Questions on natural selection: variation, selection and allele frequency

Question 1

Evaluate the claim that natural selection is accurately summed up by the phrase 'survival of the fittest'.

Accepted Answer

B1 for the phrase: it does capture that some individuals leave more descendants than others because of differences in their phenotype, which is the core of natural selection B1 against the phrase: 'fittest' is circular unless fitness is defined independently of survival, and without a definition the phrase says only that whoever survives, survives B1 against the phrase: survival alone changes nothing; an individual that lives a long time but never reproduces contributes nothing to the next generation's gene pool, so 'survival' is the wrong word for what matters B1 against the phrase: fitness is relative to a particular population in a particular environment at a particular time, so a fixed idea of 'the fittest' does not exist independently of the conditions B1 a judgement: the phrase is a poor summary of natural selection, since it omits reproduction and invites circularity; 'differential reproduction of heritable variation' is a more accurate, if less memorable, description

Question 2

A field of crops is sprayed with a new insecticide every year, and after ten years most of the insects in that field survive the spray. Explain how this change came about.

Accepted Answer

B1 genetic variation was already present in the population, produced by random mutation, so a few insects carried an allele giving resistance before the insecticide was ever used B1 the insecticide is the selection pressure: insects without the allele are killed, while those carrying it survive B1 the survivors reproduce and pass the allele to their offspring, so a greater proportion of the next generation carries it B1 repeated over many generations the frequency of the resistance allele in the gene pool rises, and that change in allele frequency is the evolution

Question 3

Compare stabilising selection with disruptive selection, referring to which phenotypes are favoured and to what happens to the mean and to the variation.

Accepted Answer

B1 stabilising selection favours the mean phenotype and acts against both extremes, whereas disruptive selection acts against the mean and favours both extremes B1 under stabilising selection the mean is unchanged, and under disruptive selection the mean may not move either, so the mean alone does not distinguish them B1 stabilising selection reduces the variation, so the curve becomes taller and narrower, whereas disruptive selection increases it and the single peak splits into two with a trough between B1 stabilising selection is the usual mode in a stable environment, whereas disruptive selection needs two distinct resources with nothing in between, as for a finch feeding on hard seeds and on soft ones

Question 4

A recessive allele causes a metabolic condition, and 1 person in every 2500 is born with it. Assuming the Hardy-Weinberg conditions hold, calculate the frequency of the recessive allele and calculate the percentage of the population who are heterozygous carriers.

Accepted Answer

M1 the homozygous recessive genotype has frequency q², so q² = 1 ÷ 2500 = 0.0004; the square root is taken of that genotype frequency, not of 2500 A1 q = the square root of 0.0004 = 0.02 M1 p = 1 − q = 0.98, and the frequency of heterozygotes is 2pq = 2 × 0.98 × 0.02 A1 0.0392, so 3.92 per cent of the population are carriers

Question 5

Before a drought, Galápagos ground finches on Daphne Major have a range of beak depths with no strong advantage to any particular depth. A severe drought leaves mainly large, hard seeds, and most small-beaked birds starve. Describe the change in the distribution of beak depth that follows, and describe how the next generation compares with the survivors.

Accepted Answer

B1 before the drought the population shows a range of beak depths with a single peak around the mean, and no particular depth is strongly favoured B1 during the drought, birds with shallow beaks cannot crack the large hard seeds that remain and mostly starve, while birds with deep, strong beaks survive B1 the survivors' distribution of beak depth is shifted towards greater depth compared with the pre-drought population, since it is drawn only from the deep-beaked part of it B1 the offspring of the survivors show most of that same shift, because beak depth is heritable, so the mean beak depth of the next generation is deeper than the mean of the population before the drought

Question 6

A textbook sentence reads: 'peppered moths adapted to soot-darkened trees by evolving darker colouring.' Explain what is wrong with the word 'adapted' in this sentence, and explain what actually happened in the peppered moth population.

Accepted Answer

B1 no individual moth can change its own colour or its own alleles during its lifetime, so 'adapted' wrongly implies that individual moths changed in response to the soot B1 the allele for dark colouring already existed in the population at low frequency before the trees darkened, produced by mutation rather than created in response to the soot B1 against a soot-darkened background, dark moths were better camouflaged from predatory birds than pale moths, so they survived and reproduced more successfully B1 it is the population, not any individual, that becomes adapted, through a rise in the frequency of the dark allele over many generations

Question 7

A recessive allele a is lethal in the homozygous state before reproduction, so aa individuals leave no offspring; AA and Aa individuals are unaffected. In a large, randomly mating population the frequency of a is 0.5. Calculate the genotype frequencies before selection, and calculate the frequency of a in the next generation after selection.

