Large islands typically have more species than comparable smaller islands.
Ecological theories, the most influential being the equilibrium theory of i
sland biogeography(1), explain the species-area relationship as the outcome
of the effect of area on immigration and extinction rates. However, these
theories do not apply to taxa on land masses, including continents and larg
e islands, that generate most of their species in situ. In this case, speci
es-area relationships should be driven by higher speciation rates in larger
areas(2-6), a theory that has never been quantitatively tested. Here we sh
ow that Anolis lizards on Caribbean islands meet several expectations of th
e evolutionary theory. Within-island speciation exceeds immigration as a so
urce of new species on all islands larger than 3,000 km(2), whereas speciat
ion is rare on smaller islands. Above this threshold island size, the rate
of species proliferation increases with island area, a process that results
principally from the positive effects of area on speciation rate. Also as
expected, the slope of the species-area relationship jumps sharply above th
e threshold. Although Anolis lizards have been present on large Caribbean i
slands for over 30 million years, there are indications that the current nu
mber of species still falls below the speciation-extinction equilibrium.