In the 1953 experiment of Miller and Urey, electric sparks were passed through a mixture of CH₄, NH₃, H₂ and water vapour. What did the experiment produce?

Amino acids such as glycine and alanine, formed from simple gases in a closed apparatus

Short self-replicating RNA molecules that copied themselves in the condensed water

Membrane-bound coacervate droplets carrying out simple metabolism with enzymes

Polypeptide chains of about a hundred residues joined by peptide bonds in solution

Which observation gives the strongest support to the hypothesis that RNA preceded both DNA and proteins in early life?

The peptidyl transferase activity of the ribosome resides in its rRNA

DNA is more stable than RNA because its sugar lacks the 2′-hydroxyl group

The peptidyl transferase activity of the ribosome resides in its rRNA

Amino acids form readily when sparks pass through a reducing gas mixture

Mitochondria contain circular DNA and divide by a process resembling fission

Which of the following observations support the endosymbiotic origin of mitochondria?

Mitochondria carry their own circular DNA molecule

Mitochondrial ribosomes resemble bacterial ribosomes and are inhibited by chloramphenicol

Mitochondrial rRNA sequences are most similar to those of α-proteobacteria

Mitochondria carry their own circular DNA molecule

Mitochondrial ribosomes resemble bacterial ribosomes and are inhibited by chloramphenicol

Mitochondrial rRNA sequences are most similar to those of α-proteobacteria

The mitochondrial genome encodes every protein the organelle needs

Which sequence lists these events from the oldest to the most recent?

Great Oxidation Event → oldest eukaryotic fossils → Cambrian explosion → end-Permian extinction

Oldest eukaryotic fossils → Great Oxidation Event → end-Permian extinction → Cambrian explosion

Great Oxidation Event → Cambrian explosion → oldest eukaryotic fossils → end-Permian extinction

Great Oxidation Event → oldest eukaryotic fossils → Cambrian explosion → end-Permian extinction

Oldest eukaryotic fossils → Cambrian explosion → Great Oxidation Event → end-Permian extinction

Cave-dwelling populations of a fish species have reduced, non-functional eyes. Which explanation is Lamarckian rather than Darwinian or neutral?

Eyes shrank in each generation through disuse in the dark, and the reduced eyes were inherited

Fish that invested less energy in costly eye tissue left more offspring, so alleles reducing the eye spread

Without selection to maintain vision, mutations that disrupt eye genes accumulated by genetic drift

Alleles that reduce the eye are linked to alleles favoured in caves and rose in frequency with them

Eyes shrank in each generation through disuse in the dark, and the reduced eyes were inherited

In a diploid population of constant size N, neutral mutations arise at a rate μ per gene copy per generation. According to the neutral theory, the rate at which neutral substitutions become fixed in the population, per generation, is —

μ, independent of population size

2Nμ, proportional to population size

μ/2N, inversely proportional to population size

4Nμ, the scaled mutation rate of the population

Which of the following observations are consistent with the neutral theory of molecular evolution?

Synonymous sites in protein-coding genes accumulate substitutions faster than non-synonymous sites

Pseudogenes, which no longer encode functional products, evolve at some of the highest rates in the genome

Histone H4, whose function tolerates almost no change, evolves very slowly

Synonymous sites in protein-coding genes accumulate substitutions faster than non-synonymous sites

Pseudogenes, which no longer encode functional products, evolve at some of the highest rates in the genome

Histone H4, whose function tolerates almost no change, evolves very slowly

Species with larger populations accumulate neutral substitutions faster per generation

In a population of seed-eating birds, the available seeds are either very small or very large and hard, and birds with intermediate bills feed poorly on both. Over several generations bill size becomes bimodal. Which mode of selection is acting?

Disruptive selection, which favours both extremes over intermediates

Stabilising selection, which favours intermediates and reduces variance

Directional selection, which shifts the mean towards one extreme

Disruptive selection, which favours both extremes over intermediates

Sexual selection, which favours traits that raise mating success

A harmless, palatable hoverfly carries yellow-and-black bands like those of a stinging wasp. Field studies show that the hoverfly gains less protection from birds where it is much more common than the wasp. Which phenomenon does this illustrate?

Batesian mimicry, whose benefit falls as the mimic becomes common relative to its model

Müllerian mimicry, in which two defended species share the cost of educating predators

Aposematism by the hoverfly, whose colours honestly advertise its own unpalatability

Crypsis, in which the bands break up the outline of the fly against flowers and leaves

Batesian mimicry, whose benefit falls as the mimic becomes common relative to its model

Which of the following statements about adaptation are correct?

