Evolution & Natural Selection

Question 1 (10)

(a) (3) — Directional selection. (1) The trait mean shifts consistently toward one extreme (larger head). (1) Justification: hard-shelled prey favours only larger heads/bite force, so one phenotypic extreme is repeatedly selected, moving the average upward. (1)

(b) (4)

• Variation: Natural genetic variation existed for bite force/muscle among individuals. (1)
• Overproduction: More lizards were born than the limited (hard) food could support. (1)
• Differential survival: Individuals with greater bite force ate the hard prey more successfully, survived, and reproduced more. (1)
• Inheritance over generations: Alleles for high bite force increased in frequency, raising the population mean. (1)

(c) (3) — Homologous. (1) It is inherited from a shared ancestor (present at low frequency ~1% in a relative), indicating common ancestry rather than independent origin. (1) It is not vestigial (it is functional/newly enhanced) and not analogous (analogous implies independent evolution from unrelated ancestry). (1)

Question 2 (12)

(a) (3) — Convergent evolution. (1) Unrelated species in similar (desert) environments experience similar selective pressures (1), so natural selection independently produces similar adaptations (hopping, water conservation, camouflage). (1)

(b) (4)

• Homologous: same evolutionary origin/underlying structure, may differ in function. (1)
• Analogous: similar function/appearance but different evolutionary origin. (1)
• The long hind legs of these two are analogous (1) because they evolved independently in separate lineages for the same function (hopping). (1)

(c) (5)

• The forelimb bone arrangement (pentadactyl limb) is homologous across all mammals — inherited from a common mammalian ancestor, hence structurally similar regardless of relatedness. (2)
• Myoglobin differing by 34% reflects accumulated molecular divergence: many generations of independent mutation since their lineages split. (2)
• No contradiction: deep shared ancestry preserves the conserved skeletal plan, while neutral/independent mutations accumulate in protein sequence — large sequence divergence indicates the two are only distantly related. (1)

Question 3 (10)

(a) (2) — Coevolution. (1) Two species reciprocally exert selective pressure on each other, so evolutionary change in one drives change in the other. (1)

(b) (4)

• Moths with longer tongues reach nectar more efficiently → better fed → survive/reproduce more, so tongue length increases. (2)
• Flowers with deeper tubes force the moth's head/body to contact anthers/stigma, increasing pollination success → deeper-tubed plants leave more offspring → tube depth increases. (2)

(c) (4) — Any two, 2 marks each:

• Reduced/failed pollination → lower seed set → population decline, because the exclusive pollinator is gone. (2)
• Selective pressure for deep tubes removed → possible shift toward shorter tubes or other pollinators / or local extinction. (2)

Question 4 (10)

(a) (4) — Sexual selection. (1) Female mate choice (intersexual selection) favours long tails. (1) Long-tailed males gain more matings, so the reproductive advantage outweighs the survival disadvantage. (1) Genes for long tails increase because reproductive success (not just survival) determines fitness. (1)

(b) (3) — Artificial selection: humans deliberately choose which organisms breed for desired traits. (1) Sexual selection: the "chooser" is the females of the species, not humans; the selecting agent is natural. (1) Worked distinction: e.g. a farmer breeding the longest-tailed birds = artificial; females mating with the longest-tailed birds = sexual (natural). (1)

(c) (3) — Predator kills longest tails, females prefer long tails → both extremes disfavoured, intermediate tails favoured. (1) Distribution narrows around an intermediate optimum. (1) This is stabilizing selection. (1)

Question 5 (8)

(a) (2) — Adaptive radiation. (1) Rapid diversification of one ancestral species into many forms, each adapted to a different niche/resource. (1)

(b) (4)

• Ancestor colonises islands; populations become geographically isolated. (1)
• Each island offers different food → different selective pressures on beak shape. (1)
• Natural selection favours beaks suited to each island's food. (1)
• Over generations, isolated populations diverge into distinct forms (divergent evolution / speciation). (1)

(c) (2) — Homologous. (1) All beaks derive from the same ancestral finch beak (common ancestry), differing in form due to divergent selection. (1)

[
  {"claim":"Directional selection shifts population mean; modeled trait mean increases from 40 to 55 mm under selection",
   "code":"before=40; after=55; result = after > before"},
  {"claim":"Allele frequency for high-bite-force increases when it confers survival advantage (selection raises p)",
   "code":"p0=0.30; w_A=1.0; w_a=0.6; q0=1-p0; wbar=p0*w_A+q0*w_a; p1=p0*w_A/wbar; result = p1 > p0"},
  {"claim":"Myoglobin 34% divergence corresponds to 66% identity, consistent with distant relationship (<70% identity)",
   "code":"divergence=0.34; identity=1-divergence; result = identity < 0.70"},
  {"claim":"Stabilizing selection reduces variance: intermediate favoured lowers spread of tail length",
   "code":"var_before=Rational(25); var_after=Rational(9); result = var_after < var_before"}
]