Two species of frogs, each found on one of two neighbouring islands yet with the same genus, are identified. Scientists believe that the two species originated from a common ancestor. Explain how these species could have evolved.

Questions about speciation are at their core based on Darwin's theory of evolution. As a result, we must recall GCSE knowledge of natural selection to combine with the new concepts in speciation introduced at A level. The context given in the question should also be considered in order to understand why two distinct species arose from one.In the below answer I have highlighted key marking points often found in exam mark schemes in bold, and GCSE knowledge of natural selection in italics:1) The original population of the common species of frog may have been divided into two groups as the ocean divided a single island into the two found today. This would result in the two groups being geographically isolated from one another.2) This physical barrier would means gene flow between the two groups would not occur as individuals from the different populations could not breed together.3) Within each group of frogs there would be variation due to random genetic mutations. 4) Each of the two groups of frogs would experience different selection pressures due to differences in the environment of the two islands.5) This means that different mutated characteristics would be an evolutionary advantage to each group.6) In each group, individuals with advantageous characteristics would be more likely to survive and reproduce, passing on their mutation to their offspring.7) This would cause the allele frequencies in both populations to change in different ways.8) As a result, over time the two groups would become so genetically dissimilar due to multiple occasions of natural selection in each group that the two groups could not breed together the produce fertile offspring. This means the two groups would have become different species.