Random mutations occur within a species of a Galapagos Finch, causing several types of Finch in a region
An area has limited resources so it is unable to host all types of animals, Since areas all differ, the food source would be different so only certain finches have the phenotype to suit the environment
Only some of the finches are able to survive to the environment they are currently at
Those better suited to the environment thrive and reproduce
As generations pass, the trait becomes the common allele and the other types die off