Selection and evolution

Every objective in this topic is printed under "A Level subject content" in the 9700 syllabus, so all of it is A Level and none of it is AS. It is examined on Paper 4. Paper 1 is the AS multiple-choice paper and does not reach this topic, so no multiple-choice practice on this site is tagged to it.

Syllabus points

17.1 Variation

• Explain, with examples, that phenotypic variation is due to genetic factors or environmental factors or a combination of genetic and environmental factors.
• Explain what is meant by discontinuous variation and continuous variation.
• Explain the genetic basis of discontinuous variation and continuous variation.
• 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 Natural and artificial selection

• 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.
• Explain how environmental factors can act as stabilising, disruptive and directional forces of natural selection.
• Explain how selection, the founder effect and genetic drift, including the bottleneck effect, may affect allele frequencies in populations.
• Outline how bacteria become resistant to antibiotics as an example of natural selection.
• 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).
• Describe the principles of selective breeding (artificial selection).
• 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 Evolution

• 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.
• Discuss how DNA sequence data can show evolutionary relationships between species.
• Explain how speciation may occur as a result of genetic isolation by: geographical separation (allopatric speciation); ecological and behavioural separation (sympatric speciation).

Where phenotypic variation comes from

Phenotype is genotype plus environment, and any given characteristic may owe its variation mostly to one, mostly to the other, or to both.

• Genetic only. Human ABO blood group. Nothing you eat or do changes it.
• Environmental only. A scar, or the language you speak.
• Both. Human height, body mass, skin colour, and the yield of a crop. Genes set a range and the environment decides where in that range an individual falls. Two genetically identical plants grown in different light intensities reach different heights.

The genetic contributions themselves come from mutation, crossing over, random orientation in meiosis and random fertilisation, all covered in topic 16.

Discontinuous and continuous variation

Discontinuous variation puts individuals into a small number of distinct categories with nothing in between. Blood group, the ability to roll the tongue, sex. Plotted, it gives a bar chart with separate bars.

Continuous variation gives a range of values with all intermediates present. Height, mass, leaf length. Plotted, it gives a normal distribution curve.

The genetic basis of the difference is what the syllabus actually asks for:

• Discontinuous characteristics are controlled by one gene, or a small number of genes, each with a large effect. The environment has little or no influence on them.
• Continuous characteristics are polygenic: many genes at different loci each contribute a small additive effect, so the possible totals form an almost unbroken range. The environment modifies the outcome as well, smoothing the distribution further.

The t-test

Use the t-test to decide whether the difference between the means of two samples is significant, when the data are continuous and approximately normally distributed. The formula is given in the exam.

The method:

1. State the null hypothesis: there is no significant difference between the two means.
2. Calculate the mean and the standard deviation of each sample.
3. Substitute into the formula to get t.
4. Work out the degrees of freedom, which is the total number of measurements in both samples minus two.
5. Compare t with the critical value at p = 0.05 for those degrees of freedom.

Worked example of the decision. Leaf lengths are measured on 12 plants in shade and 12 in full sun, and t comes out at 2.94.

degrees of freedom = 12 + 12 - 2 = 22

The critical value at 22 degrees of freedom and p = 0.05 is 2.07. Since

2.94 > 2.07

the calculated value exceeds the critical value, so the null hypothesis is rejected: the difference between the two means is significant, and there is a probability of less than 0.05 that a difference this large arose by chance.

Note that this is the opposite way round from chi-squared in feel but the same rule in fact: in both tests, calculated above critical means significant.

Natural selection

The argument, as Darwin set it out, is a chain and it should be written as one:

1. Populations produce more offspring than can survive.
2. There is variation between individuals, much of it heritable.
3. Resources are limited, so there is competition, the struggle for existence. Predation, disease and abiotic factors act as selection pressures too.
4. Individuals with advantageous alleles are more likely to survive to reproductive age.
5. They therefore pass those alleles on to more offspring.
6. Over generations the frequency of the advantageous allele rises in the population's gene pool.