Feathers, which arose before flight and were later used for it, are an example of exaptation

The dark form of the peppered moth increased in soot-darkened woods and declined after air pollution fell

Feathers, which arose before flight and were later used for it, are an example of exaptation

The dark form of the peppered moth increased in soot-darkened woods and declined after air pollution fell

Any trait that is common in a population must have been favoured by natural selection

Insecticide resistance appears because exposure to the insecticide induces protective mutations

In crosses between two closely related bird species, the F1 hybrids of one sex are sterile while those of the other sex are fertile. According to Haldane's rule, which sex is expected to be sterile?

Females, because in birds the female is the heterogametic sex (ZW)

Males, because hybrid males are the sterile sex in every animal group

Males, because in birds the male carries two different sex chromosomes

Either sex with equal likelihood, because birds lack sex chromosomes

Two populations of a fruit fly live in the same orchards. One lays eggs on apples and the other on hawthorn fruits; flies mate on the kind of fruit on which they developed, and apples ripen several weeks earlier than hawthorn fruits. Hybrids are viable and fertile in the laboratory. Which interpretation fits best?

Sympatric divergence driven by a host shift, with habitat and temporal isolation acting before mating

Allopatric speciation, because the two hosts form separate islands of habitat that act as geographic barriers

Sympatric divergence driven by a host shift, with habitat and temporal isolation acting before mating

Speciation completed by postzygotic isolation, since hybrids would be sterile if they formed in the orchards

Parapatric speciation, because the populations occupy adjacent zones along an environmental gradient

Which of the following are prezygotic isolating barriers?

Two frog species breed in the same pond but in different months of the year

Sperm of one sea-urchin species cannot bind to the egg coat of a related species

Two frog species breed in the same pond but in different months of the year

Sperm of one sea-urchin species cannot bind to the egg coat of a related species

Horse–donkey hybrids develop normally but are sterile

F1 hybrids are fertile, but many of their offspring are weak and sterile

Evolution: Origin and History of Life, Theories, Natural Selection, Adaptation and Speciation

This chapter answers Section XL4.2 of XL4 Zoology, one of the five optional sections of the Life Sciences (XL) paper: “Origin and history of life on earth, theories of evolution, natural selection, adaptation, speciation.” It teaches the chemical origin of life and the RNA world, the major events of the geological record with their approximate ages, the theories from Lamarck and Darwin to the modern synthesis and neutral theory, the arithmetic of fitness and selection, what does and does not count as an adaptation, and how reproductive isolation and new species arise. The numericals are relative fitness and selection coefficients, the change in allele frequency under selection, the equilibrium under heterozygote advantage, and divergence times from a molecular clock. All ages are approximate and are written with “about”.

1. Origin of life

Earth formed about 4.5 billion years ago, and the oldest widely accepted traces of life, stromatolites built by microbial mats and microfossils, are about 3.5 billion years old. In the 1920s Oparin and Haldane independently proposed that life arose by chemical evolution: in an early atmosphere with little or no free oxygen and rich in reducing gases, energy from lightning and ultraviolet light formed simple organic molecules that accumulated in the sea as a “prebiotic soup”. In 1953 Stanley Miller, working with Harold Urey, circulated CH₄, NH₃, H₂ and water vapour past electric sparks; within a week the condensed water held amino acids such as glycine and alanine. Many geochemists now think the early atmosphere was less reducing (CO₂ and N₂), which turns attention to settings such as hydrothermal vents and mineral surfaces. Amino acids in meteorites such as Murchison show that such synthesis also happens in space.

From monomers the path runs to polymers (concentrated by evaporation or adsorbed on clays), to compartments (Oparin's coacervates, Fox's proteinoid microspheres, self-assembling lipid vesicles) and finally to a replicating system. The RNA world hypothesis holds that RNA came first because it can both store information and catalyse reactions. The evidence: ribozymes such as self-splicing introns and RNase P are catalysts; the peptide bond in the ribosome is made by rRNA; coenzymes such as NAD⁺, FAD and coenzyme A carry nucleotide parts; and deoxyribonucleotides are still made from ribonucleotides. DNA later took over storage because it is chemically more stable, and proteins took over most catalysis because twenty amino acids offer more chemistry than four bases.

⚠️ Exam trap

Miller's experiment produced amino acids, not proteins, nucleic acids or cells, and it shows what a reducing atmosphere could yield, not that the early atmosphere was reducing. Panspermia, seeding of Earth from space, does not explain the origin of life; it only moves it elsewhere.

