Selection and evolution

• Mutation makes new alleles at random; selection and chance change how common they are; populations evolve, individuals do not.
• Mutation makes alleles, selection and chance change their frequencies, and populations evolve, individuals do not; start Hardy–Weinberg from q²; for t, v = n1 + n2 − 2.

• 17.1.1 I can explain — explain, with examples, that phenotypic variation is due to genetic factors or environmental factors or a combination of genetic and environmental factors
• 17.1.2 I can explain — explain what is meant by discontinuous variation and continuous variation
• 17.1.3 I can explain — explain the genetic basis of discontinuous variation and continuous variation
• 17.1.4 I can use — use the t-test to compare the means of two different samples (the formula for the t-test will be provided, as shown in the Mathematical requirements)
• 17.2.1 I can explain — explain that natural selection occurs because populations have the capacity to produce many offspring that compete for resources; in the 'struggle for existence', individuals that are best adapted are most likely to survive to reproduce and pass on their alleles to the next generation
• 17.2.2 I can explain — explain how environmental factors can act as stabilising, disruptive and directional forces of natural selection
• 17.2.3 I can explain — explain how selection, the founder effect and genetic drift, including the bottleneck effect, may affect allele frequencies in populations
• 17.2.4 I can outline — outline how bacteria become resistant to antibiotics as an example of natural selection
• 17.2.5 I can use — use the Hardy–Weinberg principle to calculate allele and genotype frequencies in populations and state the conditions when this principle can be applied (the two equations for the Hardy–Weinberg principle will be provided, as shown in the Mathematical requirements)
• 17.2.6 I can describe — describe the principles of selective breeding (artificial selection)
• 17.2.7 I can outline — outline the following examples of selective breeding: • the introduction of disease resistance to varieties of wheat and rice • inbreeding and hybridisation to produce vigorous, uniform varieties of maize • improving the milk yield of dairy cattle
• 17.3.1 I can outline — outline the theory of evolution as a process leading to the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation
• 17.3.2 I can discuss — discuss how DNA sequence data can show evolutionary relationships between species
• 17.3.3 I can explain — explain how speciation may occur as a result of genetic isolation by: • geographical separation (allopatric speciation) • ecological and behavioural separation (sympatric speciation)

A conclusion, which a discuss answer needs. DNA sequence data are now the most powerful evidence of evolutionary relationships between species, above all when several genes give the same tree and agree with other evidence such as anatomy. A tree built from one gene should be treated with caution.

