Population Genetics & Speciation

Question 1 (12)

(a) Pale = bb = $q^2 = 720/5000 = 0.144$. (1) $q = \sqrt{0.144} = 0.3795 \approx 0.38$. (1) $p = 1 - q = 0.62$. (1)

(b) Heterozygotes $2pq = 2(0.62)(0.38) = 0.4712$. (1) Expected number $= 0.4712 \times 5000 = 2356$ rabbits. (2)

(c) New $q^2 = 1250/5000 = 0.25 \Rightarrow q = 0.5$. (1) Frequency of b rose from 0.38 to 0.50. (1) If only allele frequency changed but genotypes still fit $p^2:2pq:q^2$, HWE proportions can hold at the new frequencies — but the change in frequency itself shows an evolutionary force is acting; population is not at long-term equilibrium/not evolving-free. Award mark for stating allele frequencies have changed → HWE assumption of no evolution violated. (1)

(d) Any two, each with mechanism (1.5 each):

• Natural selection favouring pale (or against dark) — differential survival/reproduction raises b.
• Genetic drift — random sampling change (weaker in N=5000 but possible).
• Gene flow/migration — immigration of pale-carrying individuals raises b.
• Mutation — too slow to explain this large shift (valid if explicitly rejected). Marks for correct process + correct directional effect on q. (3)

Question 2 (12)

(a) Bottleneck effect (1). Bottleneck = drastic reduction of an existing population by a catastrophe (1); founder effect = a few individuals colonise a new area starting a new population (1). Both reduce genetic diversity via sampling, but origin differs.

(b) The 60 survivors carry only a random subset of the original alleles (1); rare alleles are likely lost, and by chance some allele frequencies shift dramatically (1). Even after recovery to 30,000 the population "remembers" the reduced diversity — heterozygosity stays low because you cannot regenerate lost alleles by breeding (1).

(c) Offspring from random mating with $q(a)=0.85$, $p(A)=0.15$: $AA = p^2 = 0.0225$; $Aa = 2pq = 2(0.15)(0.85)=0.255$; $aa = q^2 = 0.7225$. (3, 1 each) (0.0225 + 0.255 + 0.7225 = 1 ✓)

(d) Introduce migrants / translocate individuals from another population → gene flow reintroduces lost alleles and raises heterozygosity ("genetic rescue") (2 for named intervention + principle), (1 for correctly linking to increased diversity/reduced inbreeding).

Question 3 (12)

(a) Confirm X and Y do not interbreed to produce fertile viable offspring in nature (no gene flow) → reproductively isolated (2).

(b) Allopatric speciation (1). Justification: populations separated by a physical barrier (dam) preventing gene flow (1); divergence accumulated independently by selection/drift in separate lakes (1); reproductive isolation arose as a by-product of geographic isolation (1).

(c) The trait shown = prezygotic, specifically behavioural isolation (courtship signals mismatched so mating does not occur) (2). Contrast with a postzygotic mechanism, e.g. hybrid inviability/sterility — here mating does occur and a zygote forms, but the hybrid dies or is sterile (2).

(d) Force/artificially fertilise eggs of X with Y sperm (or vice versa): if viable fertile offspring form, isolation is purely prezygotic (behavioural); if hybrids die or are sterile, a postzygotic barrier also exists (2).

Question 4 (12)

(a) Synapomorphy = a shared derived character inherited from a common ancestor, used to define a clade (1). Characters c1 and c2 are present in P, Q, R (but need care: c2 also absent in S — c1 groups P,Q,R,S? Check). c1 = 1 in P,Q,R,S → c1 groups all ingroup. The character grouping P, Q, R together to the exclusion of S is c2 (1). Marks: definition (1), correct identification that c2 unites P,Q,R (1), recognition c1 unites all four ingroup taxa (1).

(b) Rooted cladogram (nested by number of shared derived states):

       ┌── O
   ────┤
       │        ┌── S (c1, c5)
       └────────┤ (c1)
                │      ┌── P (c1,c2)
                └──────┤ (c1,c2)
                       │   ┌── Q (c1,c2,c3)
                       └───┤ (c1,c2,c3)
                           │  ┌── Q? 
                           └──┴── R (c1,c2,c3,c4)

Correct topology: O outgroup; S branches first within ingroup (shares only c1); then P, then Q, then R nested (each adds c3, c4). (4) — 1 for correct root, 1 for S basal, 1 for P–Q–R nested order, 1 for overall consistency.

(c) c2 is not a synapomorphy for the whole ingroup because S lacks it (1). Either S never had c2 (c2 arose after S diverged → c2 is a synapomorphy only for P+Q+R, consistent) OR S secondarily lost c2 (reversal) (1). The data best fit c2 arising once on the P+Q+R branch, so no homoplasy needed — c2 is a valid synapomorphy of P+Q+R only (1).

(d) Gradualism → R's fossils show slow continuous morphological change with many intermediates (1). Punctuated equilibrium → long unchanging (stasis) intervals interrupted by rapid change at speciation events, so few intermediates (1).

Question 5 (12)

(a) Significance: showed organic building blocks (amino acids) can form abiotically from simple gases + energy, supporting chemical/prebiotic origin of life (2). Criticism: the assumed strongly reducing atmosphere (CH₄, NH₃, H₂) may not reflect the real early-Earth atmosphere (thought more neutral, CO₂/N₂); also no self-replicating molecules produced (2).

(b) Any two (2 each): RNA can store genetic information (base sequence) and catalyse reactions (ribozymes), so one molecule does both jobs; RNA is self-replicating/can template copies; RNA is central to translation (rRNA is catalytic, tRNA), a molecular "fossil" of an RNA world.

(c) Mutation is the only process that creates new alleles — without it there'd be no variation for other forces to act on, hence "ultimate source" (2). But mutation rates are tiny, so alone it changes allele frequencies negligibly per generation; it needs selection, drift and gene flow to spread/shape the variation it supplies — hence weak as a standalone directional force (2).

[
  {"claim":"Q1a: q=sqrt(0.144)~0.38, p~0.62","code":"q=sqrt(Rational(720,5000)); p=1-q; result=(abs(float(q)-0.3795)<0.01 and abs(float(p)-0.6205)<0.01)"},
  {"claim":"Q1b: 2pq*5000 ~ 2356","code":"q=sqrt(Rational(720,5000)); p=1-q; n=2*p*q*5000; result=abs(float(n)-2356)<5"},
  {"claim":"Q1c: q new = 0.5","code":"q=sqrt(Rational(1250,5000)); result=q==Rational(1,2)"},
  {"claim":"Q2c: genotype freqs with q=0.85 sum to 1 and aa=0.7225","code":"q=Rational(85,100); p=1-q; aa=q*q; het=2*p*q; AA=p*p; result=(aa==Rational(7225,10000) and simplify(AA+het+aa)==1)"}
]