A2 17 Selection and evolution | A Level Biology 9700

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🎯What you need to be able to do

Explain that phenotypic variation is due to genetic factors, environmental factors or both.
Explain discontinuous and continuous variation and their genetic basis.
Use the t-test to compare the means of two samples.
Explain natural selection in terms of overproduction, competition and differential survival.
Explain stabilising, disruptive and directional selection.
Explain how selection, the founder effect and genetic drift, including the bottleneck effect, affect allele frequencies.
Outline antibiotic resistance in bacteria as an example of natural selection.
Use the Hardy–Weinberg principle to calculate allele and genotype frequencies, and state when it applies.
Describe selective breeding and outline the named examples, and explain how speciation occurs.

📚The biology

Variation

Phenotypic variation arises from genetic factors, environmental factors, or a combination. Human height is genetic and environmental — alleles set a range, nutrition determines where within it you fall. Blood group is purely genetic. The scar on your knee is purely environmental.

Discontinuous variation — distinct categories with no intermediates, such as ABO blood group. Controlled by one or a few genes, with the environment having little or no effect.
Continuous variation — a range of values with no clear categories, such as height or mass, usually giving a normal distribution. Controlled by many genes (polygenic), each contributing a small additive effect, and usually influenced by the environment as well.

The t-test

Use the t-test to decide whether the means of two samples differ significantly. The formula is provided; the conditions are not, and they carry marks:

the data are continuous;
the populations are normally distributed;
the standard deviations are approximately the same.

Degrees of freedom \( \nu = n_1 + n_2 - 2 \), which you must know without being given it. Compare the calculated t with the critical value at p = 0.05. If t is greater than the critical value, the difference between the means is significant and the null hypothesis is rejected. Note that this is the opposite direction of comparison from chi-squared’s interpretation being about differences from expected — but the rule is the same in both: calculated value above critical value means reject the null hypothesis.

Related descriptive statistics: standard deviation measures the spread of the data; standard error measures the reliability of the mean; and 95% confidence intervals can be approximated as \( \bar{x} \pm (2 \times \mathrm{SE}) \). If the 95% confidence intervals of two means do not overlap, the difference is likely to be significant; if they overlap substantially, it is unlikely to be.

Natural selection

The argument, in the order that earns the marks:

Populations produce more offspring than can survive.
There is variation within the population, much of it genetic and ultimately arising from mutation.
Individuals compete for limited resources — the “struggle for existence”.
Those with alleles conferring an advantage in the prevailing conditions are more likely to survive to reproduce.
They pass those alleles to the next generation, so the frequency of the advantageous allele in the population increases over generations.

The three forms of selection differ in which part of the distribution is favoured:

Stabilising — the mean is favoured and both extremes selected against. The mean stays the same and the range narrows. Human birth mass is the standard example: very small and very large babies both have lower survival. This is what happens in a stable, unchanging environment.
Directional — one extreme is favoured, so the mean shifts in that direction. Antibiotic resistance and peppered moth colouration are examples. This happens when the environment changes.
Disruptive — both extremes are favoured and the mean selected against, so the distribution becomes bimodal. This can be a first step towards speciation.

Genetic drift, the founder effect and bottlenecks

Genetic drift is a change in allele frequencies caused by chance rather than selection: which individuals happen to reproduce, and which alleles happen to end up in gametes. Its effect is much greater in small populations, where chance events can change a frequency dramatically, and negligible in very large ones.

Founder effect — a few individuals establish a new population. They carry only a small, unrepresentative sample of the original gene pool, so allele frequencies in the new population differ from the old and genetic diversity is reduced. An allele that was rare may become common by chance.
Bottleneck effect — a population is drastically reduced by a catastrophe such as disease, hunting or habitat loss. The survivors are a small, chance sample, so genetic diversity falls sharply and remains low even after the population recovers, because the alleles lost cannot be recovered. This is why bottlenecked species — cheetahs are the usual example — are vulnerable to new diseases and to environmental change.

The Hardy–Weinberg principle

\( p + q = 1 \)

\( p^{2} + 2pq + q^{2} = 1 \)

where \( p \) is the frequency of the dominant allele and \( q \) that of the recessive; \( p^{2} \) is the frequency of the homozygous dominant genotype, \( 2pq \) the heterozygous and \( q^{2} \) the homozygous recessive. Both equations are provided in the exam.

The method is almost always the same: you are told the frequency of the recessive phenotype, which is \( q^{2} \), because only the homozygous recessive shows it. Take the square root to get \( q \), subtract from 1 to get \( p \), then compute whatever is asked.

The principle applies only where: the population is large; mating is random; there is no selection for or against any genotype; there is no mutation; there is no migration in or out; and the organism is diploid and reproduces sexually. In practice these are rarely all true, which is itself useful: a population that departs from Hardy–Weinberg expectations is telling you that one of the conditions is being violated.

