Island biogeography (MacArthur & Wilson, 1967) established that species diversity on islands reflects a dynamic equilibrium between colonization and extinction. When a founding population reaches a new island, it experiences the founder effect — genetic drift in a small founder population rapidly fixes alleles, diverging from the ancestral lineage.
Geographic isolation (allopatry) prevents gene flow between island populations, allowing independent evolution under local selection pressures. Over time, populations accumulate enough genetic divergence that interbreeding becomes impossible — reproductive isolation is achieved, completing speciation. Darwin's finches (Galapagos) and Hawaiian honeycreepers are canonical examples of rapid adaptive radiation.
Sympatric speciation — divergence within a single population via ecological competition for resources — is more controversial but supported by cichlid fish radiation in East African Rift lakes. The speciation clock accelerates with increased mutation rates, stronger selection, and smaller island populations.