physically separated (mountain, ocean, desert) → different selective pressures → accumulate differences → reproductive isolation develops. Example: Darwin's finches, Hawaiian honeycreepers. Sympatric speciation: new species form without geographic isolation. Rarer in animals, more common in plants. Mechanisms: polyploidy (especially in plants, e.g., wheat = hexaploid), habitat differentiation, assortative mating. Species concept: Biological species concept (Mayr): group of organisms that can interbreed and produce fertile offspring. Limitations: doesn't apply to asexual organisms, allopatric populations, fossils. Other concepts: morphological, phylogenetic, ecological. Hybrid zones: areas where two partially reproductively isolated species meet and occasionally hybridise. Can be stabile or expand/collapse.

Mutation: changes in DNA sequence. Point mutations (substitution, insertion, deletion). Chromosomal mutations (inversion, translocation, deletion, duplication). Most mutations are neutral or slightly deleterious. Positive mutations rare but crucial. Mutation rate: ~10⁻⁸ to 10⁻⁶ per nucleotide per replication. Recombination: crossing over during meiosis → new allele combinations. Sexual reproduction greatly amplifies variation available for selection. Genetic drift: random changes in allele frequency. Important in small populations. Founder effect: small group establishes new population → limited genetic diversity. Example: Old Order Amish (Ellis-van Creveld syndrome). Bottleneck effect: population reduced drastically → loss of alleles. Example: cheetahs (extremely low genetic diversity). Gene flow/Migration: movement of individuals/gametes between populations → homogenises allele frequencies → reduces population differentiation.

H-W Law: in ideal population (large, random mating, no mutation, no selection, no migration): allele frequencies remain constant. If allele A has frequency $p$ and allele a has frequency $q$ ($p + q = 1$): genotype frequencies = $p^2$ (AA) + $2pq$ (Aa) + $q^2$ (aa) = 1. Uses: (1) Baseline to measure evolution (deviations from HW = evolution). (2) Calculate allele frequencies from phenotype data. Example: if 1% of population shows recessive phenotype ($q^2 = 0.01$): $q = 0.1$, $p = 0.9$, carrier frequency $= 2pq = 2(0.9)(0.1) = 18\%$. Agents of evolutionary change (disturb H-W): mutation, natural selection, genetic drift, gene flow (migration), non-random mating. NEET application: calculate carrier frequency for autosomal recessive conditions.

Primates: order including prosimians, monkeys, apes, humans. Evolved from insectivore-like ancestors ~80 million years ago (MYA). Key evolutionary events: 55 MYA: first primates. 35 MYA: Old World monkeys separate. 25 MYA: gibbons separate. 15 MYA: orangutans. 10 MYA: gorillas. 7 MYA: chimpanzee and human lineages diverge. ~7% DNA difference between human and chimp (98%+ similar). Hominid evolution: Australopithecus (~4 MYA, Africa, bipedal, small brain ~400 cc). Homo habilis (2.5-1.5 MYA, first tool use, brain ~650 cc). Homo erectus (1.8-0.3 MYA, spread out of Africa, fire, brain ~900-1100 cc). Homo heidelbergensis (0.7-0.2 MYA). Homo neanderthalensis (400-40,000 years ago, Europe/Middle East, brain ~1450 cc, ritual burial). Homo sapiens (anatomically modern: 300,000 years ago; behaviourally modern: 50,000 years ago). Out of Africa hypothesis: modern humans evolved in Africa then spread worldwide and replaced archaic humans.