D4.1 Natural selection. Practice questions with markscheme.

• A: survival was almost the same at 40 and 400 beetles per m² (22 % and 20 %; 71 % and 69 %), so the frost is a density-independent abiotic factor, and beetles with high antifreeze survived about three times as often, so the frost selects for the heritable trait; the proportion killed did not rise with crowding (B), a change acquired during life is not inherited (C), and death was not random with respect to the trait (D);

• A: with no predators, the only selection pressure acting on colour is female mate choice, so the increase in brightness shows that bright males had greater reproductive success; predation was absent in these ponds, colour is inherited rather than induced, and mutation rate is not affected by predators;

• C: adding nest sites alone raised the breeding population, so nest holes were a limiting resource that set the carrying capacity and pairs competed for them; food was plentiful and unchanged (A), the rapid doubling shows that more offspring were produced than could previously breed (B), and a population held below its potential by a shortage of holes means some pairs failed to obtain one (D);

• (a) [3 max]
  • the frequency of the allele (for silent wings) rises;
  • calling males are found by the flies and killed by the larvae, so many die before they have mated much (natural selection against calling);
  • silent males cannot be located by the flies, so they survive longer;
  • (so) silent males leave more offspring than calling males / have greater fitness, although each obtains only a few matings at a time;
  • the allele is heritable / is passed on to sons and daughters, so more of the next generation inherit it, OWTTE;

  The first point is needed for full marks. Do not accept 'crickets stop calling to avoid the flies' or any other change made by an individual cricket during its life.

• (b) [2 max]
  • the frequency of the allele falls (again) / calling males become more common;
  • with no fly there is no survival cost to calling, and females choose calling males, so calling males obtain more matings (sexual selection);
  • silent males have lower reproductive success / fitness, so fewer copies of the allele are passed on in each generation, OWTTE;
• (c) [2 max]
  • the allele has no effect on females, so females that carry it are not selected against and pass it on;
  • silent males still obtain some matings (by intercepting females), so their fitness is low but not zero;
  • the allele can arise again by (random) mutation of the gene;

  Beyond this subtopic. Award if given; the marks above can be earned without it.

  • D3.2 if the allele is recessive, it is hidden in heterozygous males, which can still call;

  Mark the first two reasons only.

• asexual reproduction produces offspring genetically identical to the parent / clones, so no new combinations of alleles are formed;
• (in the clonal colony) mutation is the only source of new variation, and mutations are rare / random / most are harmful or neutral;
• sexual reproduction produces new combinations of alleles in every generation (through crossing over / random orientation of chromosomes in meiosis / random fertilization), OWTTE;
• (so) a sexually produced population contains more different genotypes / phenotypes for selection to act on / a clonal colony may have no individual with an allele that would survive a change in conditions;

Do not accept 'the clones have no variation': mutation still occurs. Accept a named mechanism of meiosis or fertilization for the third point; a bare 'sexual reproduction mixes genes' needs the idea of new combinations of alleles.

• evolution is a change in the heritable characteristics of a population over generations;
• selection means that better-adapted individuals survive longer and reproduce more;
• only differences with a genetic basis (alleles) are passed to offspring via the gametes;
• characteristics acquired during an individual's lifetime (from the environment, e.g. training, injury or nutrition) are not (normally) transmitted, so however strongly they affect survival, the next generation is unchanged, OWTTE;

• individuals in a population vary in their (heritable) characteristics;
• variation arises from mutation and sexual reproduction (meiosis + fertilization);
• organisms produce more offspring than the environment can support;
• (so) there is competition / a struggle for survival;
• individuals whose characteristics suit the environment survive better and reproduce more;
• their alleles are passed on to more offspring;
• over generations the population's (heritable) characteristics change;
• named example correctly used, e.g. antibiotic resistance / peppered moth / Galápagos finch beaks;
• (selection acts on the phenotype; evolution is the resulting change in the population’s heritable characteristics), OWTTE;

• D: the difference disappears when the plants share an environment, so it was not genetic; only heritable variation can respond to selection;

• A: sexual selection favours traits that win mates (display, weaponry), explaining showy characteristics that natural (survival) selection alone would not;

• A: selection against both extremes (stabilizing selection) removes the tails of the distribution, so variation decreases while the mean stays near the optimum; a shifted mean is directional selection, two peaks is disruptive selection, and selection against the extremes cannot widen the range;

• B: with no predators, the only selection pressure acting on colour is female mate choice, so the increase in brightness shows that bright males had greater reproductive success; predation was absent in these ponds, colour is inherited rather than induced, and mutation rate is not affected by predators;

• C: the gene pool is all the genes and all their alleles present in the population, so an allele carried by heterozygotes is part of it even though it is hidden in their phenotype; a recessive allele can persist in carriers for many generations (B, C), and the gene pool belongs to the population, not to one individual (D);

• C: adding nest sites alone raised the breeding population, so nest holes were a limiting resource that set the carrying capacity and pairs competed for them; food was plentiful and unchanged (A), the rapid doubling shows that more offspring were produced than could previously breed (B), and a population held below its potential by a shortage of holes means some pairs failed to obtain one (D);

• B: q = 1 − 0.7 = 0.3, so heterozygote frequency = 2pq = 2 × 0.7 × 0.3 = 0.42;

• long-rooted plants are better adapted: they reach deeper water and survive drought better;
• (so) the better-adapted individuals live longer / grow more and produce more offspring/seeds (differential survival and reproduction);
• if root length is heritable, the offspring (tend to) inherit longer roots;
• (so) alleles for longer roots increase in frequency over generations, and the population becomes better adapted to drought, OWTTE;

• (random) mutation produces rare resistant variants (before/independently of spraying);
• spraying kills susceptible insects, so resistant ones survive and reproduce / differential survival;
• resistance alleles are inherited, so their frequency rises over generations;
• heavy use = stronger/more constant selection pressure, susceptible genotypes are removed faster;
• insects' short generation times and large numbers speed the process, OWTTE;

• strength: both changes have been observed directly, in real time, and measured (resistance %, morph frequencies), rather than only inferred from fossils;
• strength: both fit the mechanism. Heritable variation exists, a selection pressure (the antibiotic, or bird predation on the more conspicuous morph) changes survival, and the favoured type increases over generations;
• strength: the favoured type changes when the selection pressure changes (resistance rises with antibiotic use; the favoured moth form depends on bark colour), so predictions of the theory can be tested;
• strength: the observations can be / have been repeated in different places and by different scientists, OWTTE;
• limitation: in the field, other factors (e.g. other predators, migration of moths between areas) are hard to control, so the cause of the change is partly inferred from correlation;
• limitation: some early moth experiments were criticised for methodology (e.g. unnatural resting positions, release densities), showing the need for careful controls;
• limitation: each case involves one simple trait over a few decades, so extending the conclusion to complex structures over long time scales is an extrapolation;
• overall judgement: strong evidence that natural selection changes the heritable characteristics of populations, though on its own not evidence for every claim about evolution, OWTTE;

Beyond this subtopic. Award if given; the marks above can be earned without it.

• A4.1 limitation: they show change within an existing species, not the origin of new species;
• A4.1 the conclusion is corroborated by independent lines of evidence (fossils, molecular data);

Nature of Science: evaluating the strength of evidence for evolution.