A-Level Biology AQA Notes

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Variation

Within any population of a species there will be phenotypic variation
Characteristics that show continuous variation are normally polygenic (determined by many gene loci that have additive effects on each other).
Characteristics that show discontinuous variation are usually monogenic (determined by a single gene loci).
Variation is due to genetic and environmental factors.
The main source of genetic variation is mutations, which can produce different alleles of genes.
Further sources of genetic variation include meiosis (independent assortment and crossing over) and the random fertilisation of gametes during sexual reproduction to create new allele combinations.
The environment can influence the way an organism’s genes are expressed. This can be because of biological factors such as predators or non-biological factors such as sunlight.

The Effect of Selection on Allele Frequencies

Predation, disease and competition means that not all individuals within a population survive to get a chance to reproduce. This differential survival and reproduction is the process by which natural selection acts.
The organisms with phenotypes that provides a selective advantage are more likely to reproduce and thus pass on their favourable alleles to the next generation. This means that the proportional of individuals with the favourable allele will
increase in the next generation (increase the allele frequency) within the population. The population evolves.
Evolution is the change in allele frequencies in a population over time.
Directional selection results in the increase of a favoured allele over time

Speciation

Speciation is the evolution of new species from existing ones.
Reproductive isolation followed by accumulation of genetic changes through natural selection can result in the formation of a new species. This is because the populations become genetically distinct with different allele combinations, making them unable to breed to produce fertile offspring.

Allopatric speciation is the formation of two species from an original one due to geographical isolation.
Sympatric speciation is the formation of two species from one original species due to reproductive isolation whilst occupying the same geographical location. This can be by:
- Temporal variation- breeding seasons at different times.
- Behavioural variation- mutations affecting courtship.
- Mechanical variation- anatomical differences preventing mating.
- Gametic variation- results in genetic or biochemical incompatibility.
- Hybrid sterility- cannot produce viable gametes.

Genetic Drift

Genetic drift describes change in allele frequencies in the gene pool of a population (evolution) due purely to chance events and not selection pressures
Due to the random nature of gamete production and fertilisation, certain alleles may increase in the population due to chance.
The effect of genetic drift is more prominent within small populations because chance has a greater influence, whereas in larger populations the random fluctuations even out across the whole population.
A genetic bottleneck is when an event causes a big reduction in a population’s size and gene pool. Certain alleles may be due to the event and the population will also be subject to genetic drift.
When a new population is established by a small number of individuals, the founding population will have low genetic diversity and be heavily influenced by genetic drift. This is the founder effect.

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