Scientists studying a group of related bird species on an island archipelago discover that these species are always more closely related to each other within an island than they are between any two islands. The most likely explanation for this result is that:

A. each bird species evolved different adaptations in allopatry and then colonized the other islands.
B. one bird species colonized each island and then evolved into distinct species on that island through character displacement.
C. each bird species colonized one island and then no further speciation occurred.
D. many ancestral bird species colonized each island.