Abstract
Aim The theory of island biogeography provides a predictive framework relating species richness to island size and distance from the mainland. However, the theory as originally formulated does not necessarily scale to large islands and continental landmasses that are capable of generating species through in situ speciation (rather than entirely by colonization), nor does it necessarily account for how human introduction of species alters traditional biogeographical patterns. Here, we examine the ecological (colonization and extinction), evolutionary ( in situ speciation) and human‐mediated (deliberate introductions) determinants of species richness in a taxonomic group that has undergone a radiation on Caribbean islands: live‐bearing fishes of the family Poeciliidae. Location The Caribbean. Methods We created a database of both native and introduced poeciliid species occurrence on Caribbean islands through literature review, and estimated the number of colonizations versus speciation events on each island using a molecular phylogeny. Linear regression and other statistical tests were used to explore species–area and species–isolation relationships. Results Species richness on small islands results entirely from colonization and does not significantly increase with island area, whereas on larger islands species richness increases dramatically as a function of area due primarily to in situ speciation. Poeciliid fishes have been introduced widely, both as a by‐product of their popularity in the aquarium hobby and as a means of mosquito control. We show that such establishments have occurred disproportionately on islands depauperate in native species, and that introduced species richness is positively correlated with economic interconnectedness (shipping traffic) and human population size. Main conclusions On large Caribbean islands in situ speciation has elevated the number of poeciliid species beyond that predicted from ecological processes alone. Introduced species significantly alter biogeographical patterns.