Selective breeding

Artificial selection follows the same logic as natural selection except that humans, not the environment, choose which individuals breed. The principles:

Select individuals showing the desired characteristic most strongly.
Breed them together.
Select the best of the offspring.
Repeat over many generations.

Three examples are named:

Disease resistance in wheat and rice — a resistant variety, often a wild relative, is crossed with a high-yielding one, and offspring combining both are selected and bred on.
Inbreeding and hybridisation in maize — repeated inbreeding produces homozygous, uniform but weak lines; crossing two such lines gives an F₁ that is uniform and vigorous, an effect called hybrid vigour. Farmers must buy fresh seed each year, because the F₂ segregates and loses uniformity.
Milk yield in dairy cattle — bulls are selected on the milk yields of their female relatives, since a bull produces no milk himself, and artificial insemination lets one bull sire very large numbers of offspring.

The cost is reduced genetic diversity: an inbred, uniform population is vulnerable to a new disease or a change in conditions, which is one reason seed banks and rare breeds matter (Topic 18).

Evolution and speciation

Evolution is the formation of new species from pre-existing species over time, as a result of changes in the gene pools of populations from generation to generation.

DNA sequence data shows evolutionary relationships: the more similar the base sequences of a gene in two species, the more recently they shared a common ancestor, because differences accumulate by mutation over time. Comparing amino acid sequences of proteins does the same job. This evidence is independent of morphology, so it can confirm or overturn classifications based on appearance alone.

Speciation requires genetic isolation: two populations must stop exchanging alleles, so that mutation, selection and drift can act on them independently until they can no longer interbreed to produce fertile offspring.

Allopatric — isolation by a geographical barrier such as a river, a mountain range or the sea. Different selection pressures on either side lead to divergence.
Sympatric — isolation without a geographical barrier, by ecological or behavioural separation: populations occupy different niches within the same area, or breed at different times, or have courtship behaviours that no longer match.

✏️Worked example

Cystic fibrosis is caused by a recessive allele. In a population of 90 000 people, 36 have the condition. (a) Calculate the frequency of the recessive allele. (b) Calculate the expected number of carriers in this population. (c) State three conditions that must hold for the Hardy–Weinberg principle to apply, and comment on whether they are likely to hold here.

(a) Only the homozygous recessive genotype shows the condition, so the frequency of affected individuals is \( q^{2} \):

\[ q^{2} = \frac{36}{90\,000} = 0.0004 \]

\[ q = \sqrt{0.0004} = 0.02 \]

So the recessive allele has a frequency of 0.02, or 2%. Then \( p = 1 - q = 0.98 \).

(b) Carriers are the heterozygotes, whose frequency is \( 2pq \):

\[ 2pq = 2 \times 0.98 \times 0.02 = 0.0392 \]

In a population of 90 000 that is \( 0.0392 \times 90\,000 = \mathbf{3528} \) carriers.

Notice how many more carriers there are than affected individuals — 3528 against 36, almost a hundred times as many. That is the general result for a rare recessive allele, and it is why recessive conditions persist in populations: the great majority of copies of the allele are hidden in healthy heterozygotes, where selection cannot act on them.

(c) Any three of: the population is large; mating is random; there is no selection for or against any genotype; there is no mutation; there is no migration into or out of the population.

Here the population of 90 000 is reasonably large and mating with respect to this gene is effectively random, since carriers are unaware of their genotype. But selection is operating: individuals with cystic fibrosis have reduced survival and reduced likelihood of reproducing, so the recessive allele is being selected against. The calculation should therefore be treated as an estimate rather than an exact figure.

Check it. Check that all three genotype frequencies sum to 1: \( p^{2} = 0.9604 \), \( 2pq = 0.0392 \), \( q^{2} = 0.0004 \), and 0.9604 + 0.0392 + 0.0004 = 1.0000 ✓. That single check catches almost every arithmetic error in a Hardy–Weinberg question. Check also that q lies between 0 and 1: a frequency cannot exceed 1, and a value like 20 means you forgot to divide by the population size before taking the root.

Taking the frequency of affected individuals as q rather than q². The phenotype frequency you are given is a genotype frequency, and it is the homozygous recessive one, so it equals q². Using 0.0004 as q directly gives p = 0.9996 and a carrier frequency of 0.0008, which is wrong by a factor of about fifty. Second trap: 2pq is the frequency of carriers only, not of everyone carrying at least one copy — do not add q² to it unless the question asks for all individuals possessing the allele.

📝Practise

Work through these, then reveal the answer. Each question targets a different objective from the list above.