2. History of life: the geological record

Ages come from radiometric dating: a parent isotope decays with a fixed half-life, so the fraction left after n half-lives is (1/2)ⁿ. Carbon-14 (half-life about 5,730 years) dates material up to roughly 50,000 years old; uranium–lead and potassium–argon date rocks billions of years old.

Landmarks in the history of life (all ages approximate)
About how long ago	Event
4.5 billion years	Earth forms
3.5 billion years	Oldest widely accepted fossils: stromatolites and prokaryotic microfossils
2.4 billion years	Great Oxidation Event: oxygen from cyanobacterial photosynthesis accumulates in the atmosphere
1.8 billion years	Oldest widely accepted eukaryotic fossils
570 million years	Ediacaran biota: large soft-bodied multicellular organisms
540 million years	Cambrian explosion: most animal phyla with hard parts appear in a geologically short time
375 million years	Devonian lobe-finned fishes give rise to tetrapods (Tiktaalik)
252 million years	End-Permian extinction, the largest; ends the Palaeozoic
230 to 200 million years	Triassic: first dinosaurs and, late in the period, first mammals
150 million years	Jurassic Archaeopteryx, a feathered dinosaur with bird features
66 million years	End-Cretaceous extinction after an asteroid impact removes non-avian dinosaurs; mammals radiate
300,000 years	Homo sapiens in Africa

The Phanerozoic has three eras. The Palaeozoic (Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian) saw the rise of animal phyla, fishes (the Devonian is the “Age of Fishes”), land plants, amphibians and the first amniotes. The Mesozoic (Triassic, Jurassic, Cretaceous) was the age of reptiles, with the first mammals, birds and flowering plants. The Cenozoic (Paleogene, Neogene, Quaternary) is the age of mammals. Five mass extinctions stand out: end-Ordovician, Late Devonian, end-Permian, end-Triassic and end-Cretaceous. Each emptied niches that survivors filled by adaptive radiation.

📖 Fingerprints of endosymbiosis

Mitochondria and plastids have two membranes, their own circular DNA, and ribosomes that resemble bacterial ribosomes and are inhibited by antibiotics such as chloramphenicol; they divide by fission and cannot be made afresh by the cell. Their rRNA places mitochondria among the α-proteobacteria and plastids among the cyanobacteria. Most of their original genes have since moved to the nucleus.

3. Theories of evolution and the molecular clock

Theories of evolution
Theory	Core claim	Verdict
Lamarckism (Lamarck, 1809)	Organs grow with use and shrink with disuse, and such acquired characters are inherited	Rejected: Weismann's germ-plasm theory separates soma from germ line; mice whose tails were cut for several generations still bore tailed young
Darwinism (Darwin and Wallace, 1858; On the Origin of Species, 1859)	Overproduction, heritable variation and the struggle for existence let the better-suited leave more offspring; all life shares common descent	Core accepted; Darwin lacked a correct theory of heredity
Mutation theory (de Vries, early 1900s)	New species arise in one step by large mutations (Oenothera lamarckiana)	His “mutations” were mostly chromosomal changes; speciation is rarely a single jump
Modern synthesis (1930s–1940s: Fisher, Haldane, Wright, Dobzhansky, Mayr, Simpson)	Evolution is change in allele frequencies by mutation, selection, drift and gene flow; Mendelian genetics supplies the variation	The framework of present-day evolutionary biology
Neutral theory (Kimura, 1968)	Most molecular differences are selectively neutral and fixed by drift; neutral substitution rate = neutral mutation rate	Explains the molecular clock; selection still shapes adaptive traits
Punctuated equilibrium (Eldredge and Gould, 1972)	Long stasis broken by rapid change concentrated at speciation	Contrasted with phyletic gradualism; both patterns occur

The evidence for descent with modification is concordant: transitional fossils (Tiktaalik, Archaeopteryx); homologous and vestigial structures (the pentadactyl limb; the human coccyx); pharyngeal arches in all vertebrate embryos; island species resembling those of the nearest mainland; and molecular homology, from the near-universal genetic code to sequence differences (as in cytochrome c) that match the tree built from anatomy.

Molecular clock. If substitutions accumulate at a steady rate r per site per year in each lineage, the distance between two species that split T years ago is K = 2rT, because both lineages change after the split; hence T = K/(2r). Worked example: K = 0.1 substitutions per site and r = 1 × 10⁻⁹ per site per year give T = 0.1/(2 × 10⁻⁹) = 50 million years. Rates differ between genes and lineages (faster in species with short generations), so a clock is calibrated with dated fossils and tested, for example by a relative-rate test against an outgroup.

