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Flashcards · 9700 · A Level Topic 17

Selection and evolution (A Level) flashcards

16 cards on A Level Topic 17, subtopics 17.1 to 17.3. Pick a subtopic or study them all.

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17.1 Variation

Discontinuous vs continuous variation
Discontinuous: a few distinct classes; one or a few genes with large effects; little environmental effect; bar chart (ABO blood group). Continuous: a range; many additive polygenes plus a large environmental effect; histogram, often normal (height).
Genetic basis of continuous variation
Many unlinked genes each have additive alleles that add a small amount. The large number of combinations, smoothed by the environment, gives a continuous range.
When is a t-test used?
To test whether the means of two samples differ significantly. The data must be continuous, from normally distributed populations with similar standard deviations.
t-test: degrees of freedom and interpreting t
df = n₁ + n₂ − 2. If t is greater than the critical value at p = 0.05, the difference is significant and the null hypothesis is rejected; if smaller, the difference is not significant.
Example of variation caused by both genes and environment
Human height and body mass, or plant height (alleles plus light, water and mineral ions). ABO blood group is genetic only; a scar is environmental only.

17.2 Natural and artificial selection

Natural selection in five steps
Over-production of offspring; variation, partly genetic; competition for limited resources; the best adapted are more likely to survive and reproduce; they pass on their alleles, so advantageous allele frequencies rise.
Stabilising vs directional vs disruptive selection
Stabilising: against both extremes; mean unchanged, narrower range; stable environment. Directional: against one extreme; mean shifts; changing environment. Disruptive: against the middle; two peaks.
Genetic drift, founder effect and bottleneck
Genetic drift: random change in allele frequency by chance, strongest in small populations. Founder effect: a few individuals start a new population with a non-representative sample of alleles. Bottleneck: a population is suddenly reduced, leaving low diversity.
Hardy–Weinberg: how to find allele frequencies from phenotypes
q² = number with the recessive phenotype ÷ total. q = √q²; p = 1 − q. Heterozygote frequency = 2pq; homozygous dominant = p². Conditions: large population, random mating, no mutation, migration or selection.
Natural vs artificial selection
Natural: the environment is the selection pressure; favours survival and reproduction; usually slow. Artificial: humans choose which individuals breed for useful features; can be fast; often reduces genetic diversity.
Why do maize farmers grow F1 hybrids from two inbred lines?
The F1 is heterozygous at many loci, giving hybrid vigour, and all plants have the same genotype, giving a uniform crop. The F1 does not breed true, so new seed is bought each year.
How are dairy bulls selected?
By progeny testing: bulls carry milk-yield alleles but cannot show them, so they are judged by their daughters' milk yield. High-yield cows are then inseminated artificially.

17.3 Evolution

Evolution and gene pool
Evolution: the formation of new species from pre-existing species over time, through changes to gene pools from generation to generation. Gene pool: all the alleles of all the genes in a population.
How can DNA sequence data show relationships between species?
Compare the base sequence of the same gene. Fewer differences means a more recent common ancestor (more closely related). If mutations build up at a roughly steady rate, the number of differences estimates the time since divergence.
Allopatric vs sympatric speciation
Allopatric: populations separated by a geographical barrier (mountains, sea). Sympatric: isolation in the same area, by ecological or behavioural differences (food, breeding time, courtship).
Steps in speciation after isolation
No gene flow between populations → different mutations, selection pressures and drift → gene pools diverge → the populations can no longer interbreed to produce fertile offspring.

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