Evolution

🎯 Key Points

• Miller-Urey (1953): proved amino acids CAN form from inorganic gases (NH₃,CH₄,H₂,H₂O) + electric spark — supports chemical evolution, but doesn't prove it's exactly how life began
• Lamarck = inheritance of ACQUIRED characteristics (disproven); Darwin = natural selection acting on EXISTING heritable variation — don't conflate the two theories
• Homologous organs: SAME structure, DIFFERENT function (divergent evolution, e.g. forelimbs of bat/whale/human); Analogous organs: DIFFERENT structure, SAME function (convergent evolution, e.g. wings of bird/insect) — classic confusion pair
• Allopatric speciation (geographic isolation) is the MOST common route to new species; Sympatric speciation (same area, e.g. via polyploidy) is rarer, mainly seen in plants
• Hominid sequence: Dryopithecus → Australopithecus → Homo habilis → Homo erectus → Homo sapiens; modern humans originated in Africa (~200,000 years ago, "Out of Africa")

Origin of Life

• Early Earth: reducing atmosphere (NH3, CH4, H2O, H2); no free O2
• Oparin-Haldane hypothesis: chemical evolution; organic molecules formed in primordial soup
• Miller-Urey experiment (1953): synthesized amino acids from inorganic compounds using electrical discharge
• RNA world hypothesis: RNA was first self-replicating molecule
• First cells: ~3.5 billion years ago (prokaryotes); eukaryotes ~2 billion years ago

Theories of Evolution

• Lamarck: inheritance of acquired characteristics (wrong but historically important)
• Darwin: natural selection + variation = evolution; The Origin of Species (1859)
• Modern Synthesis: Darwinism + genetics (Mendelian inheritance)

Natural Selection

• Variation exists in populations; some variations are heritable
• More offspring produced than can survive (overproduction)
• Individuals with advantageous traits survive and reproduce more (differential reproduction)
• Types: directional, stabilizing, disruptive selection

Speciation

• Allopatric: geographic isolation leads to new species (most common)
• Sympatric: new species in same area (polyploidy in plants)
• Reproductive isolation: prevents gene flow between populations

Evidence for Evolution

• Fossil record: shows sequence of life forms; transitional fossils
• Homologous organs: same structure, different function (forelimbs of whale, bat, human): divergent evolution
• Analogous organs: different structure, same function (wings of birds and insects): convergent evolution
• Vestigial organs: reduced/functionless remnants (human appendix, coccyx, ear muscles)
• Molecular evidence: cytochrome c protein sequence similarity; DNA hybridization
• Biogeography: distribution of species supports evolution

Human Evolution

• Hominid evolution: Dryopithecus → Australopithecus → Homo habilis → Homo erectus → Homo sapiens
• Out of Africa hypothesis: modern humans originated in Africa ~200,000 years ago

Hardy-Weinberg Principle

• The Hardy-Weinberg principle states that in a large, randomly mating population, the allele (gene) frequencies and genotype frequencies remain constant from generation to generation; such a population is said to be in genetic equilibrium, and the sum of all allele frequencies is 1.
• For two alleles with frequencies p and q, the genotype frequencies are given by the binomial expansion (p + q)², that is p² + 2pq + q² = 1, where p² is the frequency of the homozygous dominant genotype, q² is the frequency of the homozygous recessive genotype, and 2pq is the frequency of the heterozygotes.
• Genetic equilibrium is maintained only when there is no mutation, no gene migration (gene flow), no genetic drift, random (panmictic) mating, and no natural selection. When the measured allele frequencies differ from the expected equilibrium values, the population is said to be evolving; this disturbance of Hardy-Weinberg equilibrium is used as a measure of the extent of evolutionary change.
• Five factors are known to disturb Hardy-Weinberg equilibrium and hence bring about evolution: gene migration (gene flow), genetic drift (random change in allele frequencies, pronounced in small populations), mutation, genetic recombination, and natural selection.
• Founder effect: sometimes the change in allele frequency in a new colony, founded by a small number of individuals, is so different from that of the original population that the new population becomes a separate species. The original drifted population is referred to as the founder, and this special case of genetic drift is called the founder effect.

Types of Natural Selection

• When a heritable trait shows a range of variation, natural selection can act on it in three ways, best described by the shape of the resulting distribution curve:
• Stabilising selection: favours intermediate (average) phenotypes and eliminates individuals at both extremes; more individuals acquire the mean character value, variation is reduced and the population norm is maintained (the mean does not shift).
• Directional selection: favours one of the extreme phenotypes, so more individuals acquire a value other than the mean and the mean shifts in one direction; the classic example is industrial melanism in the peppered moth (Biston betularia), where dark-coloured moths were selected after industrial soot darkened the tree bark in England.
• Disruptive selection: favours both extreme phenotypes and selects against the intermediates, so more individuals acquire peripheral character values at both ends of the distribution curve, producing a two-peaked (bimodal) distribution that can eventually lead to two distinct forms.

Adaptive Radiation

• Adaptive radiation is the process of evolution of different species, starting from a common ancestral point (a single ancestral stock) and radiating into different habitats within a geographical area.
• Darwin's finches: on the Galapagos Islands, Charles Darwin observed a variety of small black birds. Their original seed-eating ancestral form gave rise to many varieties with altered beaks suited to new feeding habits (insect-eating, vegetarian, etc.) — a classic example of adaptive radiation.
• Australian marsupials: a number of marsupials, each distinct from the other, evolved from a common ancestral stock, all within the Australian continent — another well-known example of adaptive radiation.
• When more than one adaptive radiation appears to have occurred in an isolated geographical area (representing different habitats), it is described as convergent evolution — for example, the striking resemblance between certain Australian marsupials and unrelated placental mammals (such as the marsupial "wolf" and the placental wolf).

🚀 NEET Advanced Edge

Why natural selection is NOT "survival of the strongest" but "survival of the most reproductively successful": Selection acts purely on differential reproductive output — a trait that helps an organism survive longer but produce fewer offspring than a competitor will still be selected AGAINST over generations; fitness in evolutionary biology means reproductive contribution to the next generation, not physical strength or longevity.

Why vestigial organs are evidence FOR evolution, not just biological curiosities: Structures like the human appendix or coccyx are functionally reduced remnants of organs that were fully functional in ancestral species — their presence despite no current use is best explained by descent from an ancestor where they served a purpose, a pattern that special creation (each species designed independently) does not predict.

Worked reasoning: Two populations of the same fish species become separated when a river changes course, isolating them in different lakes for thousands of generations. Decades later, individuals from the two lakes can no longer interbreed successfully. Identify the speciation type and the underlying mechanism. Answer: Allopatric speciation — geographic isolation prevented gene flow, allowing the two populations to accumulate independent genetic changes (via mutation and selection) until reproductive isolation arose.