OxfordAQA International AS and A-level Biology: Natural selection and speciation (9610)

This guide follows the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1. It covers sections 3.3.8.1 (the theory of evolution) and 3.3.8.2 (selection and speciation). Both sit in Unit 3, Populations and genes, which the specification lists as International A-level only.

Once you have worked through it, condense with the revision notes and test yourself with the practice questions. The course hub is OxfordAQA A-level Biology, the printable list is the checklist, and the free diagnostics show where to start.

What this unit covers

Section	What you must be able to do
3.3.8.1	Define evolution as a change in allele frequencies; describe phenotypic variation; link predation, disease and competition to differential survival and reproduction; explain changes in allele and phenotype frequency from data
3.3.8.2	Describe the effects of directional, disruptive and stabilising selection; explain how reproductive separation changes gene pools; explain how new species arise; distinguish allopatric and sympatric speciation

This unit builds on the gene pool and allele frequency ideas of section 3.3.7. The specification’s Unit 3 principles include “selection leads to the accumulation of genetic differences in populations”, which is the thread running through this guide.

3.3.8.1 The theory of evolution

Evolution is a change in allele frequency

The specification says evolution “may be considered as a change in the allele frequencies in a population”. A population shares one gene pool. The allele frequency is the fraction of all copies of a gene that are one particular allele. If that fraction changes from one generation to the next, the population has evolved.

Individuals change during life (a runner gets fitter), but their alleles do not change. Evolution is a property of populations over generations, not of individuals.

Variation, then differential survival and reproduction

1. Phenotypic variation. The individuals in a population differ widely in their observable traits. Genetic differences (from mutation, meiosis and sexual reproduction) contribute, and so does the environment.
2. Selection pressures. Predation, disease and competition for resources mean that not all individuals survive or reproduce equally. This is differential survival and reproduction.
3. Selective advantage. Individuals whose phenotypes give a selective advantage are more likely to produce more offspring and pass on their favourable alleles.
4. Change in the gene pool. Those alleles become more frequent in the next generation. This may alter the frequency of specific alleles in the gene pool.

Only the heritable part of variation responds to selection. If a trait is caused only by the environment, survival of its bearers does not change allele frequencies.

Explaining selection from data

You are expected to explain changes in allele and phenotype frequency from data. Work in this order:

• Calculate allele frequency before and after. For a gene with alleles D and d, frequency of D = (2 × number of DD + number of Dd) ÷ (2 × total individuals).
• State the change, and give it as a percentage change if asked.
• Say which phenotype or genotype survived better and why that gives an advantage.
• Link survivors to reproduction, inheritance and a higher frequency of the allele in the next generation.

Worked example 1. A lizard species lives on dark volcanic rock. Scale colour is controlled by one gene with incomplete dominance: DD is dark, Dd is intermediate and dd is pale. Birds hunt by sight. A sample of 250 hatchlings and, a year later, the 150 survivors from that sample were genotyped.

Genotype	Hatchlings	Survivors
DD	40	36
Dd	100	70
dd	110	44

Frequency of D in hatchlings = (2 × 40 + 100) ÷ (2 × 250) = 180 ÷ 500 = 0.36. Frequency of D in survivors = (2 × 36 + 70) ÷ (2 × 150) = 142 ÷ 300 = 0.47 (0.473). Change = 0.473 − 0.360 = +0.11, a relative increase of 0.113 ÷ 0.360 × 100 = 31%.

Survival by genotype: DD 36 ÷ 40 = 90%, Dd 70 ÷ 100 = 70%, dd 44 ÷ 110 = 40%.

Explanation: pale lizards on dark rock are more visible to predatory birds, so fewer survive (40%). Dark lizards survive best (90%). Survivors breed and pass on D, so D should be more frequent in the next generation. The change you measure is across one generation of survivors; the offspring generation would need genotyping to confirm the new gene pool.

3.3.8.2 Selection and speciation

Three types of selection

Selection acts on a phenotype that varies continuously, so the population forms a bell-shaped distribution. What matters is which part of the distribution survives and reproduces best.

Type	Which phenotypes are favoured	Effect on the distribution
Stabilising	Those near the mean	Mean stays the same; the range narrows; extremes are removed
Directional	One extreme	Mean shifts towards that extreme
Disruptive	Both extremes	Two peaks form; the mean may stay similar but the middle is lost

Stabilising selection operates when the environment is stable. Directional selection operates when conditions change or a new pressure appears. Disruptive selection operates when the environment offers two different niches and intermediates do poorly in both.

Worked example 2: directional. Tail length (mm) was measured in 100 fish before and after a predator that catches long-tailed fish was introduced.

