Evolution — Origin of Life, Evidences, Darwinism and the Hardy–Weinberg Principle

Evolution mixes a few conceptual ideas with a lot of terminology, so the strategy is to nail the concepts and then hang the terms off them. Two ideas do most of the work. First, natural selection acts on existing variation — it does not create new traits to order; this single point separates Darwin from Lamarck and answers a whole family of questions. Second, the Hardy–Weinberg principle describes a population that is not evolving, so any departure from it signals evolution — and the equation p² + 2pq + q² = 1 is a guaranteed numerical mark if you can set it up. Around these sit the evidences (homology versus analogy is the key distinction) and the classic experiments (Miller–Urey, Darwin's finches, industrial melanism). Learn the two big ideas and the vocabulary organises itself.

Origin of Life and Evidences

  • The Oparin–Haldane hypothesis proposed that life arose from simple molecules in a reducing early atmosphere ("chemical evolution"). Miller and Urey tested it: sparking a mixture of CH₄, NH₃, H₂ and water vapour produced amino acids, supporting abiotic synthesis of organic molecules.
  • Homologous organs have the same basic structure and origin but different functions (the forelimbs of a whale, bat and human) — evidence of divergent evolution from a common ancestor. Analogous organs have different origins but similar function (a butterfly's and a bird's wing) — evidence of convergent evolution.
  • Other evidences: fossils (direct record, dated by rock strata), vestigial organs (the appendix, the vermiform tail bones), and comparative embryology and biochemistry (shared molecules like DNA and ATP across all life).
⚠️ Homologous versus analogous — same origin or same function, never both
This pair is the single most tested distinction in the chapter, and it turns on separating origin from function. Homologous organs share a common origin and basic structure but may do different jobs — the forelimb of a human (grasping), a whale (swimming), a bat (flying) and a horse (running) are built on the same bone plan inherited from a common ancestor. They point to divergent evolution: one ancestral structure diverging into many uses. Analogous organs share a function but not an origin — the wing of an insect and the wing of a bird both fly, but they are built quite differently and evolved separately. They point to convergent evolution: unrelated lineages arriving at the same solution. The test to apply is: are they built the same but used differently (homologous, divergent) or used the same but built differently (analogous, convergent)? A whale's flipper and a bat's wing are homologous despite their different jobs; a shark's fin and a whale's flipper are analogous despite their shared job, because the shark is a fish and the whale a mammal. Getting this right is worth several marks a paper.

Darwinism and Mechanisms of Evolution

  • Darwin's natural selection: individuals vary; more are born than can survive; those with favourable variations survive and reproduce more ("survival of the fittest"); over generations the favourable traits accumulate. Fitness means reproductive success, not strength.
  • Lamarck's theory — inheritance of acquired characters (use and disuse) — is rejected: traits gained in a lifetime are not passed on, because variation is not directed by need. This Darwin-versus-Lamarck contrast is heavily examined.
  • Evidence in action: industrial melanism in the peppered moth (dark moths favoured on soot-darkened trees), Darwin's finches of the Galapagos (adaptive radiation from one ancestor), and antibiotic/pesticide resistance (selection in real time).
🎯 The moth did not "turn" dark — the dark ones were already there
Industrial melanism is the cleanest demonstration of how natural selection actually works, and it exposes the commonest misconception. Before industrialisation, the peppered moth was mostly light-coloured, which camouflaged it on lichen-covered tree trunks; a few dark moths existed but were easily spotted by birds and eaten. When soot from factories blackened the bark, the situation reversed: now the dark moths were camouflaged and the light ones stood out and were eaten. Over generations the population became predominantly dark. The crucial point is that the dark variant already existed before the pollution — selection did not create it in response to need. Pollution simply changed which pre-existing variant survived better. This is exactly where Darwin differs from Lamarck: the moths did not darken because they needed to (Lamarck), they were already varied and the environment picked the winners (Darwin). The same logic explains antibiotic resistance — the resistant bacteria were present by chance before the drug, and the drug merely selected them — which is why "the bacteria developed resistance in order to survive the antibiotic" is the wrong, Lamarckian phrasing that questions try to catch.

The Hardy–Weinberg Principle

ℹ️ A population that is NOT evolving — and why that is useful
The Hardy–Weinberg principle sounds paradoxical: it describes a population in which allele frequencies stay constant generation after generation — that is, one that is not evolving. Its value is precisely as a baseline. If the frequency of an allele is p and its alternative is q, then p + q = 1, and the genotype frequencies are given by p² + 2pq + q² = 1 (p² homozygous dominant, 2pq heterozygous, q² homozygous recessive). This equilibrium holds only under five idealised conditions: no mutation, no gene flow (migration), no genetic drift (a large population), random mating, and no natural selection. Real populations rarely meet all five, so allele frequencies do change — and that change is exactly what evolution is. So the principle is useful as the "nothing is happening" reference: any observed departure from Hardy–Weinberg equilibrium is evidence that one of the five forces is operating, i.e. that the population is evolving. In the exam the equation is a reliable numerical question: given the frequency of affected (recessive) individuals as q², take the square root to get q, find p = 1 − q, and read off carriers as 2pq. Practising that one calculation banks a near-certain mark.
  • The five forces that disturb equilibrium (and so drive evolution): gene mutation, genetic recombination, natural selection, genetic drift, and gene migration (flow). Drift matters most in small populations, where chance alone changes allele frequencies.
  • Speciation is the formation of new species, often by reproductive isolation (a barrier stopping gene flow between populations). Adaptive radiation is one ancestral species diversifying into many, as with Darwin's finches and Australian marsupials.