Everything hinges on step 6. Natural selection changes allele frequencies in a population; it does not change an individual. An individual does not adapt to its environment during its lifetime and cannot pass on characteristics acquired during it.

Three forms of selection

Stabilising selection is the normal state of affairs in an unchanging environment. Human birth mass is the standard example: very small babies and very large babies both have higher mortality, so the mean stays where it is and the variation is reduced.

Directional selection acts when the environment changes. The peppered moth in industrial Britain, and antibiotic resistance, are directional.

Disruptive selection acts when two different environments or niches exist and the intermediate is worst suited to both.

Genetic drift, the founder effect and bottlenecks

Selection is not the only thing that changes allele frequencies.

Genetic drift is change in allele frequency by chance alone, because only a sample of the gametes produced actually forms the next generation. In a large population these chance effects cancel out. In a small population they do not, and an allele can be lost, or become fixed, with no reference to whether it is advantageous.

The founder effect occurs when a small number of individuals colonise a new area. The alleles they happen to carry are only a sample of the parent population's gene pool, so allele frequencies in the new population differ from the start, and rare alleles may be over-represented or absent.

The bottleneck effect occurs when a population is reduced dramatically, by disease, hunting or a natural disaster, and then recovers. The survivors carry only a sample of the original alleles, so the recovered population has low genetic diversity even once its numbers are back. Cheetahs and northern elephant seals are the usual examples, and the consequence is a population less able to adapt to a future change, because the variation to select from is not there.

Antibiotic resistance

This is natural selection observed within a human lifetime, and the sequence must be written in the right order:

1. Within a bacterial population there is variation, arising by random mutation. A few cells happen to carry an allele giving resistance, for example one coding for beta-lactamase, which hydrolyses penicillin.
2. An antibiotic is applied. It is the selection pressure.
3. Non-resistant bacteria are killed; the resistant ones survive.
4. The survivors reproduce, by binary fission, and pass the allele to their offspring.
5. The frequency of the resistance allele rises, and eventually the whole population is resistant.

The mutation is not caused by the antibiotic. It was already there, at low frequency, and the antibiotic revealed it by removing everything else. Bacteria also spread resistance sideways by conjugation, passing plasmids carrying resistance genes between cells, even between species, which is why resistance spreads faster than mutation rates alone would suggest.

The Hardy-Weinberg principle

The principle lets you calculate allele and genotype frequencies from phenotype data. Two equations, both provided in the exam:

p + q = 1

p² + 2pq + q² = 1

where p is the frequency of the dominant allele and q the frequency of the recessive allele, p² is the frequency of the homozygous dominant genotype, 2pq the heterozygous and q² the homozygous recessive.

Worked example. Albinism is recessive and affects 1 person in 10 000. What proportion of the population are carriers?

Only the homozygous recessive shows the condition, so:

q² = 1 / 10000 = 0.0001

Taking the square root:

q = 0.01

p = 1 - 0.01 = 0.99

frequency of carriers = 2pq = 2 × 0.99 × 0.01 = 0.0198

as a percentage:

0.0198 × 100 = 1.98%

So roughly 2 people in 100 carry the allele, against 1 in 10 000 who show the condition. That ratio is the point of the calculation: recessive alleles are far commoner than the phenotype suggests, because almost all of them are hidden in heterozygotes. It is also why a rare recessive condition cannot be bred out of a population.

Always start from q², because the homozygous recessive is the only genotype you can identify by looking.

The principle holds only if five conditions apply:

• The population is large.
• Mating is random.
• There is no selection for or against any genotype.
• There is no mutation creating new alleles.
• There is no migration into or out of the population.

Real populations rarely satisfy all five, which is what makes the principle useful: a measured departure from the predicted frequencies is evidence that one of the conditions is being broken, usually selection.

Selective breeding

Artificial selection is the same mechanism with a human deciding the selection pressure:

1. Select individuals with the desired characteristic from a varied population.
2. Breed them together.
3. Select the offspring showing the characteristic most strongly.
4. Repeat over many generations, so the frequency of the favourable alleles rises.

