Summary Biology A2 level 'variation and evolution' notes (WJEC)
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Course
Variation and evolution
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WJEC
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WJEC Biology for A2: Study and Revision Guide
Biology A2 level 4.4 'variation and evolution'. These notes were formulated using class notes, the WJEC study guide and past papers. These notes helped me to achieve an A* at A level biology!
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4.4 Variation and evolution
Types of variation
● Variation in phenotypes may be due to;
○ Having different genotypes
○ Having the same genotype but different epigenetic modifications
○ Being exposed to different environments
● Heritable variation (can be passed onto offspring) arises from;
○ Gene mutations
○ Crossing over during prophase I of meiosis
○ Independent assortment during metaphase I & II of meiosis
○ Random mating i.e. that any organism can mate with another
○ Random fusion of gametes
○ Environmental factors leading to epigenetic modifications
■ Can lead to non-heritable variation e.g. diet
● Two types of variation
○ Continuous variation e.g. height
■ Range of phenotypes, from one extreme to the other / no distinct
categories
■ Controlled by many genes (polygenic)
■ Follows a ‘normal’ distribution - is ‘bell shaped’
■ Environmental factors have a large influence
○ Discontinuous variation e.g. blood group
■ Characteristic fit into distinct categories (nominal data) - no intermediate
values
■ Usually controlled by one gene with 2+ alleles (monogenic)
■ Environmental factors have little influence on gene expression
, T-test
● Significant value between the mean values of two continuous data sets
● Mean - a measure of central tendency
● Standard deviation - measure of variation either side of the mean
● t-value must exceed the critical value for results to be significant
○ p=0.05
○ Degrees of freedom as the number of observations-2
Causes of variation
Environmental factors
● Environmental influences affect genotype expression ⇾ different phenotypes
● E.g. industrial melanism in the peppered moth (Biston betularia)
○ In polluted habitats where trees are covered in soot, dark form is most common
○ In unpolluted habitats where lichens are found, speckled form is more common
○ Colour provides camouflage, giving a selective advantage
■ More likely to survive, reproduce transfer advantageous alleles to
offspring, numbers within population increase
● Biotic & abiotic factors
● Density dependent & density independent factors
Competition
● Intraspecific competition
○ Members of the same species compete for the same resources in an ecosystem
● Interspecific competition
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