What you'll learn
Variation, natural selection and speciation explains how populations change over time and how new species arise. It is the unifying theory of biology, and at CAPE level it is treated quantitatively as well as descriptively: you are expected to distinguish the types of selection, apply the Hardy–Weinberg principle to calculate allele frequencies, and explain the conditions under which those frequencies remain stable. By the end of this topic you should be able to distinguish continuous from discontinuous variation and identify their causes, explain natural selection and the three types of selection pressure, apply the Hardy–Weinberg equations, distinguish allopatric from sympatric speciation, explain genetic drift and the founder effect, and evaluate examples including antibiotic resistance and sickle cell.
Key terms and definitions
Variation — differences between individuals of the same species
Continuous variation — variation showing a range of values with no distinct categories, controlled by many genes
Discontinuous variation — variation falling into distinct categories, usually controlled by one or few genes
Polygenic — controlled by many genes acting together
Gene pool — all the alleles of all the genes in a population
Allele frequency — the proportion of a particular allele in the gene pool
Selection pressure — an environmental factor affecting the chance of survival and reproduction
Directional selection — selection favouring one extreme of a range, shifting the mean
Stabilising selection — selection favouring the mean and acting against both extremes
Disruptive selection — selection favouring both extremes against the mean
Speciation — the formation of new species
Reproductive isolation — the prevention of interbreeding between populations
Genetic drift — random change in allele frequency, significant in small populations
Founder effect — reduced genetic diversity when a small group establishes a new population
Core concepts
Types and causes of variation
Continuous variation shows a range of values between two extremes with no distinct categories, such as height, mass and skin colour. It is controlled by many genes acting together, described as polygenic, and is usually strongly influenced by the environment. Plotted as a frequency distribution it typically gives a normal, bell-shaped curve.
Discontinuous variation falls into clearly separate categories with no intermediates, such as ABO blood group, ability to roll the tongue, and sex. It is controlled by one or a few genes and is largely unaffected by the environment. Plotted as a frequency distribution it gives distinct bars.
Genetic causes of variation are meiosis, specifically crossing over and independent assortment; random fertilisation; and mutation, which is the only source of entirely new alleles.
Environmental causes include nutrition, light, temperature and disease. Most characteristics result from an interaction of both: an individual's genotype sets a potential range and the environment determines where within that range the phenotype falls.
Only genetic variation is heritable and therefore available to natural selection, which is a point examiners frequently test.
Natural selection
Natural selection follows from a small number of observations and their logical consequence, and the argument should be set out in order.
Organisms produce more offspring than can survive to maturity.
There is genetic variation within the population, arising ultimately from mutation.
Environmental factors act as selection pressures, so that resources such as food, light, space and mates are limited and there is competition for them.
Individuals with alleles conferring advantageous characteristics are more likely to survive this competition and to reproduce successfully.
They therefore pass those alleles to their offspring, and over many generations the frequency of the advantageous alleles in the gene pool increases.
The population as a whole becomes better adapted to its environment. Note carefully that individuals do not adapt during their lifetime; the population changes because differential survival alters allele frequencies.
The three types of selection
Directional selection occurs when the environment changes or a new selection pressure appears, favouring one extreme of the range. The mean of the population shifts in that direction over generations. Antibiotic resistance in bacteria and the peppered moth during industrialisation are standard examples.
Stabilising selection occurs in a stable environment, favouring the mean and selecting against both extremes. The range narrows and variation is reduced. Human birth mass is the standard example: very low birth mass carries a high risk of mortality, while very high birth mass increases the risk of complications during delivery, so intermediate masses are favoured.
Disruptive selection favours both extremes against the intermediate, and can split a population into two distinct groups. It is the type most likely to lead to speciation, since it creates two divergent groups within one population.
Recognising which type is operating from a graph of frequency distribution before and after selection is a standard question: a shifted peak indicates directional, a narrowed peak stabilising, and two peaks disruptive.
Antibiotic resistance as an example
Antibiotic resistance illustrates natural selection over an observable timescale and is examined regularly.
Random mutation in a bacterium produces an allele conferring resistance. The mutation occurs by chance and is not caused by the antibiotic — a point candidates very often get wrong.
When the antibiotic is applied it acts as a selection pressure. Non-resistant bacteria are killed, while the resistant bacterium survives.