Three named examples:

• Disease resistance in wheat and rice. A wild or unimproved variety carrying a resistance allele is crossed with a high-yielding cultivated variety, and the offspring are repeatedly backcrossed and selected until a line combines the resistance with the yield.
• Maize. Inbreeding produces homozygous lines that breed true and are uniform, but inbreeding depression reduces vigour because harmful recessive alleles become homozygous. Two different inbred lines are then crossed, and the hybrid offspring are uniform and unusually vigorous, an effect called hybrid vigour. The seed has to be bought fresh each year because the next generation does not breed true.
• Dairy cattle. Cows are selected on measured milk yield. Bulls cannot be measured directly, so they are chosen from the yields of their female relatives, a progeny test, and semen from the best bulls is used in artificial insemination across very many herds.

The cost of all this is reduced genetic diversity, since the same few high-performing individuals become the ancestors of the whole population, leaving it vulnerable to a new disease.

Evolution

Evolution is the process by which new species form from pre-existing ones over time, through changes to a population's gene pool from generation to generation. The unit that evolves is the population, not the individual.

DNA sequence data as evidence

Comparing the base sequence of the same gene in different species measures relatedness directly. The reasoning is:

• Mutations accumulate at a roughly steady rate over time.
• Two species that separated recently have had less time to accumulate differences, so their sequences are more similar.
• Two species that separated long ago show more differences.

The same argument works for amino acid sequences of a common protein such as cytochrome c, and for mitochondrial DNA, which mutates faster and is therefore useful for closely related groups. Building a tree from the number of differences gives a phylogenetic tree whose branch points estimate when lineages separated.

This evidence is powerful because it is independent of appearance. Two organisms can look alike through convergent evolution while their DNA shows they are distantly related.

Speciation

A species forms when two populations become genetically isolated, so that gene flow between them stops. Once no alleles pass between them, mutation, selection and drift act on the two gene pools separately, they diverge, and eventually they can no longer interbreed to produce fertile offspring.

Allopatric speciation, by geographical separation:

1. A physical barrier such as a river, a mountain range or a stretch of sea divides a population.
2. The two populations experience different selection pressures and accumulate different mutations.
3. Their gene pools diverge.
4. If they meet again, they can no longer interbreed successfully, so they are separate species.

Sympatric speciation, without a physical barrier, in the same area:

• Ecological separation, where two groups come to occupy different niches within the same area, for example feeding on different host plants or at different depths in a lake, so they rarely meet to breed.
• Behavioural separation, where differences in courtship display, mating call or the timing of flowering or breeding mean the two groups do not mate with each other even though they could.

In both cases isolation comes first and the divergence follows. Reversing that order is the commonest error in an answer on speciation.

Common mistakes

• Saying organisms adapt to their environment during their lifetime, or that a characteristic acquired in life is inherited.
• Saying the antibiotic, or the environment, causes the mutation. Mutations are random and pre-existing; the selection pressure only selects among them.
• Saying bacteria "become immune". They become resistant, and it is the population that changes, not an individual cell.
• Confusing stabilising with directional selection, or forgetting that stabilising selection leaves the mean where it is.
• Treating genetic drift as a form of selection. Drift is chance, and it acts most strongly in small populations.
• Confusing the founder effect with a bottleneck. A founder effect starts a new population from a few colonists; a bottleneck crushes an existing one.
• Starting a Hardy-Weinberg calculation from the dominant phenotype. Only q² can be read directly from the data.
• Forgetting to square-root q² before using p + q = 1, so p is calculated from q² instead of from q.
• Stating the Hardy-Weinberg conditions as though they hold in every real population.
• Saying selective breeding increases genetic diversity. It reduces it.
• Describing speciation without ever saying that gene flow between the populations stopped.
• Saying a hybrid maize crop can be re-sown from its own seed and stay uniform.