Sequential speciation and the diversity of parasitic insects

A major issue in biology is what generates biodiversity. An important part of the answer may be biodiversity itself. As new species form they may often create new niches, providing an impetus for other organisms to adapt and speciate. This hypothesis of 'cascading' or 'sequential' speciation has been argued to be particularly relevant for host-plant-specific phytophagous insects and their associated parasites. Here, we discuss the empirical and theoretical bases for sequential speciation and its potential significance for understanding the genesis of new life forms. We examine the application of the concept that biodiversity begets biodiversity by palaeontologists, ecologists, and evolutionary biologists to explain current and past distributions of organisms. We look at factors that may promote and restrict sequential speciation and review evidence from phytophagous insects for the process in nature. In particular, we discuss a case study involving the apple maggot fly, Rhagoletis pomonella (Diptera: Tephritidae) and its wasp parasitoid Diachasma alloeum (Hymenoptera: Braconidae). We conclude by outlining a research agenda for testing for sequential speciation in the field.