Evolution
Evolution is the gradual change in the inherited characteristics of populations over successive generations. The PMDC MDCAT 2026 syllabus expects you to contrast Lamarckism with Darwinism, explain natural selection and its modern (Neo-Darwinian) form, cite the main lines of evidence, and handle simple Hardy–Weinberg calculations. Expect 2–4 MCQs from this chapter.
Lamarckism
Jean Baptiste Lamarck (1809) proposed the inheritance of acquired characters: organs strengthened by use enlarge, organs neglected shrink, and these lifetime modifications pass to offspring. His stock example was the giraffe stretching for high foliage.
Darwinism
Charles Darwin published On the Origin of Species in 1859 after his voyage on HMS Beagle. Alfred Russel Wallace reached the same idea independently in the Malay Archipelago; the two presented jointly to the Linnean Society in 1858. Darwin was also influenced by Malthus on population growth and Lyell on geology.
The argument runs:
- Organisms produce far more offspring than can survive (over-production).
- Resources are limited → struggle for existence.
- Individuals of a species vary, and variations are heritable.
- Those with favourable variations survive and reproduce more — survival of the fittest.
- Over generations favourable variations accumulate → new species.
Natural Selection
Selection acts on the phenotype of the individual; allele frequencies in the population change as a consequence. Mutation is random with respect to fitness — the environment does not create the variation it favours, it merely sieves what is already there.
| Type | Favours | Effect on variation | Example |
|---|---|---|---|
| Stabilising | Intermediate phenotypes | Reduced | Human birth weight |
| Directional | One extreme | Mean shifts | Antibiotic resistance |
| Disruptive | Both extremes | Becomes bimodal | Beak size in variable seed supply |
Heterozygote advantage keeps otherwise harmful alleles in circulation: sickle-cell heterozygotes resist malaria while avoiding severe anaemia, so both alleles persist where the parasite is endemic. Similarly, a deleterious recessive survives at low frequency because it is hidden from selection in carriers.
Natural Selection in Action
- Industrial melanism
- On soot-blackened bark the dark peppered moth was better camouflaged and survived bird predation; its frequency rose. After the Clean Air Acts restored lichens, the pale form recovered — the direction of selection depends on the environment.
- Antibiotic and pesticide resistance
- Resistance alleles arise by chance before exposure. The drug then kills susceptible individuals, leaving resistant ones to multiply. The drug selects; it does not create.
Sources of Variation
Mutation is the only source of genuinely new alleles. Recombination processes merely reshuffle existing ones:
- Crossing over at chiasmata in prophase I — separates linked alleles.
- Independent assortment of homologous pairs in metaphase I.
- Random fertilisation of one gamete by another.
Only mutations in the germ line matter for evolution; somatic mutations die with the individual.
Evidence of Evolution
| Line of evidence | What it shows | Example |
|---|---|---|
| Palaeontological | Change in form over geological time | Horse series: size up, toes reduced to one, teeth adapted to grazing |
| Connecting links | Intermediate forms between groups | Archaeopteryx — feathers and wings with teeth, clawed digits and a bony tail |
| Comparative anatomy | Shared ancestry despite different function | Pentadactyl limb of whale, bat, horse, human |
| Vestigial organs | Reduced remnants of ancestral structures | Appendix, coccyx, nictitating membrane |
| Embryology | Common developmental stages | Pharyngeal gill slits in terrestrial vertebrate embryos |
| Biochemical | Conserved molecules | Cytochrome c, universal genetic code, ATP |
Artificial Selection
Humans, not the environment, choose which individuals breed — producing the many varieties of dog, pigeon, wheat and cattle. Darwin used it as his analogy for natural selection. Its drawback is a narrowed gene pool: selected varieties are often less able to survive without human protection.
Population Genetics
Modern theory treats the population and its gene pool as the unit that evolves — an individual’s genotype is fixed at fertilisation.
p + q = 1 and p2 + 2pq + q2 = 1. Allele frequencies stay constant provided the population is large, mating is random, and there is no mutation, migration or selection. Any departure from these conditions means evolution is happening.
Worked example. If 1 in 10 000 people show a recessive condition, then q2 = 0.0001, so q = 0.01 and p = 0.99. Carriers = 2pq = 2 × 0.99 × 0.01 ≈ 0.0198, i.e. about 2% — roughly 198 carriers for every affected individual.
- Genetic drift
- Random change in allele frequencies, significant only in small populations. Alleles can be lost regardless of their value.
- Founder effect
- A few colonists carry only a fraction of the parent gene pool — a special case of drift.
- Bottleneck
- A population crash leaves survivors with reduced diversity, which persists even after numbers recover.