⚠️ Exam trap

Lamarck was wrong about inheritance, not about the environment mattering: selection also works through the environment, but changes acquired by the body during life are not written into the germ line. “Fittest” means leaving the most surviving offspring, not being the strongest.

4. Natural selection: fitness and allele-frequency change

Natural selection needs variation, heritability and differential reproductive success. It acts on phenotypes but changes genotype and allele frequencies. Absolute fitness W of a genotype is its mean number of offspring that survive to breed, the product of survival and fecundity. Relative fitness w = W/W_max sets the best genotype to 1, and the selection coefficient s = 1 − w measures its disadvantage. Example: if AA, Aa and aa survive to adulthood with probabilities 0.8, 0.8 and 0.5 and lay 50, 50 and 40 eggs, then W = 40, 40 and 20, w = 1, 1 and 0.5, and s = 0.5 against aa.

Allele-frequency change. Multiply each Hardy–Weinberg genotype frequency by its fitness and divide by the mean fitness w̄ = p²w_AA + 2pq·w_Aa + q²w_aa. For selection against a recessive (fitnesses 1, 1, 1 − s), w̄ = 1 − sq² and q′ = (pq + q²(1 − s))/w̄, so Δq = −sq²(1 − q)/(1 − sq²). Worked example: q = 0.4 and s = 0.5. Genotype frequencies are 0.36, 0.48 and 0.16; w̄ = 1 − 0.5 × 0.16 = 0.92; q′ = (0.24 + 0.08)/0.92 = 0.348, so Δq = −0.052 in one generation. As q falls, most copies of a hide in heterozygotes where selection cannot see them, and the decline slows. For a recessive lethal (s = 1), q_t = q₀/(1 + tq₀), so the generations needed are 1/q_t − 1/q₀: halving q from 0.02 to 0.01 takes 50 generations.

Modes of selection
Mode	What is favoured	Example
Directional	One extreme; the mean shifts	Industrial melanism; insecticide resistance
Stabilising	The intermediate; variance falls	Human birth weight, with infant survival highest at intermediate weights
Disruptive	Both extremes over intermediates; can split a population	Black-bellied seedcracker finches, whose large- and small-billed birds each crack a different seed type
Balancing	Keeps two or more alleles	Heterozygote advantage; negative frequency-dependent selection
Sexual	Traits that raise mating success	Male combat with antlers (intrasexual); female choice of the peacock's train (intersexual)

Heterozygote advantage gives a stable polymorphism. With fitnesses 1 − s (AA), 1 (Aa) and 1 − t (aa), the equilibrium frequency of a is q̂ = s/(s + t), whatever the starting frequency. In malarial regions HbA/HbS heterozygotes resist severe Plasmodium falciparum malaria, HbA/HbA people suffer more malaria and HbS/HbS people have sickle-cell anaemia, so HbS persists at high frequency. For quantitative traits the response to selection is R = h²S, where S is the selection differential and h² the narrow-sense heritability.

5. Adaptation

An adaptation is a heritable trait that raises fitness in a given environment and was shaped by natural selection for its present role. Studied cases: industrial melanism in the peppered moth Biston betularia, whose dark (carbonaria) form rose to high frequency in soot-darkened woods of industrial Britain because birds took more of the conspicuous pale moths, and declined again after clean-air laws; bill depth in Darwin's ground finches, which rose after a drought on the Galápagos left mainly large, hard seeds; and DDT resistance in mosquitoes. In each case selection acts on variants already present; the agent does not induce the needed mutation.

Protective adaptations. Crypsis hides an animal against its background; aposematism advertises unpalatability with conspicuous warning colours. In Batesian mimicry a palatable species (hoverflies) copies a defended model (wasps); the benefit depends on predators meeting the model more often, so it falls as the mimic becomes common, a case of negative frequency dependence. In Müllerian mimicry two or more defended species (several Heliconius butterflies) share one warning pattern and so share the cost of educating predators.

Not every trait is an adaptation. A trait may be a by-product of another feature, may have been fixed by drift, or may reflect historical constraint: the vertebrate retina is wired with its nerve fibres in front of the photoreceptors, leaving a blind spot. An exaptation evolved for one role and was later co-opted for another; feathers, which arose before flight for insulation or display, are the standard case. Convergent evolution of one solution in unrelated lineages (the streamlined shape of sharks, ichthyosaurs and dolphins) is strong evidence that a trait is adaptive. Adaptive radiation is the rapid diversification of one lineage into many niches: Darwin's finches, Hawaiian honeycreepers and the cichlid fishes of the African Great Lakes.

