Sequential speciation and the diversity of parasitic insects

Abstract

Abstract.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.

Sequential divergence and the multiplicative origin of community diversity

“…Therefore, population divergence may not always be an isolated process, as the differentiation of one taxon could beget the divergence of many others. Such "sequential" or "cascading" divergence events may be particularly relevant to understanding why some groups of organisms, like plants, the insects that feed on them, and the parasitoids that attack the insects, are more diverse and species-rich than other groups (8,9,(12)(13)(14)(15). Specifically, when phytophagous insects diversify by adapting to new host plants, they create a new habitat for their parasitoids to exploit (Fig.…”

“…1B). However, few convincing examples of sequential divergence exist (12)(13)(14)(15), and in no study is there both genetic and ecological evidence for sequential divergence multiplicatively amplifying biodiversity.…”

“…1). If a parasitoid shifts to the new habitat, it can encounter the same divergent ecological selection pressures as its insect host, which could result in the parallel divergence of insect host and parasitoid (12)(13)(14)(15) (Fig. 1A).…”

“…1A). Moreover, sequential divergence may have multiplicative effects in generating biodiversity, as the shift of an insect to a new plant may open a new niche opportunity for not just one but the entire community of parasitoids attacking the insect host (12,13) (Fig. 1B).…”

“…For example, when the genotype/phenotype of a "foundation" species influences the relative fitness of other species, evolutionary change(s) in this genotype/phenotype may affect organisms in adjacent trophic levels (10,11). If these evolutionary changes are linked to ecological adaptation and reproductive isolation (RI), associated organisms may diverge in parallel, potentially creating entire coevolved communities distinct from one another (12)(13)(14)(15). Therefore, population divergence may not always be an isolated process, as the differentiation of one taxon could beget the divergence of many others.…”