Everything from the 5 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
Phenotypic variation is caused by genetic factors (different alleles, produced by mutation and shuffled by meiosis and random fertilisation), by environmental factors (diet, light, soil minerals, temperature), or by a combination of both. Discontinuous variation gives a few distinct classes with no intermediates (ABO blood group); it is controlled by one gene or a few genes with large effects and is little affected by the environment. Continuous variation gives a range with no clear classes (height, mass, leaf length); it is controlled by many genes (polygenes), each with a small additive effect, and the environment adds further variation. To decide whether the means of two samples of continuous data differ significantly, calculate t and compare it with the critical value at p = 0.05 for n1 + n2 − 2 degrees of freedom.
Variation caused only by the environment is not passed on to offspring, because it does not change the alleles in the gametes. Only genetic variation can be acted on by natural selection.
Continuous variation is not caused by alleles blending. Each gene is still inherited as discrete alleles; it is the number of genes and the environment that make the overall range look smooth.
A large difference between means is not enough on its own. If the data are very spread out (large standard deviations) or the samples are small, t can still be below the critical value.
Populations have the capacity to produce many more offspring than can survive, so individuals compete for limited resources such as food, mates and territory. Because of variation, some individuals have alleles that make them better adapted to the conditions. In this struggle for existence they are more likely to survive and reproduce and pass on their alleles, so the frequency of those alleles rises in the next generation. Environmental factors act as selection pressures that may be stabilising (the mean is favoured), directional (one extreme is favoured) or disruptive (both extremes are favoured). Allele frequencies also change by chance: genetic drift, the founder effect and the bottleneck effect all have the greatest effect in small populations. Antibiotic resistance is directional selection in action.
Organisms do not change in order to survive, and an individual does not evolve. Variation already exists; the environment selects among it, and it is the population's allele frequencies that change.
Drift is not caused by an allele being harmful. A neutral or even an advantageous allele can be lost by chance in a very small population.
Bacteria do not become resistant because they are exposed to the antibiotic or get used to it. The resistant mutants were already present; the antibiotic simply removes their competitors.
For a gene with a dominant allele A (frequency p) and a recessive allele a (frequency q): p + q = 1 (allele frequencies) and p2 + 2pq + q2 = 1 (genotype frequencies), where p2 = AA, 2pq = Aa and q2 = aa. Only the recessive phenotype reveals its genotype, so the usual method is: q2 = frequency of the recessive phenotype, q = √q2, p = 1 − q, then calculate 2pq (carriers) and p2. The principle predicts that allele frequencies stay constant from generation to generation only if the population is large, mating is random, there is no selection, no mutation and no migration (gene flow) in or out, and the organism is diploid and reproduces sexually. If observed frequencies differ from the prediction, one of these conditions is not being met.
Do not take the square root of the dominant phenotype frequency to find p. People with the dominant phenotype are AA and Aa, so their frequency is p2 + 2pq, not p2. Always start from the recessive phenotype.
The Hardy-Weinberg principle does not say that the dominant allele becomes more common. Without selection, a dominant allele stays at the same frequency, however rare it is.
In selective breeding (artificial selection), humans choose individuals with desirable characteristics and allow only these to breed. Over many generations the frequency of the alleles for the desired features rises. It differs from natural selection because the selection pressure is set by humans, and the selected features need not help the organism survive. Disease resistance is introduced into wheat and rice by crossing a high-yielding but susceptible variety with a resistant variety, then backcrossing repeatedly to the high-yielder and selecting resistant, high-yielding offspring. Maize is inbred for several generations to give homozygous, uniform inbred lines, then two lines are crossed (hybridisation) to give F1 hybrids with hybrid vigour that are also uniform. The milk yield of dairy cattle is improved by selecting high-yielding cows and bulls chosen by progeny testing, using artificial insemination. Selective breeding reduces genetic diversity.
Selective breeding does not create new alleles. It can only increase the frequency of alleles that already exist in the breeding population or its wild relatives.
The vigorous hybrids are not the inbred lines. Inbreeding makes plants uniform but weak; it is crossing two inbred lines that gives vigour, and the F1 is uniform because both parents are homozygous.
A bull does carry alleles affecting milk yield even though he produces no milk. That is exactly why progeny testing works: his daughters show the effect of the alleles he passes on.
The theory of evolution states that new species form from pre-existing species over time, as a result of changes to gene pools from generation to generation, driven by mutation, natural selection and genetic drift. DNA sequence data give evidence of relationships: mutations accumulate at a roughly steady rate, so the fewer the differences between the base sequences of two species, the more recently they shared a common ancestor. A species is a group of organisms that can interbreed to produce fertile offspring. Speciation needs genetic isolation, so that there is no gene flow between two populations. In allopatric speciation a geographical barrier separates them; in sympatric speciation they live in the same area but are separated ecologically (different habitats or food) or behaviourally (different courtship or breeding times). Different mutations, selection pressures and drift then change each gene pool until the populations can no longer interbreed.
Humans did not evolve from chimpanzees. The tree shows that humans and chimpanzees share a common ancestor more recently than either does with gorillas; both lineages have changed since the split.
Separation alone does not make a new species. The populations must become reproductively isolated: unable to interbreed to produce fertile offspring. Two populations separated briefly that can still interbreed are still one species.