Free Evolution and Natural Selection OCR A Level Biology revision notes – covering specification point 6.1.2(e).
Evolution is a change in the characteristics of a species over time.
Evolution occurs when the frequencies of alleles in a population’s gene pool change over time.
When the allele frequencies in a population change significantly from those of the ancestral population so that reproduction no longer produces fertile offspring, they are considered to have evolved into a new species.
The evolution of a new species is known as speciation.
The Role of Natural Selection
Natural selection is the process by which organisms with advantageous characteristics are more likely to survive and reproduce, passing on their alleles for these advantageous traits to the next generation.
Types of Natural Selection
The 3 primary types of natural selection are directional selection, disruptive selection and stabilising selection:
- Directional selection favours phenotypes at one extreme, shifting the population mean in that direction.
- Disruptive selection* favours both extremes simultaneously, reducing the frequency of intermediate phenotypes.
- Stabilising selection favours intermediate phenotypes and selects against extremes, maintaining the population mean.
*Disruptive selection, unlike directional selection and stabilising selection, is not on the OCR A Level Biology specification; however, its effect has appeared in official OCR A Level Biology question papers and been named in the official mark scheme.
Random Mechanisms
Genetic drift is the random change in allele frequency caused by chance events that prevent organisms from passing on their alleles.
Genetic drift can be caused by the following events:
- Chance events like extreme weather (e.g. lightning strikes) or accidents (e.g. being crushed in a landslide).
- Random reproduction success (e.g. seeds land on bare rock).
- Genetic bottleneck events (a catastrophic event drastically reduces the population, greatly reducing the genetic diversity in the surviving population).
- Founder effect (a small group colonises a new area, carrying only a random subset of alleles).
Speciation
Speciation occurs when a population becomes genetically distinct enough from its ancestral population that it can no longer interbreed to produce fertile offspring.
Speciation is caused by evolution occurring within an isolated population, which leads to changes in allele frequencies.