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
- Stabilising selection maintains genetic polymorphisms in the population
- Disruptive selection also maintains genetic polymorphisms in the population
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.