1. Distinguish between continuous and discontinuous variation, and explain the genetic basis of each.

Discontinuous variation gives distinct categories with no intermediates — an individual falls clearly into one group, as with ABO blood group or the ability to roll the tongue. It is controlled by one gene, or a small number of genes, each with a large effect, and the environment has little or no influence on which category an individual falls into. Continuous variation gives a range of values with no distinct categories, usually approximating a normal distribution, as with height or body mass. It is polygenic: many genes each contribute a small, additive effect, so the combined effect of many loci produces a spectrum of values rather than discrete classes. The environment also influences the phenotype — nutrition affects height — which smooths the distribution further. A useful test: if a characteristic can be counted into categories it is discontinuous; if it must be measured on a scale it is continuous.

2. Explain the difference between stabilising and directional selection, giving an example of each and stating the conditions under which each occurs.

Stabilising selection favours the mean phenotype and selects against both extremes. The mean stays the same but the range of variation narrows. Example: human birth mass — very small babies have poor survival because of immaturity, very large ones because of difficult delivery, so intermediate masses are favoured. It occurs in a stable, unchanging environment, where the current mean is already well matched to conditions. Directional selection favours one extreme, which is selected for while the other extreme and often the mean are selected against, so the mean of the population shifts in that direction over generations. Example: antibiotic resistance in bacteria, or the increase in dark peppered moths in polluted areas. It occurs when the environment changes, so that a phenotype which was previously disadvantageous becomes advantageous.

3. Explain what is meant by a genetic bottleneck and why it leaves a species vulnerable.

A bottleneck occurs when a population is drastically reduced in size by an event such as disease, over-hunting, natural disaster or habitat destruction. The survivors are a small and largely random sample of the original population, so many alleles present in the original gene pool are lost entirely and the frequencies of those remaining are altered by chance rather than by selection — this is genetic drift acting powerfully because the population is small. Even after the population recovers in numbers, its genetic diversity remains low, because lost alleles cannot be regained except by new mutation, which is slow. The species is then vulnerable because less variation means less raw material for natural selection: if the environment changes or a new pathogen appears, it is less likely that any individual carries an allele conferring resistance, so the whole population may be affected. Inbreeding also becomes more likely, increasing the frequency of homozygous recessive genetic disorders.

4. Describe how a farmer could use selective breeding to increase the milk yield of a dairy herd, and give one disadvantage of the process.

Select the cows in the herd with the highest milk yields, and select bulls on the basis of the milk yields of their female relatives — mothers, sisters and daughters — since a bull produces no milk himself and can only be judged on his progeny. Breed the selected animals together, often using artificial insemination so that one outstanding bull can sire very large numbers of calves across many herds. From the offspring, select again those with the highest yields, and repeat over many generations. Over time the mean milk yield of the herd rises, because the alleles associated with high yield increase in frequency. Disadvantage: repeatedly breeding from a small number of selected individuals reduces genetic diversity and increases inbreeding, so harmful recessive alleles are more likely to become homozygous and the herd becomes more vulnerable to disease or to a change in conditions. There are also welfare concerns, since very high-yielding cows suffer more from mastitis and lameness.

5. Explain how allopatric speciation occurs, and how it differs from sympatric speciation.

In allopatric speciation, a population is divided by a geographical barrier — a river changing course, a mountain range rising, sea level separating islands from a mainland. The two populations are genetically isolated: no gene flow occurs between them. Each is subject to different selection pressures, because conditions on either side differ, and each also accumulates different mutations and is subject to independent genetic drift. Over many generations their gene pools diverge, and changes in structure, physiology or behaviour accumulate until, even if the barrier is removed, members of the two populations can no longer interbreed to produce fertile offspring. They are then separate species. Sympatric speciation achieves the same genetic isolation without any geographical barrier — the populations occupy the same area. Isolation arises ecologically or behaviourally: they exploit different niches or food sources, breed at different times of year, or develop different courtship behaviours or mating calls that members of the other group no longer recognise or respond to.

6. Two samples of leaves are measured and their mean lengths compared with a t-test. State the null hypothesis, the formula for degrees of freedom, and how the result is interpreted.

Null hypothesis: there is no significant difference between the mean leaf lengths of the two populations from which the samples were taken; any difference observed is due to chance. Degrees of freedom: \( \nu = n_1 + n_2 - 2 \), where \( n_1 \) and \( n_2 \) are the two sample sizes — this formula is not provided in the exam and must be known. Interpretation: compare the calculated value of t with the critical value at p = 0.05 for that number of degrees of freedom. If the calculated t is greater than or equal to the critical value, the probability that the difference arose by chance is less than 0.05, so the difference is significant and the null hypothesis is rejected. If t is less than the critical value, the difference is not significant and the null hypothesis is accepted. The test is only valid if the data are continuous, drawn from normally distributed populations, and the two standard deviations are approximately equal.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

Understanding Evolution (University of California Museum of Paleontology) — a reliable, well-illustrated account of selection, drift and speciation
Any online Hardy–Weinberg calculator — useful for checking your own working, though the exam requires you to show the steps
Documented case studies of bottlenecks — the cheetah, the northern elephant seal, the Mauritius kestrel — which give you concrete evidence to cite in a discussion question