Natural selection

Natural selection is a theory devised by Charles Darwin concerning the survival of the fittest, whereby organisms that are more adapted to the environment (most fit) will live on to survive and produce fertile offspring. Natural selection comprises of four elements. When answering questions regarding natural selection all of the elements should be present in the answer.

Elements of natural selection

  • Phenotypic variation must exist within the population.
  • When there is a change in environmental conditions, organisms with the favoured phenotype will have an improved chance of surviving and reproducing compared to those with the less favoured phenotype. Individuals with the favoured phenotype are said to have a selective advantage.
  • Individuals who possess the favoured characteristics are more likely to survive to the next generation.
  • The individuals who have survived will reproduce to produce fertile offspring, and hence the frequency of individuals with the desired characteristics will increase in the population over time.

All questions regarding natural selection should incorporate the aforementioned elements. However, when answering the question do not refer to the elements in a generic way; make sure your answers are specific to the question.

Example of a natural selection question

This is example and sample answer has been sourced from VCAA Biology Examination 2 2010.


The blue mussel, Mytilus edulis, lives along the northeastern coastline of the USA. A species of Asian shore crab, Hemigrapsus sanguineus, was accidentally introduced into the area about 15 years ago. The Asian shore crab has only migrated to the southern half of the total area inhabited by the blue mussel. The Asian shore crab feeds off the blue mussels. The thinner the mussel shell, the easier it is for the crab to crush and eat the mussel. In recent times, scientists have observed that the overall population of the southern blue mussel has a thicker shell than that of the northern blue mussel. This contrasts with 15 years earlier when there was no difference in the range of shell thickness in northern and southern blue mussel populations.

Explain the process of natural selection that has occurred in the population of southern blue mussels over the last 15 years that has resulted in thicker shells.


There was variation in the mussel population or thick-shelled mussels had a selective advantage the crabs eat more thin-shelled mussels or crabs are a selective pressure. The thicker-shelled mussels survived, reproduced and passed their alleles on to the next generation.

Sourced form the 2010 VCAA examiners report for Examination 2 VCE Biology.

As can be noted from the answer, all the elements are included in a concise manner and are written specific to the question.

See also