6. Speciation

Under the biological species concept a species is a group of interbreeding natural populations reproductively isolated from other such groups, so speciation is the evolution of reproductive isolation. Barriers act before or after a zygote forms.

Reproductive isolating barriers
Barrier	Stage	Example
Habitat	Prezygotic	Populations in one area use different habitats and rarely meet
Temporal	Prezygotic	Related species breed in different seasons or at different times of day
Behavioural	Prezygotic	Species-specific courtship songs, displays or firefly flash patterns
Mechanical	Prezygotic	Incompatible genitalia in many insects
Gametic	Prezygotic	Sea-urchin sperm bindin binds only eggs of its own species
Hybrid inviability	Postzygotic	Hybrid embryos die during development
Hybrid sterility	Postzygotic	The mule (horse 2n = 64, donkey 2n = 62, mule 2n = 63) cannot complete normal meiosis
Hybrid breakdown	Postzygotic	F1 hybrids are fertile but F2 or backcross progeny are weak or sterile

Allopatric speciation starts with a geographic barrier. In vicariance the barrier divides a widespread population, as when the Isthmus of Panama, which closed about 3 million years ago, split marine animals into Pacific and Caribbean sister species. In peripatric speciation a small founder population at the edge of the range diverges fast through drift and new selection, as on islands. In parapatric speciation adjacent populations along an environmental gradient diverge despite some gene flow across a hybrid zone. Sympatric speciation needs no geographic separation: by polyploidy (common in plants, rare in animals), by a shift to a new host, as in the apple and hawthorn races of the fly Rhagoletis pomonella, which mate on their own host fruit, or by mate choice, as among cichlids. Where incompletely isolated populations meet and hybrids are unfit, selection strengthens prezygotic isolation: reinforcement.

⚠️ Haldane's rule

When in F1 hybrids one sex is absent, rare or sterile, it is the heterogametic sex. That is the male in mammals and Drosophila (XY) but the female in birds and butterflies (ZW), so “hybrid males are sterile” is right only for male-heterogametic groups.

Key takeaways

Life arose by chemical evolution at least about 3.5 billion years ago; Miller–Urey made amino acids under reducing conditions, and ribozymes and rRNA catalysis of the peptide bond support an RNA world.
Approximate ages: Earth 4.5 billion years, first fossils 3.5 billion, Great Oxidation 2.4 billion, eukaryotes 1.8 billion, Cambrian explosion 540 million, end-Permian 252 million and end-Cretaceous 66 million years ago.
Inheritance of acquired characters failed; natural selection, joined to Mendelian genetics in the modern synthesis, stands. Neutral theory makes the substitution rate equal the neutral mutation rate, the basis of the molecular clock T = K/(2r).
w = W/W_max and s = 1 − w; after selection q′ is the fitness-weighted share of a divided by w̄. Selection against a rare recessive is slow, and heterozygote advantage holds q̂ = s/(s + t).
Adaptations are selected heritable traits, not responses to need; distinguish exaptations, by-products and constraints. Batesian mimicry is frequency-dependent; Müllerian mimicry shares the cost among defended species.
Speciation is the evolution of reproductive isolation through prezygotic and postzygotic barriers, in allopatric, peripatric, parapatric or sympatric settings; reinforcement and Haldane's rule are the classic exam points.

Practice questions (18)

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.

