Population Genetics & Speciation

Question 1. (4 marks)

• Principle: In a large, randomly mating population that is not subject to evolutionary forces, allele and genotype frequencies remain constant from generation to generation. (2)
  • Frequencies follow $p^2 + 2pq + q^2 = 1$, where $p+q=1$.
• Assumptions (any four, ½ each = 2): (1) No mutation; (2) No natural selection; (3) No gene flow/migration; (4) Random mating; (5) Very large (infinite) population — no genetic drift. Why: Each assumption prevents one force that would otherwise change allele frequencies.

Question 2. (4 marks) (a) $q^2 = \frac{1}{400} = 0.0025$, so $q = \sqrt{0.0025} = 0.05$. (2) (b) $p = 1 - q = 1 - 0.05 = 0.95$. (1) Heterozygotes $2pq = 2(0.95)(0.05) = 0.095$ (≈9.5%). (1) Why: $aa$ genotype frequency equals $q^2$; the rest follows from $p+q=1$ and the HW expansion.

Question 3. (4 marks) (a) Genetic drift: random change in allele frequencies from one generation to the next due to chance sampling of gametes, most pronounced in small populations. (2) (b) Gene flow: the transfer of alleles between populations through the movement (migration) of individuals or gametes, which tends to make populations genetically more similar. (2)

Question 4. (4 marks)

• Bottleneck effect: a drastic reduction in population size (e.g. by disaster) leaves a small surviving group whose allele frequencies differ from the original by chance; reduces genetic diversity. Example: northern elephant seals / cheetahs. (2)
• Founder effect: a new population is started by a few individuals carrying only a fraction (non-representative sample) of the parent population's alleles. Example: Amish population and high frequency of Ellis–van Creveld syndrome. (2) Why: Both are special cases of drift; distinction is cause (crash vs colonisation).

Question 5. (4 marks)

• Mutation is the ultimate source of new alleles / genetic variation, providing the raw material on which selection and drift act. (2)
• Weak alone: mutation rates are very low, so the change in allele frequency per generation is tiny. (1)
• Essential: without mutation there would be no new variation to be selected or to arise; other forces only redistribute existing alleles. (1)

Question 6. (4 marks)

• Biological species concept: a species is a group of actually or potentially interbreeding natural populations that produce fertile offspring and are reproductively isolated from other such groups. (2)
• Limitations (any two, 1 each): cannot apply to asexual/self-fertilising organisms; cannot apply to fossils/extinct species; difficult where hybridisation occurs; hard to test "potentially interbreeding". (2)

Question 7. (6 marks)

Award 2 for correct geographic distinction, 2 for mechanisms, 2 for examples.

Question 8. (5 marks) (a) Prezygotic: isolating mechanisms that prevent mating or fertilisation, so no zygote forms. Postzygotic: mechanisms acting after fertilisation, reducing viability or fertility of the hybrid. (2) (b) (½ each, rounded to 3):

• Temporal isolation → prezygotic
• Hybrid sterility → postzygotic
• Mechanical isolation → prezygotic
• Hybrid inviability → postzygotic (3)

Question 9. (5 marks)

• Gradualism: evolution proceeds by slow, steady, continuous accumulation of small changes; predicts many transitional forms in the fossil record. (2)
• Punctuated equilibrium: long periods of stasis (little change) interrupted by short bursts of rapid change, often at speciation; few transitional forms — sudden appearance in fossil record. (2)
• Valid comparison of rate/fossil pattern for the final mark. (1)

[
  {"claim": "q = sqrt(1/400) = 0.05", "code": "q = sqrt(Rational(1,400)); result = (q == Rational(1,20))"},
  {"claim": "p = 1 - q = 0.95", "code": "q = Rational(1,20); p = 1 - q; result = (p == Rational(19,20))"},
  {"claim": "Heterozygote frequency 2pq = 0.095", "code": "q = Rational(1,20); p = 1 - q; result = (2*p*q == Rational(19,200))"},
  {"claim": "HW genotype frequencies sum to 1", "code": "q = Rational(1,20); p = 1 - q; result = (p**2 + 2*p*q + q**2 == 1)"}
]