Evolution
The gradual change in inherited characteristics of populations over successive generations, driven by natural selection — proposed by Charles Darwin in 1859.
A. Darwin's Theory of Natural Selection
Four Key Principles
- Overproduction: All organisms produce more offspring than can possibly survive — resources are limited
- Variation: Individuals within a population show natural variation in their traits (heritable differences)
- Struggle for Existence: Competition for limited resources — food, mates, territory
- Survival of the Fittest: Individuals with advantageous traits survive longer and reproduce more successfully
- Heredity: Favourable traits are inherited by offspring — passed to the next generation
- Speciation: Over many generations, accumulated changes can lead to the formation of new species
⚡ "Survival of the Fittest" coined by Herbert Spencer (not Darwin). "Fittest" means best adapted to the environment — not necessarily strongest or fastest.
B. Key Evolutionary Concepts
Definitions
| Concept | Definition |
|---|---|
| Natural Selection | The process by which organisms with favourable heritable traits survive and reproduce more than those without |
| Variation | Heritable differences among individuals of the same species — the raw material for evolution |
| Adaptation | An inherited trait that increases an organism's fitness (survival and reproduction) in a specific environment |
| Speciation | Formation of a new, distinct species from an existing ancestral species over time |
| Mutation | A change in the DNA sequence — the primary source of new heritable variation in populations |
| Genetic Drift | Random change in allele frequencies in a population, especially significant in small populations (bottleneck effect) |
| Gene Flow | Transfer of genetic material between populations through migration of individuals |
| Fitness | An organism's ability to survive AND reproduce successfully in its environment — measured by reproductive success |
C. Evidence for Evolution
Six Lines of Evidence
| Evidence | Explanation | Example |
|---|---|---|
| Fossil Record | Shows gradual morphological changes in species over geological time | Horse fossils showing increase in size and reduction of toe bones |
| Homologous Structures | Same underlying bone structure, different function — same evolutionary origin (common ancestor) | Human arm, whale flipper, bat wing, horse leg — all have same bones (humerus, radius, ulna) |
| Analogous Structures | Same function, different evolutionary origin (convergent evolution) | Bird wing vs insect wing — both for flight but evolved independently |
| Vestigial Organs | Reduced, non-functional remnants of structures that were useful in ancestors | Human appendix, coccyx (tailbone), wisdom teeth, whale hip bones |
| Molecular Biology | DNA and protein sequence similarities between species indicate common ancestry | Humans and chimpanzees share ~98.7% of DNA |
| Embryology | Early embryos of different vertebrate species look strikingly similar — suggests common ancestry | Fish, reptile, bird and human embryos all have gill slits and tails at early stages |
⚡ MCQ Trick Homologous = same ORIGIN, different function. Analogous = same FUNCTION, different origin. Vestigial = reduced/non-functional remnant. Mutation = PRIMARY source of variation.
D. Lamarck vs Darwin
Comparison Table
| Feature | Lamarck | Darwin |
|---|---|---|
| Theory | Inheritance of Acquired Characteristics | Natural Selection |
| Mechanism | "Use and disuse" of organs — used organs grow stronger; unused organs shrink | Natural variation + selection pressure + heredity |
| Giraffe example | Giraffes stretched necks during lifetime → offspring born with longer necks | Random variation in neck length → longer-necked giraffes survived better → passed trait to offspring |
| Inheritance | Acquired traits (gained during lifetime) are inherited | Only heritable (genetic) traits are passed on |
| Accepted? | ❌ Rejected — acquired traits are NOT inherited (disproved) | ✅ Widely accepted — supported by genetics and molecular biology |
Quick MCQ Revision
| Fact | Answer |
|---|---|
| Darwin's theory | Natural Selection (1859) |
| Lamarck's theory | Inheritance of Acquired Characteristics — REJECTED |
| Primary source of new variation | Mutation (change in DNA sequence) |
| Homologous structures | Same origin, different function (e.g. human arm, whale flipper) |
| Analogous structures | Same function, different origin (e.g. bird wing, insect wing) |
| Vestigial organs | Non-functional remnants of ancestral structures (e.g. human appendix) |
| Fitness | Ability to survive AND reproduce in an environment |
| Genetic drift | Random allele frequency change — significant in small populations |