- Gene flow
- Migration exchanges alleles between populations, making them more alike and opposing divergence.
Speciation
A biological species is a group whose members can interbreed to produce fertile offspring. Speciation therefore means the evolution of reproductive isolation.
| Mode | Geographical barrier? | Typical route |
|---|---|---|
| Allopatric | Yes — river, mountain, sea | Isolated populations diverge under different selection |
| Sympatric | No — same area | Polyploidy in plants; a polyploid cannot backcross fertilely with its diploid parents |
| Isolating mechanism | Acts | Examples |
|---|---|---|
| Prezygotic | Before fertilisation | Different breeding season, habitat, courtship behaviour, incompatible genitalia |
| Postzygotic | After the zygote forms | Hybrid inviability; hybrid sterility (the mule); hybrid breakdown in F2 |
Divergence is fastest when gene flow is blocked and the two habitats impose strongly different selection pressures.
Adaptive Radiation
One ancestral stock diversifies into many forms occupying different niches. The Galapagos finches differ in beak shape according to food source; Australian marsupials radiated into grazing, burrowing and carnivorous forms. It happens most readily where a coloniser meets unoccupied niches and little competition.
Neo-Darwinism
The modern synthesis marries Darwin’s selection with genetics. Agents of change: mutation and recombination generate variation; natural selection sorts it; genetic drift and gene flow modify frequencies further. Evolution is measured as a shift in allele frequencies within a gene pool over generations.
Origin of Life
The Oparin–Haldane hypothesis proposes chemical evolution in the early oceans. A reducing (oxygen-free) atmosphere was essential — free oxygen would have oxidised organic molecules as fast as they formed. Miller and Urey (1953) sparked a mixture of methane, ammonia, hydrogen and water vapour and obtained amino acids, showing the building blocks could form abiotically.
The RNA world idea holds that RNA preceded DNA and protein, because RNA can both carry information and act catalytically — one molecule doing both jobs before the two roles specialised.
Worked MCQs
Five MCQs covering the high-yield testing patterns for evolution. Read every explanation carefully.
Q1. In a population at Hardy–Weinberg equilibrium the recessive allele frequency is 0.2. The frequency of heterozygotes is:
q = 0.2, so p = 0.8. Heterozygotes = 2pq = 2 × 0.8 × 0.2 = 0.32. The distractors are the other terms of the expansion: q2 = 0.04 (homozygous recessive) and p2 = 0.64 (homozygous dominant); 0.16 is the trap for forgetting to double pq.
Q2. The similar streamlined body form of a shark, a dolphin and an extinct ichthyosaur is best explained by:
A cartilaginous fish, a mammal and a reptile are only distantly related. Fast swimming imposes the same hydrodynamic demands on all three, so each independently evolved the same shape. The resemblance is analogous, not homologous.
Q3. Antibiotic resistance spreads through a bacterial population because the antibiotic:
Resistance alleles arise by chance mutation whether or not the drug is present. The antibiotic kills susceptible cells and leaves the already-resistant ones to multiply. Mutation is random with respect to need — the environment selects, it does not instruct.
Q4. The sterility of the mule, a horse–donkey hybrid, is an example of:
Mating and fertilisation both succeed, so nothing prezygotic has intervened. The barrier appears afterwards: the parental chromosome sets cannot pair properly in meiosis, so the hybrid cannot form viable gametes.
Q5. Whose experiment on removing the tails of mice over many generations argued against Lamarckism?
Weismann amputated tails for generations and offspring were still born with tails, supporting his germ-plasm theory that somatic changes never reach the gametes. De Vries is associated with mutation theory, not this experiment.
Quick Recap
- Lamarck = inheritance of acquired characters (use and disuse); refuted by Weismann’s germ-plasm theory.
- Darwin (Origin of Species, 1859, HMS Beagle) + Wallace independently → natural selection.
- Darwin’s weakness: no mechanism for the origin or inheritance of variation.
- Mutation is the only source of new alleles; crossing over and assortment only reshuffle.
- Homologous = same origin, different function. Analogous = same function, different origin.
- Archaeopteryx = reptile–bird link; horse series = gradual change; appendix = vestigial.
- Hardy–Weinberg: p2 + 2pq + q2 = 1; needs large population, random mating, no mutation/migration/selection.
- Drift matters in small populations; founder effect and bottleneck are special cases.
- Allopatric = with barrier; sympatric = without (polyploidy). Prezygotic before fertilisation, postzygotic after (mule).
- Neo-Darwinism: mutation + recombination + selection + drift + gene flow, acting on the gene pool.