In the 1953 experiment of Miller and Urey, electric sparks were passed through a mixture of CH₄, NH₃, H₂ and water vapour. What did the experiment produce?
1. Amino acids such as glycine and alanine, formed from simple gases in a closed apparatus
2. Short self-replicating RNA molecules that copied themselves in the condensed water
3. Membrane-bound coacervate droplets carrying out simple metabolism with enzymes
4. Polypeptide chains of about a hundred residues joined by peptide bonds in solution
Show answer
Answer: A — Amino acids such as glycine and alanine, formed from simple gases in a closed apparatus
The spark-discharge apparatus yielded simple organic monomers, notably amino acids such as glycine and alanine, showing that such molecules form abiotically under reducing conditions. It did not produce self-replicating RNA, protocells or long polypeptides; polymerisation and compartments are later steps studied in other experiments such as those on coacervates and proteinoid microspheres.
Which observation gives the strongest support to the hypothesis that RNA preceded both DNA and proteins in early life?
1. DNA is more stable than RNA because its sugar lacks the 2′-hydroxyl group
2. The peptidyl transferase activity of the ribosome resides in its rRNA
3. Amino acids form readily when sparks pass through a reducing gas mixture
4. Mitochondria contain circular DNA and divide by a process resembling fission
Show answer
Answer: B — The peptidyl transferase activity of the ribosome resides in its rRNA
The RNA world needs a molecule that both carries information and catalyses; the fact that the ribosome's peptide-bond-forming centre is RNA is a molecular fossil of that stage. DNA's greater stability explains why DNA later took over storage, which is the tempting answer, but it says nothing about RNA coming first. Spark-discharge synthesis concerns monomers, and mitochondrial DNA is evidence for endosymbiosis.
Which of the following observations support the endosymbiotic origin of mitochondria?
1. Mitochondria carry their own circular DNA molecule
2. Mitochondrial ribosomes resemble bacterial ribosomes and are inhibited by chloramphenicol
3. Mitochondrial rRNA sequences are most similar to those of α-proteobacteria
4. The mitochondrial genome encodes every protein the organelle needs
Show answer
Answer: A — Mitochondria carry their own circular DNA molecule; B — Mitochondrial ribosomes resemble bacterial ribosomes and are inhibited by chloramphenicol; C — Mitochondrial rRNA sequences are most similar to those of α-proteobacteria
Circular genomes, bacteria-like ribosomes sensitive to chloramphenicol and an rRNA affinity with α-proteobacteria all point to a bacterial ancestor. The claim that the organelle encodes all its own proteins is false: most mitochondrial proteins are encoded in the nucleus and imported, because most genes of the endosymbiont moved to the nuclear genome.
Which sequence lists these events from the oldest to the most recent?
1. Oldest eukaryotic fossils → Great Oxidation Event → end-Permian extinction → Cambrian explosion
2. Great Oxidation Event → Cambrian explosion → oldest eukaryotic fossils → end-Permian extinction
3. Great Oxidation Event → oldest eukaryotic fossils → Cambrian explosion → end-Permian extinction
4. Oldest eukaryotic fossils → Cambrian explosion → Great Oxidation Event → end-Permian extinction
Show answer
Answer: C — Great Oxidation Event → oldest eukaryotic fossils → Cambrian explosion → end-Permian extinction
The Great Oxidation Event was about 2.4 billion years ago, the oldest widely accepted eukaryotic fossils are about 1.8 billion years old, the Cambrian explosion was about 540 million years ago and the end-Permian extinction about 252 million years ago. Placing eukaryotes before the oxygenation of the atmosphere is the commonest slip.
Which mass extinction marks the boundary between the Palaeozoic and Mesozoic eras?
1. The end-Cretaceous extinction
2. The end-Permian extinction
3. The end-Ordovician extinction
4. The Late Devonian extinction
Show answer
Answer: B — The end-Permian extinction
The end-Permian extinction, about 252 million years ago and the largest of the five, closes the Palaeozoic and opens the Mesozoic. The end-Cretaceous extinction, about 66 million years ago, is the tempting answer but marks the Mesozoic–Cenozoic boundary. The end-Ordovician and Late Devonian events fall inside the Palaeozoic.
Cave-dwelling populations of a fish species have reduced, non-functional eyes. Which explanation is Lamarckian rather than Darwinian or neutral?
1. Fish that invested less energy in costly eye tissue left more offspring, so alleles reducing the eye spread
2. Without selection to maintain vision, mutations that disrupt eye genes accumulated by genetic drift
3. Alleles that reduce the eye are linked to alleles favoured in caves and rose in frequency with them
4. Eyes shrank in each generation through disuse in the dark, and the reduced eyes were inherited
Show answer
Answer: D — Eyes shrank in each generation through disuse in the dark, and the reduced eyes were inherited
Inheritance of a change acquired through disuse during life is Lamarck's mechanism. The energy-saving explanation is Darwinian selection on heritable variation, the drift explanation is neutral evolution under relaxed selection, and the linkage explanation is indirect selection by hitchhiking; all three involve changes in allele frequencies, not inheritance of acquired characters.
