1. Evidences of Evolution
A. Morphological and Anatomical Evidence
| Type | Definition | Example | Evidence for |
|---|---|---|---|
| Homologous organs | Same basic structure (origin), different functions | Human arm, whale flipper, bat wing, horse forelimb — all have same bones (humerus, radius, ulna) | Divergent evolution; common ancestry |
| Analogous organs | Different structure (origin), same function | Wings of bird and insect; eyes of octopus and human | Convergent evolution; similar selection pressures |
| Vestigial organs | Reduced, functionless remnants of once-functional organs | Appendix (human); nictitating membrane; coccyx; body hair; ear muscles | Common ancestry; organs lost function over evolution |
B. Palaeontological Evidence (Fossil Record)
- Fossils are preserved remains or traces of organisms from past geological ages. They provide direct evidence of what organisms looked like and when they lived.
- Transitional fossils show intermediate characteristics: e.g., Archaeopteryx (had features of both reptiles — teeth, clawed wings, long bony tail — AND birds — feathers, wishbone). Shows the link between reptiles and birds.
- Horse evolution — a classic sequence: Eohippus (Hyracotherium, small, multi-toed, Eocene) → Mesohippus → Merychippus → Equus (modern horse, single-toed). Shows gradual size increase and toe reduction.
- The fossil record is incomplete (preservation is rare) but shows a clear pattern of gradual change over time.
C. Embryological Evidence
- Biogenetic Law (Haeckel's Recapitulation Theory): "Ontogeny recapitulates phylogeny" — embryonic development repeats evolutionary history.
- All vertebrate embryos (fish, frog, lizard, chicken, human) show similar features at early stages: pharyngeal gill slits, two-chambered heart, tail. This suggests common ancestry.
- Human embryos show gill slits and tails at early developmental stages — vestigial remnants of ancestral features.
D. Biogeographical Evidence
- Darwin's observations of Galápagos finches: 13–14 species with different beak shapes, all descended from one ancestral South American finch species. Each species adapted to different food sources on different islands.
- Marsupials in Australia — evolved independently from placental mammals but show similar adaptive radiation (wolf-like, mole-like, anteater-like forms).
- Continental drift separated populations → independent evolution → different species.
E. Molecular Evidence
- DNA sequence similarity reflects evolutionary closeness: Human and chimpanzee share ~98–99% DNA similarity.
- Cytochrome c (a protein) sequence comparison: the more similar the sequence, the more closely related the species.
- All living organisms use the same genetic code (64 codons), same ATP for energy, same L-amino acids → evidence of common ancestor.
2. Mechanisms of Evolution — Natural Selection in Action
Industrial Melanism — Classic Example
Peppered moth (Biston betularia):
- Before industrialisation: white-speckled form was common (camouflaged on lichen-covered grey bark); dark (melanic) form was rare.
- After industrialisation: soot blackened tree bark → white moths became visible to predators → dark moths had survival advantage.
- After decades: dark form became dominant in polluted areas. White form dominant in unpolluted areas.
- Significance: Demonstrates natural selection acting on existing variation in real time; not evolution of a new trait but selection of pre-existing variant.
Types of Natural Selection
| Type | Effect on population | Example |
|---|---|---|
| Stabilising selection | Favours intermediate phenotype; reduces variation; most common in stable environments | Human birth weight — very low and very high birth weights have higher mortality; intermediate weight has highest survival |
| Directional selection | Favours one extreme phenotype; shifts population mean in one direction | Industrial melanism in peppered moths; antibiotic resistance in bacteria |
| Disruptive selection | Favours both extremes; eliminates intermediate phenotype; can lead to two distinct populations | Beak size in Darwin's finches during drought — small and large seeds both available; medium beaks disadvantaged |
Antibiotic Resistance — Evolution by Natural Selection
- Antibiotics do NOT cause mutations — mutations occur randomly and spontaneously.
- Antibiotic resistance pre-exists in rare mutant bacteria before antibiotic exposure.
- Antibiotic kills susceptible bacteria → resistant mutants survive → reproduce → resistant population predominates.
- This is directional selection — the antibiotic is the selecting agent, not the cause of resistance.
3. Mechanisms of Evolution — Genetic Drift
Genetic drift is the change in allele frequency in a population due to random chance (random sampling error), not natural selection. It is most significant in small populations.
Founder Effect
When a small group of individuals colonises a new area, carrying only a fraction of the original gene pool's alleles. The new population has a different (and less diverse) allele frequency than the original. Example: Certain genetic diseases are common in isolated communities (e.g., Amish communities have high frequency of certain rare genetic disorders).
Bottleneck Effect
A dramatic reduction in population size due to a catastrophic event (disease, natural disaster, hunting). Survivors are a random subset → random allele frequencies. Example: Northern elephant seal population was reduced to ~20 individuals in the late 1800s; today's large population has very low genetic diversity.
| Feature | Natural Selection | Genetic Drift |
|---|---|---|
| Driving force | Environment / fitness | Random chance |
| Population size | More effective in large populations | More significant in small populations |
| Direction | Non-random; directional (toward better adaptation) | Random; can fix harmful or neutral alleles |
| Result | Adaptation | Reduced genetic diversity; possible fixation of alleles |
4. Gene Flow and Mutation
Gene flow (migration): Movement of alleles between populations through migration of individuals or gametes. Tends to reduce differences between populations and increase diversity within them.
Mutation: Random change in DNA sequence. Provides the ultimate source of new alleles. Most mutations are neutral or harmful; occasionally a beneficial mutation arises and is acted upon by natural selection.
