Natural Selection (AQA AS Biology): Revision Note

The theory of natural selection

• Genetic variation exists within populations due to the presence of different alleles

• There is differential reproductive success between the organisms with different alleles of the same gene

• Under certain environmental conditions, individuals with certain alleles will have an increased chance of survival and reproduction

• Natural selection can cause the frequency of alleles in a population to change over time

Principles of natural selection

• Random mutation can produce new alleles of a gene

• Many mutations are harmful or neutral but under certain environmental conditions, the new alleles may benefit their possessor, leading to an increased chance of survival and increased reproductive success

• The advantageous allele is passed on to the next generation through reproductive success

• As a result, over several generations, the new allele will increase in frequency in the population

Example: natural selection in rabbits

• Variation in fur colour exists within rabbit populations

• At a single gene locus, normal brown fur is produced by a dominant allele, whereas white fur is produced by a recessive allele in a homozygous individual

• Rabbits have natural predators like foxes, which act as a selection pressure

• Rabbits with a white coat do not camouflage as well as rabbits with brown fur, meaning predators are more likely to see white rabbits when hunting

• As a result, rabbits with white fur are less likely to survive than rabbits with brown fur

• Therefore, the rabbits with brown fur have a selection advantage, so they are more likely to survive to reproductive age and be able to pass on their alleles to their offspring

• Over many generations, the frequency of alleles for brown fur will increase, and the frequency of alleles for white fur will decrease