Selection and evolution

You will be able to

• Explain, with examples, that phenotypic variation is due to genetic factors, environmental factors or both.
• Explain discontinuous and continuous variation, and their genetic basis in one or a few genes of large effect or in many genes of small additive effect.
• Use a t-test, with the null hypothesis, degrees of freedom and the critical value at p = 0.05, to compare two sample means, and state what the result does and does not show.

You will be able to

• Explain how over-production, variation and competition lead to the survival and reproduction of the best adapted, who pass on their alleles.
• Explain how environmental factors act as stabilising, directional and disruptive forces of selection.
• Outline how bacteria become resistant to antibiotics as an example of natural selection, and why individuals do not evolve but populations do.

You will be able to

• Explain how genetic drift, the founder effect and the bottleneck effect change allele frequencies by chance, most strongly in small populations.
• Distinguish the consistent direction of selection from the random direction of drift in replicate populations.
• Calculate allele and genotype frequencies with the Hardy–Weinberg equations and state the conditions under which they apply.

You will be able to

• Describe the principles of selective breeding: humans choose individuals with the desired features to breed, over many generations, changing allele frequencies.
• Outline how disease resistance is bred into wheat and rice, how inbreeding and hybridisation give vigorous, uniform maize, and how the milk yield of dairy cattle is improved.
• Explain why the response to selection slows as genetic variation is used up, and why selective breeding does not alter DNA directly.

You will be able to

• Outline evolution as the formation of new species from pre-existing species, through changes to gene pools over many generations.
• Explain how geographical separation (allopatric) and ecological or behavioural separation (sympatric) lead to genetic isolation and reproductive isolation.
• Discuss how differences in DNA sequences show evolutionary relationships, and build a simple tree from a difference matrix.