Evolution & Natural Selection

Q1. (4 marks)

• (a) Homologous structures — structures in different species that share a common evolutionary origin (same basic anatomical/embryonic plan) but may perform different functions. (2)
• (b) Analogous structures — structures that perform similar functions and may look alike but have different evolutionary origins (arise by convergent evolution). (2) Why: Homology reflects common ancestry (divergence); analogy reflects similar selective pressures.

Q2. (6 marks) Any three, 2 marks each (1 for line + 1 for example):

• Fossil record — shows transitional forms and change over time, e.g. Archaeopteryx linking reptiles and birds.
• Comparative anatomy — homologous structures like the pentadactyl limb in mammals.
• Molecular evidence — similarity in DNA/protein sequences (e.g. cytochrome c) increases with relatedness; universal genetic code.
• (Also acceptable: embryology, biogeography, vestigial structures.)

Q3. (3 marks)

• (a) A vestigial structure is a reduced, functionless (or reduced-function) remnant of a structure that was functional in an ancestor. (1)
• (b) Two of: appendix, coccyx (tailbone), wisdom teeth, ear muscles, body hair (goosebumps). (1 each)

Q4. (4 marks) 1 mark each:

• Variation — individuals in a population differ in heritable traits.
• Overproduction — organisms produce more offspring than can survive.
• Struggle for existence / differential survival — competition for limited resources; individuals with favourable traits survive and reproduce more ("survival of the fittest").
• Inheritance — favourable traits are passed to offspring, so their frequency increases over generations.

Q5. (4 marks)

• Natural selection: environment selects individuals with favourable heritable traits; occurs without human intervention. Example: peppered moth colour change / antibiotic resistance in bacteria. (2)
• Artificial selection: humans deliberately choose which organisms breed to enhance desired traits. Example: dog breeds, crop varieties (e.g. maize from teosinte). (2)

Q6. (6 marks) 2 marks each:

• Directional selection — favours one extreme phenotype, shifting the mean in one direction (e.g. antibiotic resistance).
• Stabilizing selection — favours the intermediate phenotype, reducing variation (e.g. human birth weight).
• Disruptive (diversifying) selection — favours both extremes against the intermediate (e.g. beak sizes in a bimodal seed environment).

Q7. (4 marks)

• (a) Sexual selection — selection arising from differences in mating success; traits that improve an individual's chance of obtaining mates are favoured. (2)
• (b) Intrasexual — competition among members of the same sex (usually males) for mates, e.g. antler combat. Intersexual — mate choice, one sex chooses mates based on traits, e.g. peacock's tail. (2)

Q8. (4 marks)

• Convergent evolution: unrelated species evolve similar traits due to similar environments/selective pressures (produces analogous structures), e.g. wings of birds and bats/insects; streamlined body of sharks and dolphins. (2)
• Divergent evolution: related species become increasingly different over time from a common ancestor (produces homologous structures), e.g. Darwin's finches. (2)

Q9. (3 marks)

• (a) Adaptive radiation — rapid diversification of one ancestral species into many new species, each adapted to a different niche/habitat. (2)
• (b) Example: Darwin's finches on the Galápagos Islands / Hawaiian honeycreepers / Australian marsupials. (1)

Q10. (2 marks)

• Coevolution — the reciprocal evolutionary change between two interacting species, each acting as a selective pressure on the other. (1)
• Example: flowering plants and their pollinators; predator–prey; host–parasite. (1)

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  {"claim":"Q6 requests exactly three types of selection", "code":"types=['directional','stabilizing','disruptive']; result=(len(types)==3)"},
  {"claim":"Total marks sum to 40", "code":"marks=[4,6,3,4,4,6,4,4,3,2]; result=(sum(marks)==40)"},
  {"claim":"Paper has 10 questions", "code":"marks=[4,6,3,4,4,6,4,4,3,2]; result=(len(marks)==10)"},
  {"claim":"Darwin's core mechanism has four listed components", "code":"comp=['variation','overproduction','differential survival','inheritance']; result=(len(comp)==4)"}
]