Module 4: Natural Selection

These free OCR A Level Biology Natural Selection revision notes have been written for specification point 4.2.2(h).

Natural selection is a process by which those individuals who are best adapted to their environment survive and reproduce more successfully than those who are not, passing on their alleles to the next generation.

Over many generations, these alleles, and the characteristics they code for, increase in frequency in the population.

Evolution is when the characteristics in a species change due to significant changes in allele frequencies.

Speciation is when a new species evolves; the population becomes distinct enough from the ancestral precursor species that they can no longer interbreed to produce fertile offspring.

Natural Selection

The process of natural selection can be simplified into the following stages:

1. There is variation within a population caused by alleles which arise from mutation.

2. Some individuals will be better adapted (fitter) than others.

3. Selection pressures allow only the fitter individuals to survive and reproduce.

4. These individuals pass on their alleles for their traits to their offspring.

5. These characteristics, and their alleles, become more common in the next generation.

6. If enough time passes, as this process repeats over time genetic differences will accumulate within the population that it may be considered a new species from it’s ancestral one.

Natural Selection Example: Peppered Moths

Peppered moths are a historical example that occurred in England during the Industrial Revolution, as the burning of coal created soot that darkened the surfaces of trees. The white and speckled peppered moth, camouflaged to blend in with tree bark, now stood out and was easily hunted and eaten by predators. However, some individuals carried a mutated allele that caused melanism (black pigment). This trait was previously selected against and remained uncommon in the population, but now conferred a distinct survival advantage in the new environment, over time becoming the dominant characteristic in the population.

Once England’s air pollution was resolved, a reversal occurred in the population’s occurrence of phenotypes, and now the white speckled peppered moth is the most common form.

The natural selection process at work is:

1) There is variation within a population of peppered moths.

2) Some individuals have preexisting alleles, or a new mutation, that confers darker colouring (melanism).

3) The population is exposed to the selection pressure of industrial soot pollution, which darkens tree bark and makes pale moths more visible to predators, whilst darker moths are better camouflaged and less likely to be seen and eaten.

4) Those with the beneficial allele for melanism survive and reproduce, passing on the allele to their offspring.

5) The allele for dark colouring increases in frequency in the next generation, and so this new population is more likely to survive in polluted environments.

6) This process repeats over many generations, leading to the evolution of a population adapted to urban, soot-covered habitats.