Sequential Speciation and Cascading Divergence

by

Glen Ray Hood

• LAST MODIFIED: 12 January 2023
• DOI: 10.1093/obo/9780199941728-0145

Introduction

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.

General Overviews

The salient features and themes of sequential speciation which build upon the classic study on coevolution and diversification, Ehrlich and Raven 1964, are outlined in reviews and primary literature such as Abrahamson and Blair 2008; Feder and Forbes 2010; Hood, et al. 2015; and Hood and Feder 2016. These sources discuss the following in detail: (1) the general implications of sequential speciation for patterns of biodiversity, (2) how the process is defined and how it differs from similar mechanisms of coevolution, (3) the criteria that need to be met for a system to qualify as undergoing the sequential speciation process, (4) the methods used to ecologically and genetically test for the sequential speciation, and (5) examples of systems that have tested both positive and negative for sequential speciation. While the specific examples detailed within these literature reviews emphasize the process in tritrophic systems composed of plants, plant-feeding insects, and insect-feeding natural enemies, the review Brodersen, et al. 2018 focuses on detailing evidence of “upward adaptive cascades,” a process somewhat analogous to sequential speciation, in the diversification of predator-prey fish systems.

• Abrahamson, W. G., and C. P. Blair. 2008. Sequential radiation through host-race formation: Herbivore diversity leads to diversity in natural enemies. In Specialization, speciation, and radiation: The evolutionary biology of herbivorous insects. Edited by K. J. Tilmon, 188–202. Berkley: Univ. of California Press.

  DOI: 10.1525/california/9780520251328.003.0014

  A book chapter that was the first dedicated review of sequential speciation, highlighting evidence for the process in several arthropod systems. This review was also the first to lay out the criteria conducive for sequential speciation in arthropod systems.

• Brodersen, J., D. M. Post, and O. Seehausen. 2018. Upward adaptive radiation cascades: Predator diversification induced by prey diversification. Trends in Ecology and Evolution 33.1: 59–70.

  DOI: 10.1016/j.tree.2017.09.016

  A review of a process that the authors refer to as “upward adaptive cascades” among predator-prey fish systems that overlaps broadly with the central tenants of sequential speciation. Here, they compare and contrast the difference between upward adaptive cascades in fish versus insect and detail tests cases in several fish systems.

• Ehrlich, P. R., and P. H. Raven. 1964. Butterflies and plants: A study in coevolution. Evolution 18.4: 586–608.

  DOI: 10.1111/j.1558-5646.1964.tb01674.x

  This classic paper outlines the first explicit model testing for diversification through an “escape and radiate” hypothesis whereby insects speciate in response to open niche space via the evolution of novel plant diversity.

• Feder, J. L., and A. A. Forbes. 2010. Sequential speciation and the diversity of parasitic insects. Ecological Entomology 35.s1: 67–76.

  DOI: 10.1111/j.1365-2311.2009.01144.x

  An important review of the process of sequential speciation that distinguishes itself from the others listed here by elaborating upon the genetic and ecological evidence for sequential speciation in the apple maggot fly, Rhagoletis pomonella, and the parasitoid wasps, Diachasma alloeum, documented in Forbes, et al. 2009 (cited under Direct Evidence Supporting Sequential Speciation).

• Hood, G. R., and J. L. Feder. 2016. Sequential speciation. In The encyclopedia of evolutionary biology. Edited by R. M. Kliman, 41–48. Cambridge, MA: Academic Press.

  A review of sequential speciation that highlights the difference between strict cocladogenesis and sequential speciation. This review is also the most recent treatment to detail systems that have tested both positive and negative for sequential speciation.

• Hood, G. R., A. A. Forbes, T. H. Q. Powell, et al. 2015. Sequential divergence and the multiplicative origin of community diversity. Proceedings of the National Academy of Science 112.44: E5908–E5989.

  DOI: 10.1073/pnas.1424717112

  A follow up study to Forbes, et al. 2009 (cited under Direct Evidence Supporting Sequential Speciation) that documents that biodiversity can be multiplicatively (as opposed to linearly) amplified across trophic levels from one fly species to many different parasitoid taxa. In this study, the authors also outline criteria modified from Abrahamson and Blair 2008 necessary to document a sequential speciation event in insect systems.