IB Biology D4.1: Natural selection worked solutions

B. Natural selection is the greater survival and reproduction of individuals whose heritable traits suit their environment, so those traits become more common. A describes Lamarckian inheritance of acquired traits, C treats selection as fighting, and D makes mutation respond to need.

B. I and II are required. III is wrong because characteristics gained during a lifetime are not encoded in the base sequence of the gametes, so they are not heritable.

C. Selection against an extreme phenotype removes the alleles of the individuals that fail to breed. Mutation, random fertilisation and crossing over all increase variation.

B. More offspring than the habitat can support compete for the limited food and space, and many die. Natural selection and adaptation are results of that competition working on variation, not its immediate consequence, and overproduction does not cause mutation.

C. Larger lizards kept their heat better, so they survived and bred, and body size is heritable, so the allele for large size became more frequent. A and B claim that individuals change or are given a trait by the pressure, and D invents a feeding difference.

B. Females prefer long tails, so males with them mate more and leave more offspring; this is sexual selection. A, C and D give survival advantages, but a long tail reduces survival.

B. Selection acts against both extremes and favours the middle, which is stabilizing selection. Directional selection acts against one extreme and disruptive selection against the middle.

C. p=1-0.3=0.7, so 2pq=2(0.7)(0.3)=0.42. A is q^2 and D is p^2.

B. Lower survival of one genotype is natural selection, which breaks the equilibrium. Random mating, a very large population and no migration are all conditions that must hold.

B. The farmer chooses the breeding individuals, which is artificial selection. The environment is not the selecting agent, so it is not natural or sexual selection, and the choice acts on existing variation, not on mutation.

Mutation: a random change in the base sequence of a gene creates a new allele (1). Meiosis: crossing over exchanges segments between homologous chromosomes, and independent assortment sends maternal and paternal chromosomes to the gametes in different mixtures, so the gametes carry new combinations of existing alleles (1). Fertilisation: either of many gametes may fuse, which joins alleles at random (1). Result: the offspring differ from the parents and from each other, so selection has differences to act on (1). Sexual reproduction on its own produces no new alleles.

Any five of: running speed varies between individuals (1); the variation is heritable, caused by alleles (1); more offspring are produced than the habitat can support, so there is competition and predation (1); faster individuals are more likely to escape and survive (1); they reproduce more (1); they pass the alleles for speed to their offspring (1); the frequency of these alleles rises over generations, which is evolution (1). The answer keeps one trait, speed, in every link.

Salt in the soil is the selection pressure (1). The tolerance alleles arose earlier by mutation, so the variation was already present; the salt did not cause the mutation (1). Plants with the tolerance alleles survived and produced more seeds on the verge (1). They passed the alleles to their offspring, so the frequency of the tolerance alleles increased over generations (1).

(a) 9.9-9.2=0.7\,\mathrm{mm} (1). (b) Any four of: the drought is the selection pressure, removing small soft seeds (1); large hard seeds remained, and deeper beaks can open them (1); deeper-beaked birds survived and reproduced more (1); the offspring mean of 9.7\,\mathrm{mm} is higher than 9.2\,\mathrm{mm}, so beak depth is heritable (1); the frequency of the alleles for deeper beaks rose (1). The offspring mean lies between the two parent values, which is consistent with a heritable trait that also varies in the offspring.

(a) Independent variable: whether a predator is present (1). Dependent variable: percentage of males with bright fins (1). (b) Pond 2 is the control; it shows the change without the predator, so that the effect of the predator can be separated (1). (c) With the predator the percentage fell from 50 to 28, a fall of 22 percentage points; without it, the percentage rose from 50 to 71, a rise of 21 points (2). (d) Natural selection: predators remove bright males, which are conspicuous, so fewer reach breeding age and the bright-fin alleles fall (1). Sexual selection: with no predator, females choose bright males, so these males mate more and the alleles rise (1). Both pressures act on the same trait and the balance depends on the predator (1).

Females choose males with complex calls (1). These males mate more often and leave more offspring (1). The offspring inherit the alleles for the complex call, so their frequency rises (1). The trait persists while the reproductive benefit is greater than the survival cost from bats (1).

(a) A gene pool is all the genes, with all their different alleles, in a population at one time (1). (b) 500 diploid people carry 1000 copies of the gene (1); 0.15\times1000=150 copies of HbS (1). (c) Where malaria is present carriers are protected, survive and reproduce more, so HbS stays frequent (1). Populations in different places face different pressures, and an isolated population exchanges few alleles with others, so the frequencies differ (1).

