Selection and Evolution

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Cambridge International AS & A Level Biology (9700)

Selection and Evolution

The big idea: Individuals vary, more are born than can survive, and the environment decides which alleles get passed on. Over many generations allele frequencies shift, and when a population is cut off from others its gene pool can drift so far that a new species forms.

Summary — What This Chapter Covers

Phenotypic variation comes from genes, environment or both; discontinuous and continuous variation and their genetic basis.
The t-test for comparing two means.
Natural selection; stabilising, directional and disruptive selection; founder effect, genetic drift and bottlenecks; antibiotic resistance.
The Hardy–Weinberg principle and the conditions for using it.
Selective breeding of wheat, rice, maize and dairy cattle.
Evolution, DNA sequence evidence, and allopatric and sympatric speciation.

1. Variation

17.1

Causes of phenotypic variation

Genetic factors only: ABO blood group, albinism, sickle cell anaemia. Environment has no effect on the phenotype.
Environmental factors only: a scar, a language spoken, a pruned shape of a plant. Identical twins can differ in these.
Both: human height and body mass (genes set the potential; diet and health affect how much is reached); plant height (genes plus light, minerals, water); skin colour (genes plus exposure to sunlight).

Discontinuous and continuous variation

	Discontinuous	Continuous
Description	Distinct categories with no intermediates; shown on a bar chart	A range of values between two extremes; often a normal distribution, shown on a histogram
Examples	ABO blood groups, pea seed shape (round/wrinkled)	Height, mass, leaf length
Genetic basis	One gene, or a few genes, each with a large effect; different alleles give clearly different phenotypes	Many genes (polygenes) at different loci, each with a small, additive effect
Environment	Little or no effect	Large effect, which smooths out the steps between genotypes

Example of additive polygenes: if three unlinked genes each have an allele that adds 5 cm to height (A, B, C) and one that adds nothing (a, b, c), genotypes range from aabbcc (0 additive alleles) to AABBCC (6), giving 7 height classes. Environmental effects blur these into a continuous range.

The t-test

Used to test whether the difference between the means of two samples of continuous data is significant. It is valid when the data are continuous, the populations are normally distributed and the standard deviations are approximately the same. The formula is provided; you must calculate the degrees of freedom.

t-test (formula provided) t = (x̄₁ − x̄₂) / √(s₁²/n₁ + s₂²/n₂) v = n₁ + n₂ − 2

Null hypothesis: there is no significant difference between the two means. If t is greater than the critical value at p = 0.05, the difference is significant (reject the null hypothesis).

Worked Example

Leaf lengths from a sunny site: n = 10, mean 24.0 mm, s = 2.0 mm. From a shaded site: n = 10, mean 21.5 mm, s = 2.5 mm. The critical value of t at p = 0.05 for 18 degrees of freedom is 2.10. Is the difference significant?

2. Natural and Artificial Selection

17.2

Natural selection

many more offspring
vary
compete
best adapted

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Also in the full note

Stabilising, directional and disruptive selection
Other things that change allele frequencies
Antibiotic resistance as natural selection
Selective breeding (artificial selection)
The theory of evolution
Speciation

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Selection and Evolution