Key takeaways

  • Natural selection acts on pre-existing variation; it does NOT create traits on demand — this separates Darwin from Lamarck.
  • Homologous = same origin, different function (divergent, e.g. mammal forelimbs); analogous = different origin, same function (convergent).
  • Oparin–Haldane proposed chemical evolution; Miller–Urey made amino acids by sparking CH₄/NH₃/H₂/water vapour.
  • Fitness = reproductive success, not strength. Industrial melanism and antibiotic resistance are selection in real time.
  • Hardy–Weinberg describes a NON-evolving population; departure from it signals evolution.
  • p² + 2pq + q² = 1: p² dominant homozygotes, 2pq heterozygotes (carriers), q² recessive homozygotes.
  • Equilibrium needs: no mutation, no gene flow, no drift (large population), random mating, no selection.
  • Genetic drift matters most in small populations; adaptive radiation = one ancestor diversifying (Darwin's finches).

Practice questions (8)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

  1. The forelimbs of a whale, a bat and a human have the same basic bone structure but different functions. They are:

    1. Homologous organs, showing divergent evolution
    2. Analogous organs, showing convergent evolution
    3. Vestigial organs
    4. Analogous organs, showing divergent evolution
    Show answer

    Answer: A — Homologous organs, showing divergent evolution

    Same basic structure inherited from a common ancestor but different uses defines homologous organs and divergent evolution. Analogous organs share a function but not a structure, so calling them analogous contradicts the "same bone structure" given in the question.

  2. In a population at Hardy–Weinberg equilibrium, the frequency of heterozygotes is given by:

    1. 2pq
    2. p²
    3. q²
    4. p + q
    Show answer

    Answer: A — 2pq

    In p² + 2pq + q² = 1, the middle term 2pq is the heterozygote frequency, while p² and q² are the two homozygotes. Choosing p² or q² picks a homozygous class instead of the heterozygotes the question asks for.

  3. The rise of dark-coloured peppered moths after industrialisation is best explained by:

    1. Natural selection of a pre-existing dark variant on soot-darkened trees
    2. The moths turning dark because they needed camouflage
    3. Inheritance of soot acquired during the moths' lifetime
    4. A sudden mutation caused directly by pollution in every moth
    Show answer

    Answer: A — Natural selection of a pre-existing dark variant on soot-darkened trees

    The dark variant already existed; darkened bark simply let it survive better while light moths were eaten, so selection changed which variant predominated. The "turned dark because they needed to" and "inheritance of acquired soot" options are Lamarckian phrasings the question is testing you to reject.

  4. The Miller–Urey experiment supported the idea that:

    1. Organic molecules like amino acids could form abiotically on early Earth
    2. Life was created fully formed in a single step
    3. Cells arose directly from rocks
    4. DNA was the first molecule to appear
    Show answer

    Answer: A — Organic molecules like amino acids could form abiotically on early Earth

    By sparking a reducing gas mixture, Miller and Urey produced amino acids, supporting the chemical (abiotic) synthesis of organic molecules proposed by Oparin and Haldane. It did not create a cell or life itself, so the "fully formed life" option overstates the result.

  5. Genetic drift, as a factor in evolution, is most significant in:

    1. Small populations
    2. Very large populations
    3. Populations with high migration
    4. Populations with no variation
    Show answer

    Answer: A — Small populations

    In small populations, chance events can shift allele frequencies dramatically, making drift a powerful force. Large populations buffer such random fluctuations, which is exactly why Hardy–Weinberg assumes a large population to keep drift negligible.

  6. In Darwinian terms, the "fitness" of an organism refers to its:

    1. Reproductive success — leaving more offspring
    2. Physical strength
    3. Body size
    4. Lifespan alone
    Show answer

    Answer: A — Reproductive success — leaving more offspring

    Fitness measures how many surviving offspring an individual leaves, not how strong or large it is; a weaker organism that reproduces more is fitter in this sense. Equating fitness with strength is the popular misreading the question is designed to correct.

  7. Darwin's finches of the Galapagos, diversified from a single ancestral species into many forms, illustrate:

    1. Adaptive radiation
    2. Convergent evolution
    3. Genetic drift only
    4. Inheritance of acquired characters
    Show answer

    Answer: A — Adaptive radiation

    One ancestral finch diversifying into many species with different beaks suited to different foods is the classic case of adaptive radiation. Convergent evolution is the opposite pattern — unrelated species becoming similar — so it does not describe divergence from a single ancestor.

  8. A departure from Hardy–Weinberg equilibrium in a population indicates that:

    1. The population is evolving (one of the five forces is acting)
    2. The population has stopped reproducing
    3. Allele frequencies are perfectly constant
    4. No genotypes exist in the population
    Show answer

    Answer: A — The population is evolving (one of the five forces is acting)

    Hardy–Weinberg describes a non-evolving population, so any deviation shows that mutation, gene flow, drift, non-random mating or selection is at work — i.e. evolution is occurring. Constant allele frequencies describe equilibrium itself, which is the opposite of a departure from it.