Evolution is the change in the heritable characteristics of a population over successive generations. It explains the diversity of life on Earth. The modern concept integrates Darwin's natural selection with Mendelian genetics — known as the Modern Synthetic Theory or Neo-Darwinism.
Origin of Life
- Earth is approximately 4.5 billion years old. Life arose about 3.5-4 billion years ago. Key theories:
- Oparin-Haldane hypothesis (Chemical evolution): Life originated through gradual chemical evolution — inorganic molecules → organic molecules → complex biomolecules → protocells. Primitive Earth had a reducing atmosphere (CH4, NH3, H2O, H2 — no free O2).
- Miller-Urey experiment (1953): Simulated early Earth conditions; passed electric sparks through CH4, H2, NH3, H2O → produced amino acids, organic compounds. This was the first experimental evidence for abiogenesis.
Darwin's Theory of Natural Selection
- 1.Charles Darwin proposed (1859, "On the Origin of Species"):
- 2.Individuals in a population show natural variation
- 3.Resources are limited → struggle for existence
- 4.Those with favourable variations survive better (survival of the fittest) → natural selection
- 5.Favourable variations are inherited by offspring
- 6.Over many generations, this leads to speciation
Industrial Melanism in peppered moths (Biston betularia): Before industrialisation, pale moths predominated (camouflage on lichen-covered trees). After industrial pollution darkened trees, dark (melanic) moths survived better. This is natural selection in action.
Antibiotic resistance in bacteria: A few bacteria with random mutations conferring resistance survive antibiotic treatment. They multiply and pass on resistance genes. This is natural selection — not Lamarckian inheritance.
Hardy-Weinberg Equilibrium
- In an ideal population (no mutation, no migration, random mating, large size, no selection), allele frequencies remain constant. For alleles p and q (p + q = 1):
- Genotype frequencies: p2 (AA) + 2pq (Aa) + q2 (aa) = 1
- If allele frequency changes, evolution is occurring
If q (frequency of recessive allele) = 0.3, then p = 0.7. Frequency of homozygous recessive (aa) = q2 = 0.09 (9%). Frequency of carriers (Aa) = 2pq = 2 x 0.7 x 0.3 = 0.42 (42%).
- Factors disturbing equilibrium (causing evolution):
- Gene mutation: New alleles appear
- Gene migration (gene flow): Alleles move between populations
- Genetic drift (Founder effect / Bottle-neck effect): Random changes in small populations
- Genetic recombination: New combinations of alleles
- Natural selection: Differential survival and reproduction
Types of Evolution
Convergent evolution: Unrelated organisms evolve similar traits in similar environments (analogous structures). E.g., wings of bats, birds, and insects; eyes of vertebrates and cephalopods.
Divergent evolution: Related organisms evolve different traits due to different environments (homologous structures). E.g., forelimbs of whale, bat, horse, and human — same basic bone structure (humerus, radius, ulna, carpals, phalanges) but different functions.
Adaptive radiation: Multiple species evolve from a common ancestor to fill different ecological niches. E.g., Darwin's finches in Galapagos Islands; Australian marsupials.
Thorn in Bougainvillea and tendril in Cucurbita: Both are modified stems (homologous), but serve different functions (protection vs climbing). This suggests common ancestry (divergent evolution).
Wings of butterfly and wings of birds are analogous — same function (flight) but different evolutionary origins and structures. This suggests convergent evolution in different lineages.
Darwin's finches in Galapagos — all descended from a common finch ancestor, but evolved different beak shapes suited to different food sources (seeds, insects, cactus). This is adaptive radiation.
Speciation — allopatric speciation occurs when populations are geographically isolated (e.g., a mountain range), leading to genetic divergence until they can no longer interbreed. Sympatric speciation occurs without geographic isolation (polyploidy in plants).
Evidence for Evolution
- Fossil record: Shows progression of forms; transitional fossils (e.g., Archaeopteryx — intermediate between reptiles and birds)
- Comparative anatomy: Homologous organs (same structure, different function), vestigial organs (e.g., human appendix, wisdom teeth), analogous organs
- Comparative embryology: Early embryos of diverse vertebrates are remarkably similar
- Biochemical evidence: Shared DNA sequences, cytochrome-c similarity across species
Common mistakes
- Lamarck's theory (inheritance of acquired characters) is incorrect — only genetic mutations are heritable, not characters acquired during an organism's lifetime.
- Evolution is change in population allele frequencies — individual organisms do not evolve, populations do.
- Natural selection does not produce mutations — mutations are random; selection acts on them.
- Analogous structures show convergent evolution; homologous structures show divergent evolution.
Summary
Life originated ~3.5 billion years ago through chemical evolution. Darwin's natural selection explains evolution through differential survival. Hardy-Weinberg principle describes static allele frequencies; factors like mutation and selection disturb equilibrium. Evidence from fossils, anatomy, and biochemistry supports evolution. Adaptive radiation, convergent and divergent evolution explain biodiversity patterns.