Tail length class (midpoint, mm)	10	12	14	16	18
Fish before	5	20	40	25	10
Fish after (new generation)	11	34	33	17	5

Mean before = (10×5 + 12×20 + 14×40 + 16×25 + 18×10) ÷ 100 = 1430 ÷ 100 = 14.3 mm. Mean after = (10×11 + 12×34 + 14×33 + 16×17 + 18×5) ÷ 100 = 1342 ÷ 100 = 13.4 mm. The mean fell by 0.88 mm, a decrease of 0.88 ÷ 14.3 × 100 = 6.2%. The whole distribution moved towards shorter tails: directional selection.

Worked example 3: stabilising. A seabird lays 1 to 5 eggs. Average number of chicks fledged per nest by clutch size:

Clutch size	1	2	3	4	5
Chicks fledged per nest	0.8	1.6	2.1	1.7	1.1

Success peaks at a clutch of 3. A clutch of 5 fledges 1.1 ÷ 2.1 = 52%, so 48% fewer chicks than the best clutch. Parents cannot feed large broods, and a small clutch wastes the chance to rear more. Alleles for clutch sizes near 3 pass on more often: stabilising selection.

Worked example 4: disruptive. Jaw width was measured in juveniles of a lake fish. Small jaws suit tiny prey, large jaws suit snails, and medium jaws suit neither.

Jaw class	Small	Medium	Large
Juveniles	40	60	40
Survival to adult	70%	30%	65%
Adults	28	18	26

Adults number 72 out of 140 juveniles (51%). Medium is the most common juvenile class but the least common adult class, so the next generation will have two peaks. If breeding then stops between the two peaks, disruptive selection can lead to speciation.

Reproductive separation and gene pools

Populations of one species can become reproductively separated, for example by a physical barrier. Separated populations no longer exchange alleles (no gene flow). Each population then changes independently:

• Different mutations arise in each.
• Selection pressures differ, so different alleles are favoured.
• Chance events alter allele frequencies, especially in small populations.

Over many generations, the two gene pools differ.

What makes a new species

New species arise when genetic differences lead to the inability of members of the separated populations to interbreed and produce fertile offspring. Both parts matter. Offspring that are viable but sterile (a mule is a standard example) do not make the parents one species, because genes do not pass on.

Barriers that prevent interbreeding can be set out as: different breeding seasons, different courtship behaviour or signals, incompatible reproductive structures or gametes, or hybrids that die or are sterile. You do not need to memorise a named list, but you should be able to use these to explain why two populations can no longer interbreed.

Worked example 5. Two populations of a small freshwater fish, A and B, were isolated in separate lakes for many generations, then brought together. Fifty pairs of each type were set up.

Cross	Pairs producing fertile offspring
A × A	46 of 50
B × B	44 of 50
A × B	3 of 50

Within-population crosses succeed in 92% and 88% of pairs. Between populations only 6% succeed. This is strong evidence of reproductive separation, but not yet proof of two species: the three pairs that did produce fertile offspring show gene flow is still possible. The conclusion should be that they are diverging, and the species criterion is met only if fertile offspring cannot be produced.

Allopatric speciation

Allopatric means “in other places”. The sequence is:

1. A population is divided by a geographical barrier (sea, mountain range, river, new island).
2. No gene flow between the groups.
3. Different selection pressures and different mutations in each area.
4. Allele frequencies diverge, so gene pools differ.
5. If the groups later meet, they fail to produce fertile offspring together, so they count as two species.

Sympatric speciation

Sympatric means “in the same place”. There is no geographical barrier, but reproductive separation still arises. Examples include:

• Polyploidy in plants. A diploid plant with 2n = 14 can give a tetraploid (4n = 28) after a failure of chromosome separation. The tetraploid breeds with other tetraploids, but a cross with a diploid gives a triploid (21 chromosomes) which is sterile, because chromosomes cannot pair in sets at meiosis. The tetraploid is instantly reproductively separate.
• Different niches or food sources. Disruptive selection may favour two forms, and if each form breeds mainly with its own type, the gene pools separate.
• Different breeding times or behaviours in the same habitat.

The specification names allopatric and sympatric speciation without listing these mechanisms, so treat them as examples you can use in explanations.

Common errors

• Writing that individuals “adapt” or “need” a feature. Selection acts on existing variation.
• Saying the environment causes the new allele. Mutation is random; the environment selects.
• Describing allele frequency change in one lifetime. It is measured across generations.
• Defining a species only by “cannot interbreed”. Fertile offspring is part of the definition.
• Confusing stabilising selection (extremes removed) with no selection at all.
• Giving the geographical barrier as the cause of speciation. The barrier prevents gene flow; divergence comes from mutation and selection.

Official syllabus

OxfordAQA, International AS and A-level Biology (9610), specification Version 5.1, sections 3.3.8.1 and 3.3.8.2 (Unit 3, Populations and genes). The specification’s Required practicals 7 to 9 are in Unit 3 but are not part of this section.