In a diploid population of constant size N, neutral mutations arise at a rate μ per gene copy per generation. According to the neutral theory, the rate at which neutral substitutions become fixed in the population, per generation, is —
1. μ, independent of population size
2. 2Nμ, proportional to population size
3. μ/2N, inversely proportional to population size
4. 4Nμ, the scaled mutation rate of the population
Show answer
Answer: A — μ, independent of population size
Each generation 2Nμ new neutral mutations arise among the 2N gene copies, and each has a probability 1/(2N) of drifting to fixation, so the substitution rate is 2Nμ × 1/(2N) = μ. Population size cancels, which is why neutral molecular change can tick like a clock. 2Nμ is tempting because it is the number of new mutations per generation, not the number fixed; 4Nμ is the parameter θ that governs neutral heterozygosity.
Which of the following observations are consistent with the neutral theory of molecular evolution?
1. Synonymous sites in protein-coding genes accumulate substitutions faster than non-synonymous sites
2. Pseudogenes, which no longer encode functional products, evolve at some of the highest rates in the genome
3. Histone H4, whose function tolerates almost no change, evolves very slowly
4. Species with larger populations accumulate neutral substitutions faster per generation
Show answer
Answer: A — Synonymous sites in protein-coding genes accumulate substitutions faster than non-synonymous sites; B — Pseudogenes, which no longer encode functional products, evolve at some of the highest rates in the genome; C — Histone H4, whose function tolerates almost no change, evolves very slowly
Neutral theory predicts that the less constrained a site, the larger the share of its mutations that are neutral and the faster it evolves: hence fast synonymous sites and pseudogenes and a slow, highly constrained histone H4. The neutral substitution rate equals the neutral mutation rate and does not depend on population size, so faster neutral substitution in larger populations contradicts the theory.
In a randomly mating population at Hardy–Weinberg proportions, the frequency of a recessive allele a is 0.3. The relative fitnesses of AA, Aa and aa are 1, 1 and 0.6. After one generation of selection, the frequency of a is ____ (to three decimal places).
Numerical answer — type the value.
Show answer
Answer: 0.274
p = 0.7 and q = 0.3, so the genotype frequencies are AA 0.49, Aa 0.42 and aa 0.09. Mean fitness w̄ = 0.49 + 0.42 + 0.09 × 0.6 = 0.964. After selection q′ = (½ × 0.42 + 0.09 × 0.6)/0.964 = (0.21 + 0.054)/0.964 = 0.264/0.964 = 0.274. Forgetting to divide by w̄ gives 0.264.
In an insect population, genotypes AA, Aa and aa survive to adulthood with probabilities 0.9, 0.9 and 0.6, and surviving females lay on average 40, 40 and 35 eggs respectively. Taking absolute fitness as survival × fecundity, the selection coefficient against aa is ____ (to two decimal places).
Numerical answer — type the value.
Show answer
Answer: 0.42
Absolute fitness: AA = 0.9 × 40 = 36, Aa = 36, aa = 0.6 × 35 = 21. Relative fitness of aa = 21/36 = 0.583, so s = 1 − 0.583 = 0.417, or 0.42 to two decimal places. Using survival alone gives s = 1 − 0.6/0.9 = 0.33, which ignores the fecundity component of fitness.
At a locus showing heterozygote advantage, the relative fitnesses of AA, Aa and aa are 0.8, 1.0 and 0.5. The equilibrium frequency of allele a maintained by selection is ____ (to two decimal places).
Numerical answer — type the value.
Show answer
Answer: 0.29
With fitnesses 1 − s (AA), 1 (Aa) and 1 − t (aa), the stable equilibrium is q̂ = s/(s + t), where q is the frequency of a. Here s = 0.2 and t = 0.5, so q̂ = 0.2/0.7 = 0.286, or 0.29. Writing t/(s + t) = 0.71 gives the frequency of A instead, the usual mix-up: the allele whose homozygote is less fit ends up rarer.
A gene differs between two mammal species by K = 0.08 substitutions per site. Assume a constant rate of r = 2 × 10⁻⁹ substitutions per site per year in each lineage, so that the divergence time is T = K/(2r). The two species diverged ____ million years ago.
Numerical answer — type the value.
Show answer
Answer: 20
Both lineages accumulate substitutions after the split, so K = 2rT and T = K/(2r) = 0.08/(2 × 2 × 10⁻⁹) = 0.08/(4 × 10⁻⁹) = 2 × 10⁷ years = 20 million years. Dividing by r alone, as if only one lineage had changed, gives 40 million years.
In a population of seed-eating birds, the available seeds are either very small or very large and hard, and birds with intermediate bills feed poorly on both. Over several generations bill size becomes bimodal. Which mode of selection is acting?
1. Stabilising selection, which favours intermediates and reduces variance
2. Directional selection, which shifts the mean towards one extreme
3. Disruptive selection, which favours both extremes over intermediates
4. Sexual selection, which favours traits that raise mating success
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Answer: C — Disruptive selection, which favours both extremes over intermediates
When both extremes have higher fitness than intermediates, selection is disruptive and the distribution becomes bimodal. Stabilising selection is the tempting opposite: it acts against extremes. Directional selection would shift the whole distribution one way, and nothing in the scenario concerns mate choice.
