Evolution

1Chemical origin and Miller–Urey

Oparin and Haldane argued that life began with chemical evolution: a reducing atmosphere, energy inputs, and accumulation of organic molecules in a primordial soup. Miller and Urey tested a slice of that idea by sparking a mixture of methane, ammonia, hydrogen and water vapour — amino acids appeared among the products.

The experiment supports abiotic synthesis of organics under those conditions; it does not by itself prove that the early Earth had exactly that atmosphere, nor does it assemble a cell. Treat it as evidence for a step, not as a complete origin story.

Figure. Flow from reducing gases through spark energy to organic products — apparatus schematic as boxes, not glassware art.

What Miller–Urey actually did

1. GasesCH₄, NH₃, H₂ and water vapour in a closed apparatus.
2. EnergyElectric sparks simulated lightning.
3. ProductsAmino acids and other organics formed abiotically.

2Homologous and analogous organs

Homologous organs share a common ancestral plan and may diverge in function — the forelimb of a human, whale and bat. That pattern is divergent evolution and evidence of common ancestry. Analogous organs share function but not origin — bird wing and insect wing — and point to convergent evolution.

Fossils, embryology and molecular sequences add independent evidence, but the homology/analogy fork is the one that is asked as a classification under time pressure. Ask "same origin or same function only?" before you name the pattern.

Figure. Side-by-side definition boxes — not anatomical drawings of limbs or wings.

3Natural selection and industrial melanism

Darwin's mechanism needs heritable variation, more offspring than resources can support, and differential survival and reproduction — descent with modification follows. Industrial melanism in the peppered moth is the classroom illustration: dark forms rose in frequency where soot darkened tree trunks and bird predation favoured camouflage, then fell again as air cleared.

Selection changes allele frequencies; it is not "need creates adaptation" in Lamarck's sense. The moth story is about differential survival of existing variants, not moths inventing melanin because they wished for it.

Figure. Mechanism strip: variation → selection → frequency change. Not a painting of moths on bark.

Darwin's filter

1. VariationIndividuals in a population differ heritably.
2. StruggleMore are born than the environment can support.
3. Unequal successSome variants leave more offspring; the population's makeup shifts.

4Hardy–Weinberg equilibrium

Hardy–Weinberg gives the genotype frequencies expected when a diploid population is large, randomly mating, and free of selection, mutation, migration and drift. With alleles A and a at frequencies p and q (p + q = 1), the genotypes are p², 2pq and q², and those three fractions sum to 1.

Evolution, in the population-genetic sense used here, is a departure from that null. For a rare recessive disease, affected people are q² and carriers are 2pq — and carriers vastly outnumber the affected when q is small.

Figure. Genotype frequency boxes under allele-frequency conservation — schematic, not a Punnett square redraw.

5Forces that break Hardy–Weinberg

Any of mutation, migration (gene flow), selection, non-random mating or drift moves a population off the Hardy–Weinberg null — that move is evolution in the allele-frequency sense. Genetic drift is random frequency change; it is strong in small populations. Two named forms are the bottleneck (a crash shrinks the sample) and the founder effect (a few colonists start a new population).

Drift is not selection: a neutral allele can fix or vanish by chance. Saying "drift always helps the fittest" confuses the two filters.

Figure. Five forces that break the Hardy–Weinberg null. Drift is random sampling error, strongest in small populations; the others are directional or systematic.

6Outline of human evolution

The fossil sequence taught for exams runs Australopithecus → Homo habilis → Homo erectus → Homo neanderthalensis → Homo sapiens, with a general rise in brain size and tool complexity along that teaching line. Names and order are the content; absolute cranial volumes vary by specimen and should not be invented beyond what a given stem supplies.

Neanderthals are a side-branch of Homo that overlapped with modern humans in time and place, not a cartoon "missing link" that had to vanish before sapiens appeared. Binomial names stay in roman here — the platform still lacks conventional italics for species names in labels.

Figure. Exam teaching order as a linear strip — not a full phylogenetic tree with every side branch drawn.

Teaching sequence

1. AustralopithecusEarly bipedal hominin in the usual school sequence.
2. Early HomoH. habilis then H. erectus — tools and wider geographic spread.
3. Later HomoH. neanderthalensis and H. sapiens — large brains; sapiens persists.

7When a population is evolving

Put the pieces together: if measured genotype frequencies depart from p² : 2pq : q² under the Hardy–Weinberg assumptions, or if p and q themselves change across generations, the population is evolving in the genetic sense. Selection, drift, migration, mutation and non-random mating are the named causes; homology and fossils are the historical evidence that such change has been happening for a long time.

"Survival of the fittest" is a slogan for differential reproductive success, not a moral ranking and not a promise that every trait you admire was shaped for your exam definition of fitness.

Figure. A population evolves, in the genetic sense, when p (or q) changes across generations — here 0.6 to 0.45. This is not a redraw of the H–W genotype boxes.