Sequential Speciation and Cascading Divergence

A central, long-standing, and largely untested premise in evolutionary ecology is that “biodiversity begets biodiversity” in a process referred to as “sequential” or “cascading” divergence or speciation. The hypothesis of sequential speciation postulates that as populations diverge and new species form (i.e., the process of speciation), they create new niches for interacting organisms to utilize and adapt to in kind, potentially catalyzing a chain reaction of speciation events upwards across trophic levels. As a result, the process of sequential speciation has been inferred to help explain a number of observational and correlative patterns in ecology and evolutionary biology, such as adaptive radiations following mass extinctions, increased species richness in tropical climates, and the incredible diversity of one of the most speciose group of animals on the planet, specialist insects. However, the most direct evidence for the process of sequential speciation comes from tritrophic systems of interacting organisms involving plants, insects that feed on these plants, and insect natural enemies that feed on these plant-feeding insects. This article first provides a broad overview of the sequential speciation literature and a guide to important reviews that detail the current state of the field. To best understand and study sequential speciation, it is important to understand how it differs from and is similar to related concepts in evolutionary ecology. In this regard, this article provides literature that defines the process and contrasts sequential speciation and strict cocladogenesis, a coevolutionary phenomenon that can result in similar patterns of biodiversity. Next, references are provided that highlight the general implications for sequential speciation and detail indirect evidence from multiple subfields of biology that implicate the process in generating biodiversity. The article then details several resources that define the conditions conducive for sequential speciation, summarizes the primary literature providing direct evidence in support of the process, and references specific studies that test for but find no evidence of sequential speciation. Lastly, the bibliography concludes by detailing future directions and considerations for studying sequential speciation and its role for understanding patterns of species diversity.