Summary WJEC (England) Eduqas A-Level Biology 2. Continuity of Life - 6. variation & evolution
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Course
Continuity of life
Institution
WJEC
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a. Variation of phenotype = genetic (genotype / epigenetic modification) & environmental factors
b. Continuous & discontinuous variation
Continuous – normal distribution Discontinuous – bar chart
- Range of phenotypes – intermediates between - Distinct groups of characteristics – no
extremes intermediates
- Polygenic – trait controlled by many genes - Monogenic
- Environment factors = major - Environmental factors = minor
heritable & non-heritable variation
Heritable Non-heritable
- Gene mutations - Environment (non-epigenetic): diet
- Crossing over
- Independent assortment
- Random mating
- Random fertilisation
- Environment: epigenetic modifications
c. Competition = selection pressure: survival & breeding success = natural selection
Interspecific: same species
Intraspecific: different species
d. Selection
pressure: environmental factor = affects organisms ability to survive = changes allele frequency in pop when limiting
- Food supply
- Breeding/nesting site availability
- Climate
- Human impacts: habitat loss = no breeding sites
Natural selection:
- conditions change = selection pressures affect survival of phenotypes in population
- mutation = selective advantage
- survive & reproduce = pass selected alleles to offspring
- repeat over several generations = allele frequency increase over time
Stabilising: extreme values selected against = more average (higher peak)
Directional: 1 extreme value selected for/against = curve moves in one direction
Disruptive: average selected against = 2 peaks
e. Population genetics – allele frequencies in gene pool
stable environment = stable allele proportions (change with selection pressures)
gene pool: total alleles for all genes in population
genetic drift: chance variations in allele frequencies in population
- significant in small/isolated populations – small number of alleles have high frequency
founder effect: small number isolate from large pop & establish new – have reduced genetic variation = genetic drift
f. Allele frequency: expressed as proportion/percentage of total alleles for that gene
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