(a) p+q=1 because the two alleles make up the whole gene pool (1). If gametes unite at random, the genotype frequencies are the terms of (p+q)^2, which is p^2+2pq+q^2=1 (1). (b) It allows allele frequencies to be calculated from genotype frequencies, or genotype frequencies from allele frequencies (1). (c) Observed frequencies that differ from the prediction show that one or more conditions, such as random mating, no selection, no migration, no mutation or a large population, are not being met (1). Name one condition and say how its failure changes frequencies, for example selection against aa lowers q^2 (1).

(a) On pale trunks 12 pale and 31 dark models were attacked; on dark trunks 38 pale and 14 dark models were attacked (2: one for each trunk, comparing the two colours with values). (b) Evaluation, any five: the matching colour is attacked least on both trunk types, which supports camouflage against bird predation as a selection pressure (1); each model type was tested against its matching and its non-matching background, so the design has a comparison (1); the models cannot show heritability, so the study does not show that colour is inherited (1); attacked models are not deaths of living moths and say nothing about reproduction or allele frequency (1); only 48 hours and a small number of trees were used, with no repeat, so the result may not apply generally (1); the claim should be that the data are consistent with selection by predation, not that they prove it (1).

(a) q^2=1/2500=0.0004 (1); q=\sqrt{0.0004}=0.02 (1); p=1-0.02=0.98 (1). (b) Carriers are heterozygotes: 2pq=2(0.98)(0.02)=0.0392 (1), which is about 1 in 26 because 1/0.0392=25.5 (1). (c) 0.0392\times40\,000=1568 carriers (1). (d) Two of: mating is random with respect to genotype; no selection, so survival does not differ between genotypes; no migration into or out of the population; no mutation; a very large population (2).

(a) Survival is highest at 70–90 mm and low at both extremes (1), so stabilizing selection acts against both extremes (1). (b) Allele frequency changes (1), because alleles that give extreme lengths are lost with the individuals that do not survive, even though the mean length hardly moves (1). (c) The spread of lengths narrows, so the variation falls, if length is heritable (1). (d) Any two sensible pressures, one for each end: very small lizards lose heat faster or are eaten by more predators; very large lizards are easier to see or need more food than the habitat provides (2).

(a) 8\div5000\times100=0.16\% (1). (b) Far more seeds are produced than the dune can support, so seedlings compete for scarce water and nutrients (1); heavier seeds carry more stored food, so they are more likely to establish and survive (1). (c) The next generation's mean seed mass returned to 2.1\,\mathrm{mg}, so the difference between the survivors and the others was not heritable, probably an effect of the parent's environment (1). Selection acted on the phenotype, but the allele frequencies did not change, so the population did not evolve (1).

(a) A single base is replaced by another, which is a substitution or point mutation (1). (b) Cross Hh by Hh: HH, Hh, Hh, hh, so the probability of hh is 1/4 (2). (c) Carriers are less likely to die of severe malaria, so they survive and reproduce more than HH people in regions with malaria (1); each carrier passes the allele to about half its offspring, and the extra reproduction of carriers keeps the allele in the gene pool (1); homozygous hh people are disadvantaged, so the allele does not rise to fixation (1).

(a) Resistance alleles arose earlier by random mutation, and the antibiotic did not cause them (1). The antibiotic is the selection pressure and kills non-resistant bacteria (1). Resistant bacteria survive, divide and pass the allele to their descendants, so the resistance allele becomes more frequent (1). (b) Natural selection, because there is no deliberate choice of which individuals breed, and the result is an unintended consequence of human action (1). (c) Any two: complete the full course of the prescribed antibiotic so fewer survivors remain; avoid using antibiotics when they are not needed; use a combination of antibiotics so one mutation is not enough (2).

(a) More offspring are produced than the food supply can support (1); the individuals compete, and those with the more suitable beak get more food, so they survive and breed more (1). (b) Different foods give different selection pressures, so different beak alleles become common on each island (1); little interbreeding means few alleles are exchanged, so the gene pools diverge (1); if the differences become large enough that the two populations can no longer interbreed, they are separate species (1). (c) A narrow hybrid zone indicates strong selection against individuals that are poorly matched to either environment (1).