The survivor reproduces, and because bacteria reproduce rapidly by binary fission, the resistant strain increases in number quickly. Resistance alleles can also spread horizontally between bacteria on plasmids, which accelerates the process further.
The proportion of resistant bacteria in the population therefore rises, and the antibiotic becomes ineffective.
Overuse and inappropriate prescription increase the selection pressure, and failing to complete a course leaves partially resistant survivors, which is why both are discouraged.
Sickle cell and balanced polymorphism
The sickle cell allele is a particularly relevant Caribbean example and illustrates that an allele may be advantageous in one environment and harmful in another.
Individuals homozygous for the sickle allele suffer sickle cell anaemia and historically had reduced survival. Individuals homozygous for the normal allele are fully susceptible to malaria. Heterozygotes have sickle cell trait, with mild or no symptoms, and have increased resistance to malaria.
In regions where malaria is endemic, heterozygotes therefore have the highest fitness, and selection maintains both alleles in the population rather than eliminating the sickle allele. This is called balanced polymorphism or heterozygote advantage.
Where malaria is absent, the heterozygote advantage disappears and the sickle allele is selected against, which is why its frequency is lower in populations with no history of malaria.
The Hardy–Weinberg principle
The Hardy–Weinberg principle allows allele and genotype frequencies to be calculated and provides a null model against which change can be detected.
Two equations are used. The allele frequencies sum to one: p plus q equals 1, where p is the frequency of the dominant allele and q the frequency of the recessive allele.
The genotype frequencies also sum to one: p squared plus 2pq plus q squared equals 1, where p squared is the frequency of the homozygous dominant genotype, 2pq the heterozygous, and q squared the homozygous recessive.
The usual route through a calculation begins with the recessive phenotype, since only that phenotype corresponds to a single genotype. Its frequency gives q squared directly, from which q is the square root, p is 1 minus q, and the remaining genotype frequencies follow.
The principle holds only under five conditions, which must be known: the population is large; mating is random; there is no migration into or out of the population; there is no mutation; and there is no natural selection. If observed frequencies depart from those predicted, one or more of these conditions is not met, which is how the principle is used to detect that evolution is occurring.
Speciation
A species is a group of organisms that can interbreed to produce fertile offspring. Speciation requires that one population becomes two that can no longer do so.
Allopatric speciation results from geographical isolation. A physical barrier such as a river, mountain range or stretch of sea divides a population. The two groups experience different environmental conditions and therefore different selection pressures, and mutations arising in one cannot spread to the other. Over many generations the allele frequencies diverge, and eventually the two populations become so different that they could not interbreed successfully even if reunited.
Sympatric speciation occurs without geographical separation, through reproductive isolation arising within a single area. Mechanisms include temporal isolation, where the two groups breed at different times of year; behavioural isolation, where courtship behaviour diverges so that individuals no longer recognise one another as mates; mechanical isolation, where reproductive structures become incompatible; and polyploidy, which is common in plants and produces immediate reproductive isolation because the polyploid cannot produce fertile offspring with the diploid parent population.
In both cases the essential sequence is isolation, then divergence under different selection pressures or by drift, then reproductive incompatibility.
Genetic drift and the founder effect
Genetic drift is random change in allele frequency between generations, caused by chance rather than by selection. In a large population these random fluctuations largely cancel out, but in a small population they can change allele frequencies substantially and may eliminate an allele entirely regardless of whether it is advantageous.
The founder effect occurs when a small number of individuals establish a new population, for example by colonising an island. They carry only a small sample of the original gene pool, so the new population has reduced genetic diversity and allele frequencies that may differ markedly from the parent population by chance alone.
A genetic bottleneck has a similar effect, occurring when a population is drastically reduced by a catastrophe and then recovers from the few survivors.
These processes matter for conservation, because small populations lose genetic diversity and with it the capacity to adapt to environmental change.
Worked examples
Example 1: A Hardy–Weinberg calculation (5 marks)
In a population, 4 per cent of individuals show a recessive phenotype. Calculate the frequency of the dominant allele and the percentage of the population expected to be heterozygous.
Only the homozygous recessive genotype produces the recessive phenotype, so q squared equals 0.04.
Taking the square root, q equals 0.2, which is the frequency of the recessive allele.
Since p plus q equals 1, p equals 1 minus 0.2, which is 0.8. The frequency of the dominant allele is therefore 0.8.
The heterozygous frequency is 2pq, which is 2 multiplied by 0.8 multiplied by 0.2, giving 0.32. Therefore 32 per cent of the population is expected to be heterozygous.
