A4.1 Evolution and speciation. Practice questions with markscheme.

• B: the bones correspond one to one in the same arrangement despite serving different functions, which is best explained by inheritance from a common ancestor with a pentadactyl limb;

• (a) [2]
  • not (mainly) heritable / mainly caused by the environment (at the alpine site);
  • when grown in the same conditions the two populations reach similar heights / 61 cm and 58 cm;
  • the difference falls from 47 cm in the wild to 3 cm in the garden;

  The conclusion alone does not score: it must be supported by the garden data.

• (b) [2]
  • yes, in leaf hair density (but not in height);
  • the difference in leaf hairs remains when both are grown in the same conditions / 22 and 5 hairs mm⁻² in the garden;
  • so the difference in leaf hairs is heritable/genetic, and evolution is a change in the heritable characteristics of a population;
  • the height difference disappears in the garden, so it is acquired/not heritable and is not evidence of evolution;
• (c) [2]
  • dense leaf hairs are an advantage at high altitude, e.g. reducing water loss in wind / insulating against cold / shading from strong (UV) light;
  • plants in the alpine population varied in (heritable) leaf hair density;
  • plants with more hairs survived and reproduced more at the alpine site / selection favoured dense hairs;
  • the trait was passed to offspring and became more common over many generations;

  Accept any plausible advantage of hairy leaves at altitude for the first point.

• mutations cause base sequences to change / differences to accumulate over time (at a roughly steady rate);
• the more (sequence) differences between two species, the longer since they diverged from a common ancestor / the more distantly related they are;
• sequence similarities shared by all organisms indicate common ancestry / descent with modification;
• the pattern of differences can be used to construct evolutionary trees, which can be checked against other evidence (e.g. anatomy/fossils), OWTTE;

• (a) [3]
  • changes acquired during an organism's life (such as shrinking from disuse) are not heritable / are not passed to offspring;
  • there was heritable variation in leg size in the ancestral population (arising by mutation);
  • in burrows, individuals with smaller legs survived and reproduced more (e.g. because legs hindered movement or cost resources to grow);
  • so the heritable characteristic of smaller legs became more common and leg size fell over many generations;
  • evolution is change in the heritable characteristics of a population, not change in an individual, OWTTE;
• (b) [2]
  • homology depends on shared origin / inheritance from a common ancestor, not on function;
  • the python's femur has the same position and relationship to the pelvis as a lizard's femur, so both were inherited from a common ancestor with legs;
  • homologous structures can be modified by natural selection for different functions or (almost) none, OWTTE;
• (c) [2]
  • leg loss could have evolved independently in the two lineages / by convergent evolution;
  • similar selection pressures (e.g. burrowing or moving through dense vegetation) can produce similar features in distantly related groups, so one shared feature is not reliable evidence of close relationship;
  • compare base sequences of DNA (or amino acid sequences of proteins) of slow worms, snakes and lizards with legs: the fewest differences show the most recent common ancestor;

  Award one mark for the error (first or second point) and one for the evidence (third point): an answer that only explains the error scores 1.

• an ancestral population was split by the Congo River into two populations on opposite sides;
• neither ape can swim, so the river acts as a geographical barrier and gene flow between the two populations stopped;
• conditions differed on the two sides of the river (e.g. gorillas compete for ground vegetation north of the river but are absent south of it), so the populations faced different selection pressures;
• differential selection favoured different heritable characteristics in each population, e.g. less aggressive, female-led groups in bonobos but more aggressive, male-dominated groups in chimpanzees;
• (so) genetic differences accumulated and the populations diverged over many generations;
• eventually the populations became different enough to be separate species, OWTTE;

Accept other correct differences between the two species or their habitats for mp 3 and 4. Award any 4.

• D: base-sequence differences accumulate over time since divergence, so the fewest differences (R, 8) indicate the most recent common ancestor with P;

• B: same underlying structure, different function = homology, evidence of divergence from a common ancestor; analogous = similar function, different structure (convergence);

• C: similar (swimming) selection pressures produce similar adaptations independently; the two lineages are only distantly related;

• B: evolution is change in heritable characteristics of a population over generations; acquired (lifetime) changes are not inherited;

• D: a geographical barrier stops interbreeding, so alleles can no longer flow between the populations and differences produced by selection and drift accumulate;

• C: hybridization between *Persicaria* species followed by polyploidy produced a new, instantly isolated species; the doubled chromosome set restores pairing in meiosis, making the polyploid fertile with itself but not with its parents;

• A: in both, which individuals survive and reproduce depends on their heritable variation, so the population changes over generations; they differ in the selecting agent (the human breeder chooses which individuals breed in artificial selection, the environment determines which survive and reproduce in natural selection); the speed of change under artificial selection is evidence for evolution;

• A: adaptive radiation produces closely related species adapted to different vacant niches, which lets them coexist without competing and so increases biodiversity; B fails because species sharing one niche would compete; C fails because the species remain closely related; D fails because the species arose by divergence from a common ancestor, not by hybridization;

• (a) [3]
  • 1: geographical/geographic;
  • 2: temporal;
  • 3: behavioural;
• (b) [3]
  • plants flowering in May cannot pollinate/interbreed with plants flowering in July / there is no gene flow between the two groups;
  • so the two groups have separate gene pools;
  • mutations that arise in one group are not passed to the other;
  • differential selection (e.g. different conditions early and late in the season) / chance changes alter each group differently;
  • differences accumulate over many generations until the groups could not interbreed even if their flowering overlapped;
  • this is sympatric speciation, as there is no geographical barrier;

• (a) [2]
  • the shared features indicate descent from a common (marsupial) ancestor;
  • the differences in form show that they diverged, adapting to different ways of life/niches;
  • (this is) adaptive radiation (from one ancestral group);
  • the ancestral marsupials spread into many vacant niches in Australia / had few competitors;
• (b) [2]
  • convergent evolution;
  • they have the same way of life/niche (gliding between trees), so they faced similar selection pressures;
  • the gliding membranes evolved independently / are analogous structures (same function, different origin);
• (c) [2]
  • in Australia there were almost no placental mammals to compete with, so many niches were vacant;
  • in the Americas most niches were already occupied by placental mammals / competition limited the niches marsupials could enter;
  • more vacant niches allow more species to form and coexist without competing / more extensive adaptive radiation;