• “The bacteria got used to the antibiotic and built up immunity to it.” Correct A resistance allele was already present in a few bacteria, produced by random mutation. The antibiotic kills the susceptible bacteria and the resistant ones survive and divide. Immunity is a property of an animal’s immune system, not of a bacterium.
• “The drug made the bacteria mutate so that they could survive.” Correct Mutation is random and happens whether or not the antibiotic is present. The antibiotic is the selection pressure: it does not create the allele, it changes how common it becomes.
• “Natural selection means only the strongest survive.” Correct The individuals best adapted to that environment are most likely to survive and reproduce. Strength may be irrelevant; survival without reproduction passes on nothing; and “most likely” is a probability, not a certainty.
• “q is the frequency of people with the recessive phenotype.” Correct The recessive phenotype is q². Take the square root to find q, then p = 1 − q.
• “For a t-test the degrees of freedom are n − 1, and they are given in the question.” Correct For the t-test, v = n1 + n2 − 2, and you must recall it: the question gives the formula for t and the table of critical values, but not the degrees of freedom.
• “Genetic drift is when natural selection drifts the population towards the best allele.” Correct Drift is a change in allele frequency by chance. It has no direction, can remove an advantageous allele, and matters most in small populations.
• “Two populations are different species once they look different.” Correct Speciation is complete when the populations are reproductively isolated: they can no longer interbreed to produce fertile offspring. How they look is not the test.
• “Giraffes stretched their necks to reach high leaves and passed on longer necks to their offspring.” Repair That is Lamarck’s idea of the inheritance of acquired characteristics, and it is wrong: a change during an animal’s lifetime does not change the alleles in its gametes. Giraffes that already carried alleles for longer necks were more likely to survive and reproduce, so those alleles became more frequent.
• “The finches developed deeper beaks in order to crack the hard seeds.” Repair No purpose and no change within individuals. Birds that already had deeper beaks were more likely to survive the drought and reproduce, so the alleles for deeper beaks increased in frequency and the population became adapted.
• “Resistant bacteria can only pass the resistance allele to their own daughter cells.” Repair Binary fission passes it to daughter cells, but resistance genes on plasmids can also be passed by conjugation to other bacteria, even of other species.
• “Continuous variation is caused by the environment and discontinuous variation by genes.” Repair Continuous variation is caused by many genes of small, additive effect and the environment. Genes are involved in both kinds.
• “In stabilising selection the mean moves towards the best value.” Repair In stabilising selection the mean stays the same and the range narrows, because both extremes are selected against. The mean shifts in directional selection.
• “The founder effect happens because the founders are the fittest individuals.” Repair Which individuals found the new population, and which alleles they carry, is a matter of chance. The founder effect is random, like drift, not a form of selection.
• “p² + 2pq is the frequency of the dominant allele.” Repair p² + 2pq is the frequency of the dominant phenotype (AA plus Aa). The frequency of the dominant allele is p.
• “The Hardy–Weinberg equations show that the population is evolving.” Repair The equations predict the frequencies in a population that is not evolving. A population is evolving for that gene if its observed frequencies depart from the prediction, or change between generations.
• “t came out as −6.82.” Repair The numerator is |x̄1 − x̄2|, the size of the difference ignoring its sign, so t is never negative.
• “The t-test proves that shade makes ivy leaves longer.” Repair A test never proves. The difference is significant: the probability that it is due to chance is less than 0.05. And other factors that differ between the two sides may contribute.
• “Selective breeding gives a crop new alleles for disease resistance.” Repair Selective breeding creates no alleles. It brings in a resistance allele that already exists in another variety or a wild relative, by crossing and backcrossing, and makes it frequent.
• “Seed saved from F1 hybrid maize gives an equally uniform crop next year.” Repair The saved seed is the F2. The heterozygous F1 alleles separate at meiosis, so the F2 is variable and less vigorous; new F1 seed is bought each year.
• “The two species with the most base differences share the most recent common ancestor.” Repair The opposite: the pair with the fewest differences share the most recent common ancestor, because they have had the least time to accumulate different mutations.
• “In sympatric speciation there is no barrier to gene flow.” Repair There is a barrier, but it is ecological or behavioural rather than geographical, and the populations live in the same area.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. The four that open Topic 17’s outcomes, in the syllabus’s own words: explain — “set out purposes or reasons / make the relationships between things clear / say why and/or how and support with relevant evidence”; outline — “set out the main points”; describe — “state the points of a topic / give characteristics and main features”; discuss — “write about issue(s) or topic(s) in depth in a structured way”. Questions on this topic also use calculate (“work out from given facts, figures or information”), state (“express in clear terms”) and suggest (“apply knowledge and understanding to situations where there are a range of valid responses to make proposals / put forward considerations”). Use, which opens 17.1.4 and 17.2.5, is the outcome’s verb, not a command word: it means you carry out the calculation. Check any other word against the syllabus’s table.
• You may be asked to finish, not start. The syllabus says you will not be expected to carry out every step of a standard deviation or a t-test in an examination, but you may be given a partly completed calculation to finish. The studio below does exactly that, twice. And a test never proves anything: write the conclusion as a probability.
• Interleave with the chapters that use this one. When you reach chapter 18, re-answer the speciation question in worked example 6 and ask which species concept its last step uses (18.1.1); redo the ivy t-test before learning correlation (18.2.5); and explain again why a bottlenecked population is at risk (18.3.1). In chapter 19, re-answer why selective breeding creates no new alleles, and contrast it with inserting a gene. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 17 is A Level content, so it is examined in Papers 4 and 5. A Level content: examined in Paper 4 (A Level structured, which also requires the AS content) and, as practical context, Paper 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• Topic 17 has no Paper 1 or Paper 2 content: it is examined in Paper 4 structured questions. Expect to explain natural selection, a type of selection, drift or speciation as a sequence of linked points; to outline antibiotic resistance or a selective-breeding example; to calculate allele and genotype frequencies with the Hardy–Weinberg equations the question prints; and to discuss DNA sequence evidence with a conclusion.
• Bar charts and histograms of variation; before-and-after distribution curves to name as stabilising, directional or disruptive; tables of resistance or allele frequency over time to describe with figures; genotype counts for Hardy–Weinberg; tables of base differences to turn into a tree. Numerical skills: square roots, frequencies as decimals, percentages and “1 in n”, and a partly completed t-test.
• Topic 17 names no practical, but continuous variation is the natural Paper 5 context: plan how to compare two samples (leaf length in sun and shade), with a categoric independent variable, a measured continuous dependent variable, random sampling, one genotype where possible, and a large enough sample; then analyse with the t-test or with 95% confidence intervals and error bars.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. The four that open Topic 17’s outcomes, in the syllabus’s own words: explain — “set out purposes or reasons / make the relationships between things clear / say why and/or how and support with relevant evidence”; outline — “set out the main points”; describe — “state the points of a topic / give characteristics and main features”; discuss — “write about issue(s) or topic(s) in depth in a structured way”. Questions on this topic also use calculate (“work out from given facts, figures or information”), state (“express in clear terms”) and suggest (“apply knowledge and understanding to situations where there are a range of valid responses to make proposals / put forward considerations”). Use, which opens 17.1.4 and 17.2.5, is the outcome’s verb, not a command word: it means you carry out the calculation. Check any other word against the syllabus’s table.