A harmless, palatable hoverfly carries yellow-and-black bands like those of a stinging wasp. Field studies show that the hoverfly gains less protection from birds where it is much more common than the wasp. Which phenomenon does this illustrate?
1. Müllerian mimicry, in which two defended species share the cost of educating predators
2. Aposematism by the hoverfly, whose colours honestly advertise its own unpalatability
3. Crypsis, in which the bands break up the outline of the fly against flowers and leaves
4. Batesian mimicry, whose benefit falls as the mimic becomes common relative to its model
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Answer: D — Batesian mimicry, whose benefit falls as the mimic becomes common relative to its model
A palatable mimic of a defended model is a Batesian mimic. Predators learn to avoid the pattern from unpleasant encounters with the model; where mimics are common, birds meet them more often and learn less avoidance, so protection falls (negative frequency dependence). Müllerian mimicry is the tempting alternative, but it needs both species to be defended. The hoverfly is harmless, so its colours are not honest warning, and bands that attract attention are not crypsis.
Which of the following statements about adaptation are correct?
1. Feathers, which arose before flight and were later used for it, are an example of exaptation
2. The dark form of the peppered moth increased in soot-darkened woods and declined after air pollution fell
3. Any trait that is common in a population must have been favoured by natural selection
4. Insecticide resistance appears because exposure to the insecticide induces protective mutations
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Answer: A — Feathers, which arose before flight and were later used for it, are an example of exaptation; B — The dark form of the peppered moth increased in soot-darkened woods and declined after air pollution fell
Feathers are the classic exaptation, and the rise and fall of melanic Biston betularia tracked pollution through bird predation. A common trait may be a by-product, a result of drift or a historical constraint, so commonness alone does not prove adaptation. Resistance alleles are present before exposure; the insecticide selects them but does not create them.
In crosses between two closely related bird species, the F1 hybrids of one sex are sterile while those of the other sex are fertile. According to Haldane's rule, which sex is expected to be sterile?
1. Females, because in birds the female is the heterogametic sex (ZW)
2. Males, because hybrid males are the sterile sex in every animal group
3. Males, because in birds the male carries two different sex chromosomes
4. Either sex with equal likelihood, because birds lack sex chromosomes
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Answer: A — Females, because in birds the female is the heterogametic sex (ZW)
Haldane's rule names the heterogametic sex, which in birds is the ZW female, as the one absent, rare or sterile in hybrids. “Hybrid males are sterile” is the tempting overgeneralisation from mammals and Drosophila, where males are XY. Male birds are ZZ (homogametic), and birds do have sex chromosomes.
Two populations of a fruit fly live in the same orchards. One lays eggs on apples and the other on hawthorn fruits; flies mate on the kind of fruit on which they developed, and apples ripen several weeks earlier than hawthorn fruits. Hybrids are viable and fertile in the laboratory. Which interpretation fits best?
1. Allopatric speciation, because the two hosts form separate islands of habitat that act as geographic barriers
2. Sympatric divergence driven by a host shift, with habitat and temporal isolation acting before mating
3. Speciation completed by postzygotic isolation, since hybrids would be sterile if they formed in the orchards
4. Parapatric speciation, because the populations occupy adjacent zones along an environmental gradient
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Answer: B — Sympatric divergence driven by a host shift, with habitat and temporal isolation acting before mating
The populations share one geographic area, and divergence follows a shift to a new host. Mating on the natal host (habitat isolation) and the earlier ripening of apples (temporal isolation) are prezygotic barriers, so this is sympatric divergence in progress. Fertile laboratory hybrids rule out postzygotic isolation as the barrier; trees in one orchard are not geographic barriers, and there is no spatial gradient of the kind parapatry requires.
Which of the following are prezygotic isolating barriers?
1. Two frog species breed in the same pond but in different months of the year
2. Sperm of one sea-urchin species cannot bind to the egg coat of a related species
3. Horse–donkey hybrids develop normally but are sterile
4. F1 hybrids are fertile, but many of their offspring are weak and sterile
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Answer: A — Two frog species breed in the same pond but in different months of the year; B — Sperm of one sea-urchin species cannot bind to the egg coat of a related species
Temporal isolation and gametic incompatibility (species-specific sperm–egg recognition through bindin) act before a zygote forms, so they are prezygotic. Mule sterility is hybrid sterility, and weak or sterile offspring of fertile F1 hybrids are hybrid breakdown; both are postzygotic.