Evolution
Evolution is the gradual change in the inherited characteristics of populations over successive generations. The PMDC MDCAT 2026 syllabus keeps this chapter tight: explain the origin of life according to the concept of evolution, describe Lamarck’s theory of inheritance of acquired characters, and explain Darwin’s theory of natural selection. 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.
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. 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.
Q2. 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.
Q3. According to Lamarck, the long neck of the modern giraffe is the result of:
Lamarckism rests on use and disuse plus the inheritance of acquired characters: an organ strengthened during an individual’s lifetime is supposed to be transmitted to its offspring. The mutation-plus-selection option is Darwin’s explanation, not Lamarck’s.
Q4. Which condition of the primitive atmosphere was essential for the chemical evolution proposed by Oparin and Haldane?
Free oxygen is a powerful oxidising agent and would have destroyed organic molecules as fast as they formed. Oparin and Haldane therefore required a reducing atmosphere — methane, ammonia, hydrogen and water vapour — which is exactly the mixture Miller and Urey used in 1953.
Q5. Who arrived at the theory of natural selection independently of Darwin and presented jointly with him in 1858?
Wallace reached the same conclusion while working in the Malay Archipelago, prompting the joint presentation to the Linnean Society in 1858 and Darwin’s publication of On the Origin of Species in 1859. Malthus (population growth) and Lyell (geology) influenced Darwin but did not propose natural selection.
Quick Recap
- Lamarck = use and disuse + inheritance of acquired characters (giraffe’s neck); refuted by Weismann’s germ-plasm theory and his tail-removal experiment on mice.
- Darwin (Origin of Species, 1859, HMS Beagle) + Wallace independently → natural selection; influenced by Malthus and Lyell.
- Darwin’s chain: over-production → struggle for existence → heritable variation → survival of the fittest → new species.
- Darwin’s weakness: no mechanism for the origin or inheritance of variation.
- Selection acts on the phenotype; mutation is random with respect to fitness — the environment sieves variation, it never creates it to order.
- Origin of life: Oparin–Haldane chemical evolution in a reducing (oxygen-free) atmosphere; Miller–Urey (1953) made amino acids from methane, ammonia, hydrogen and water vapour.
- RNA world: RNA came before DNA and protein because it both stores information and catalyses.