Volcanic activity in the Pacific Ocean causes a new island to form. This island initially has no living residents, but after a while, droppings from passing migratory birds introduce seeds to the island. Five of these seeds come from one species of palm tree. This agent of evolutionary change has shaped the initial allele frequency on the new island . Over time, variation in several phenotypic traits, including palm height and number of fruit produced each year, accumulate. While the ancestral palm tree from the other islands occurred in a forest and needed to be tall in order to gain access to sunlight, there are few competitors on the new island. As a result, acts such that shorter trees tend to produce more offspring on the new island. However, not all trees are short, because the sea birds still migrate between islands, which allows occasional to continue between the new island and older islands.