As a check, p squared is 0.64, and 0.64 plus 0.32 plus 0.04 equals 1.00 as required.
Example 2: Identifying a type of selection (4 marks)
Before a change in climate, a population of plants showed a normal distribution of stem height with a mean of 40 centimetres. After twenty generations the distribution remained normal but the mean had shifted to 52 centimetres. Identify the type of selection and explain.
This is directional selection. The change in climate acted as a new selection pressure favouring one extreme of the existing range, in this case taller plants — perhaps because increased competition for light advantaged plants that could overtop their neighbours.
Taller plants were therefore more likely to survive and reproduce, passing the alleles for greater height to their offspring. Over generations the frequency of those alleles increased in the gene pool, so the mean stem height of the population shifted upwards.
The distribution remains normal because variation is still present; it is the position of the mean that has changed rather than the spread.
Example 3: Explaining allopatric speciation (5 marks)
Explain how a single species of lizard living on a mainland could give rise to a separate species on an offshore island.
A small number of individuals reach the island, for example carried on floating vegetation, and establish a new population. Because they represent only a small sample of the mainland gene pool, the island population begins with different allele frequencies through the founder effect.
The two populations are now geographically isolated by the sea, so there is no gene flow between them: mutations arising in one population cannot spread to the other.
The island presents different environmental conditions — perhaps different prey, predators or temperature — and therefore different selection pressures. Different alleles are advantageous in each population, so natural selection changes their allele frequencies in different directions. Genetic drift also acts more strongly on the small island population.
Over many generations the two populations diverge in genotype and phenotype. Eventually the differences are so great that individuals from the two populations could not interbreed to produce fertile offspring even if brought together, so they constitute separate species.
Common mistakes and how to avoid them
The most frequent and most heavily penalised error is stating that organisms adapt to their environment, or that mutations arise because they are needed. Mutation is random and occurs first; selection acts afterwards on the variation already present.
Students often write that antibiotics cause bacteria to become resistant. The antibiotic selects for pre-existing resistant individuals.
Another common slip is confusing the types of selection. A shifted mean is directional, a narrowed range stabilising, and two separate peaks disruptive.
In Hardy–Weinberg calculations, many candidates use the recessive phenotype frequency as q rather than as q squared. Only the square root of the recessive phenotype frequency gives the allele frequency.
Candidates frequently omit the conditions under which Hardy–Weinberg applies, which is often a separate question part worth several marks.
Finally, answers about speciation often describe isolation without describing divergence, or divergence without stating that reproductive incompatibility results. All three stages are needed.
Exam technique for "Variation, natural selection and speciation"
Set out natural selection as a numbered sequence — variation from mutation, selection pressure, differential survival, reproduction, increased allele frequency. Marks are usually allocated one per stage.
Use the language of alleles and frequencies rather than of individuals changing. Populations evolve; individuals do not.
In Hardy–Weinberg questions, always start from the recessive phenotype and work outwards, and check that the three genotype frequencies sum to one.
For speciation, name the type, state the isolating mechanism, and then give the three stages in order.
When a graph of frequency distribution is supplied, describe the change in both the mean and the spread before naming the type of selection.
Quick revision summary
Continuous variation is polygenic and environmentally influenced, giving a normal distribution; discontinuous variation is controlled by few genes and gives distinct categories. Genetic variation arises from crossing over, independent assortment, random fertilisation and mutation, and only genetic variation is heritable. Natural selection follows from overproduction of offspring, genetic variation, selection pressures and competition, leading to differential survival and reproduction and an increase in advantageous allele frequencies. Directional selection shifts the mean, stabilising selection narrows the range, and disruptive selection favours both extremes and may cause speciation. Antibiotic resistance arises by random mutation and is then selected for by the antibiotic, spreading rapidly and horizontally by plasmids. Sickle cell shows balanced polymorphism, with heterozygote advantage maintaining both alleles where malaria is endemic. The Hardy–Weinberg equations p plus q equals 1 and p squared plus 2pq plus q squared equals 1 apply only to large, randomly mating populations with no migration, mutation or selection. Allopatric speciation follows geographical isolation and sympatric speciation follows temporal, behavioural, mechanical or polyploid isolation, in each case through isolation, divergence and reproductive incompatibility. Genetic drift and the founder effect change allele frequencies by chance in small populations.