Invoking a metaphor of islands, Weiner examines the “invisible coasts,” the boundaries that exist between different species. Despite the title of his landmark work, Darwin largely ignored the question of how a species originates and what defines its borders. Darwin’s creationist detractors overstated the fixity of species as a way of refuting evolutionary theory. Evolutionists are currently examining these unresolved questions: “What holds the new species apart? What are the barriers, and what makes the barriers harder or easier to cross?” (159).

Studies indicate that species barriers are maintained by instinct and behavior. Evolution must select for behaviors that maintain these barriers; otherwise, the barriers would not persist through time. Peter Grant and a student confirmed this in an experiment observing the mating patterns of the ground finches on Daphne Major. In a thousand pairings, they observed only 26 instances of interspecies courting, all of which abruptly stopped before the ritual finished. Closer study indicated that the birds assessed mates based on their song and body type, and that females consistently preferred members of their own species and birds from their own island. Grant believes that the beak is one important factor in the finches’ recognition of their own kind and their selection of a mate.