Accepted Answer

M1 before selection, Hardy-Weinberg with p = q = 0.5 gives p² = 0.25 AA, 2pq = 0.5 Aa and q² = 0.25 aa A1 aa individuals leave no offspring, so among survivors the genotypes are in the ratio 0.25 AA : 0.5 Aa (aa removed entirely), which renormalises to ¹⁄₃ AA and ²⁄₃ Aa M1 allele frequency is counted over alleles: every AA contributes two copies of A and every Aa contributes one copy of A and one of a, so q^′ comes only from the Aa survivors, as ½ × ²⁄₃ A1 q^′ = ¹⁄₃ ≈ 0.33, so the allele frequency falls from 0.5 to about 0.33 in one generation but is not eliminated, because copies carried by heterozygotes are not exposed to selection

Question 8

Outline, in the correct order, the four steps of a complete explanation of how natural selection increases the frequency of an advantageous allele in a population.

Accepted Answer

B1 genetic variation, caused by mutation, already exists in the population before the selection pressure acts B1 a selection pressure is identified and named, such as a predator, a drought or an antibiotic B1 individuals with the advantageous phenotype survive and reproduce more successfully than individuals without it B1 the alleles responsible are passed on to offspring, so their frequency in the population's gene pool increases over generations

Question 9

An allele giving resistance to a pesticide reaches a frequency of 0.6 in a field where the pesticide is used every season. Spraying then stops, and over the following years the frequency of the allele falls. Suggest why.

Accepted Answer

B1 with no pesticide present the allele confers no advantage, so it is no longer favoured and the individuals lacking it are no longer killed B1 resistance usually carries a cost, such as a protein that does its normal job less well or energy spent running a mechanism the insect does not need, so carriers leave slightly fewer surviving offspring than the rest B1 the frequency of the allele in the gene pool therefore falls generation by generation

Question 10

A peacock with a larger, more colourful tail wins more matings, but the tail also makes him more visible to predators and costly to grow, and peacocks with large tails tend to live shorter lives on average. Suggest why a large tail could still be favoured by natural selection.

Accepted Answer

B1 fitness is measured by an individual's reproductive contribution to the next generation relative to others, not by how long it survives, so a trait that shortens life can still raise fitness if it increases mating success enough B1 a male with a larger tail may be chosen by females more often, so he fathers more offspring across his shorter life than a male with a smaller tail fathers across a longer one B1 the relevant evidence would be the number of offspring each male actually produces over his lifetime, not how long he survives

Question 11

Name the only process that produces an allele that has never existed anywhere before, and name two processes that produce new combinations of alleles that already exist.

Accepted Answer

B1 mutation, a change in the base sequence of DNA B1 crossing over in prophase I and independent assortment at metaphase I of meiosis, with random fertilisation combining any gamete of one parent with any of the other's

Question 12

State what is meant by the fitness of an individual, and state why survival on its own does not raise it.

Accepted Answer

B1 an individual's contribution to the gene pool of the next generation relative to other individuals in the same population, counted as offspring that themselves survive to reproduce B1 an individual that lives a long time and never reproduces contributes nothing to the next generation's gene pool, so survival counts only in so far as it leads to reproduction

Question 13

State what is meant by a selection pressure, and state one named example of a selection pressure other than an antibiotic or a pesticide.

Accepted Answer

B1 a selection pressure is an environmental factor that causes individuals with different phenotypes to survive and reproduce at different rates B1 a named example such as a predator, a parasite, drought, cold, or competition for food or for mates

Question 14

Give two differences between genetic variation and environmental variation in a population.

Accepted Answer

B1 genetic variation arises from differences in alleles and can be inherited by offspring, whereas environmental variation arises from differences in the conditions individuals experience and is not inherited B1 only genetic variation provides the raw material selection can act on to change a population over generations, so a population that varies only environmentally cannot evolve in response to a new selection pressure, however severe it is

Question 15

A moth population's wing-pattern score is measured before and after a change in the environment. Before the change the population has one peak in wing-pattern score, at the mean. After the change there are two peaks, one at each end of the range, and very few moths show the intermediate pattern that used to be commonest. Identify the type of selection shown, and identify one feature of the new distribution that supports your answer.

Accepted Answer

B1 the type shown is disruptive selection B1 the single peak has split into two peaks with a trough between them, showing that the intermediate phenotype, once commonest, is now selected against rather than favoured