AQA A-Level Biology: Natural Selection and Evolution

Evolution is a change in the allele frequencies of a population over time, driven mainly by natural selection. This guide explains where the variation that selection acts on comes from, how natural selection changes a population step by step, and the three patterns selection can produce. The key idea is that selection does not create variation; it acts on variation that already exists, favouring some alleles over others.

The sources of variation

Natural selection can only work if the individuals in a population vary, and that variation has genetic and environmental causes.

The genetic sources are:

• Mutations, which are the primary source of new alleles and so of new variation.
• Crossing over between homologous chromosomes during meiosis, which shuffles alleles into new combinations.
• Independent segregation of homologous chromosomes during meiosis, which mixes maternal and paternal chromosomes.
• Random fertilisation of gametes, which combines two different genomes at random.

On top of this, environmental factors, such as the food or light an organism receives, affect the phenotype. Selection acts on the phenotype, but only the genetic part of variation can be passed on and cause evolution.

The principles of natural selection

Evolution is a change in allele frequency over many generations, occurring through natural selection. Selection is driven by pressures such as predation, disease and competition for resources, which mean not every individual survives and reproduces equally. The process runs in a clear sequence.

1. Mutation. A random mutation produces a new allele of a gene.
2. Advantage. Because of a particular selection pressure, that allele gives its owner an advantage, for example better camouflage or resistance to a drug. The organism has a selective advantage.
3. Reproduction. These individuals are more likely to survive and to reproduce successfully.
4. Inheritance. They pass the advantageous allele on to their offspring.
5. Allele frequency. Over many generations, the frequency of the advantageous allele increases in the gene pool.

Because this raises the frequency of one allele over generations, it is evolution by definition.

The three types of selection

Selection acts on a range of variation in a trait, and depending on which part of that range is favoured, it produces one of three patterns.

• Stabilising selection favours the average. Individuals with alleles for the middle, or modal, value of a trait have the advantage, so alleles for extreme values become less common. Human birth weight is a classic example: very small and very large babies are less likely to survive. The range of the trait narrows.
• Directional selection favours one extreme. When conditions change, individuals at one end of the range have the advantage, so alleles for that extreme increase while alleles for the other extreme decrease. Antibiotic resistance in bacteria is an example: when an antibiotic is present, the most resistant bacteria survive and their alleles spread.
• Disruptive selection favours both extremes at once, against the average. Alleles for both extreme forms increase while alleles for the intermediate form decrease. This splits a population into two groups and can, over time, lead to speciation.

Three things are worth reading off these graphs, because they are what questions ask you to describe.

• The mean. Stabilising selection leaves it where it was; directional selection moves it towards one extreme; disruptive selection leaves no single mean, because the population now has two peaks.
• The range. Stabilising selection narrows it, which is why that curve is taller: the same number of organisms is packed into a smaller spread of the trait. Directional selection leaves the spread much as it was. Disruptive selection widens it.
• The arrows, which show where selection is acting against individuals, and so which way the curve is pushed. Describing a graph like this is a matter of saying what has happened to the mean and to the range, and then naming the type.

How this fits together

Natural selection is the engine of evolution: it takes the variation thrown up by mutation and meiosis and steadily changes a population's allele frequencies. When disruptive selection, or the isolation of two groups, drives their gene pools far enough apart, they can become separate species, which is the subject of the speciation and genetic drift guide.