Evolution & Natural Selection

Question 1 (8 marks)

(a) Logical chain (5 marks) — 1 mark each premise, 1 for conclusion:

• Overproduction: organisms produce more offspring than can survive. (1)
• Struggle for existence: resources are limited, so competition results. (1)
• Variation: individuals in a population vary in their traits. (1)
• Heritability: at least some of this variation is heritable/passed to offspring. (1)
• Conclusion — differential survival & reproduction: individuals with favourable heritable traits survive and reproduce more; over generations these traits increase in frequency (adaptation). (1)

(b) Why no variation → no selection (3 marks):

• Selection acts by differentially favouring some variants over others (1).
• If all individuals are identical, none has a survival/reproductive advantage over another — there is nothing to "select between" (1).
• Even if some die, the survivors' offspring are no different, so no directional change in trait frequency occurs across generations (1).

Question 2 (10 marks)

(a) Classification (4 marks):

• Whale flipper / bat wing / human arm = homologous — same bone arrangement (pentadactyl limb) inherited from a common ancestor, despite different functions (2).
• Bat wing / butterfly wing = analogous — same function (flight) but different underlying structure and separate evolutionary origins (2).

(b) Which supports which (4 marks):

• Homology → divergent evolution: a single ancestral structure diverges into different forms/functions in descendant lineages, so shared structure with different function indicates common ancestry (2).
• Analogy → convergent evolution: unrelated lineages independently evolve similar features due to similar selection pressures/environments, so shared function without shared structure indicates independent origins (2).

(c) Molecular evidence (2 marks):

• True homologues show high DNA/protein sequence similarity reflecting shared ancestry; analogous structures arise from unrelated genes and show low sequence similarity (1). Molecular data thus reveals ancestry independent of morphology (1).

Question 3 (9 marks)

(a) Directional selection (4 marks):

• Type: directional selection (1).
• Only large/hard seeds remain, favouring birds with deeper, stronger beaks (1).
• Mean beak depth shifts toward larger values over generations (1); the distribution peak moves to one extreme (larger beaks) (1).

(b) Disruptive selection (3 marks):

• Type: disruptive selection (1).
• Both extremes (large and small beaks) favoured, medium beaks disadvantaged (1).
• Distribution becomes bimodal — two peaks at the extremes with a trough in the middle (1).

(c) Variation change (2 marks):

• Stabilizing selection favours the mean and eliminates extremes → variance decreases (1).
• Disruptive selection favours extremes and removes intermediates → variance increases (potentially leading to two subpopulations) (1).

Question 4 (9 marks)

(a) Derivation (3 marks):

• Random mating means allele pairs combine independently. Squaring $p+q=1$: (1) $(p+q)^2 = 1 \implies p^2 + 2pq + q^2 = 1$ (1)
• Interpretation: $p^2$ = frequency of $AA$, $2pq$ = frequency of $Aa$, $q^2$ = frequency of $aa$ (1).

(b) Calculation (4 marks):

• $q^2 = 0.16 \implies q = \sqrt{0.16} = 0.4$ (1)
• $p = 1 - q = 1 - 0.4 = 0.6$ (1)
• Heterozygote frequency $= 2pq = 2(0.6)(0.4) = 0.48$ (2)

(c) Condition (2 marks):

• HW assumes no selection (all genotypes have equal fitness) (1). If natural selection favours one genotype, its allele increases in frequency each generation, so $p$ and $q$ change and equilibrium is broken (1).

Question 5 (8 marks)

(a) Natural vs artificial (4 marks):

• Natural selection: environment/nature selects favourable heritable traits; selecting agent = environmental pressures (predators, climate, food). Example: dark peppered moths favoured in soot-polluted areas (2).
• Artificial selection: humans deliberately choose which individuals breed; selecting agent = humans. Example: dog breeds / high-yield crops selectively bred by farmers (2).

(b) Coevolution (4 marks):

• Coevolution = two species exert reciprocal selective pressure, each driving evolutionary change in the other (1).
• Example: long-tubed flower and long-tongued moth/bird (1). Deeper flowers favour pollinators with longer tongues (they access nectar) (1); longer-tongued pollinators in turn select for deeper flowers (better pollen transfer/reduced nectar theft), producing a reciprocal "arms race" (1).

Question 6 (6 marks)

(a) Adaptive radiation (4 marks):

• Definition: rapid diversification of one ancestral lineage into many species adapted to different niches (1).
• Conditions: availability of many unoccupied ecological niches (1), e.g. after colonising new/isolated territory (islands) or after mass extinction (1); different selection pressures in each niche drive divergence (e.g. Darwin's finches) (1).

(b) Vestigial structures (2 marks):

• They are reduced/functionless remnants of structures that were functional in ancestors, explained by descent with modification (1); a design argument cannot readily explain non-functional leftovers, whereas evolution predicts them as retained ancestral features (1).

[
  {"claim": "q = sqrt(0.16) = 0.4", "code": "q = sqrt(Rational(16,100)); result = (q == Rational(2,5))"},
  {"claim": "p = 1 - q = 0.6", "code": "q = Rational(2,5); p = 1 - q; result = (p == Rational(3,5))"},
  {"claim": "Heterozygote frequency 2pq = 0.48", "code": "p = Rational(3,5); q = Rational(2,5); result = (2*p*q == Rational(48,100))"},
  {"claim": "HW expansion: (p+q)**2 = p**2 + 2pq + q**2", "code": "p, q = symbols('p q'); result = (expand((p+q)**2) == p**2 + 2*p*q + q**2)"},
  {"claim": "Genotype frequencies sum to 1 for given p,q", "code": "p = Rational(3,5); q = Rational(2,5); result = (p**2 + 2*p*q + q**2 == 1)"}
]