Evolutionary Biology Speciation Continuum
Sean Stankowski
  • LAST MODIFIED: 22 August 2023
  • DOI: 10.1093/obo/9780199941728-0148


Understanding the origin of species is no easy task. Academic studies of speciation usually take place over a few years, while the process itself usually occurs over evolutionary timescales. How, then, can we possibly hope to understand speciation from start to finish? The idea of a “speciation continuum” has been presented as something of a solution to this problem. Because we are surrounded by many examples of speciation—each providing a snapshot at some point during the transition from populations to species—we can use them to understand the process as a whole. This article will (1) highlight some key references that laid the foundations for continuum thinking in speciation research, (2) explore the origins and definitions of the term “speciation continuum” and (3) highlight some outstanding empirical studies rooted in the speciation continuum.

Origins and Development of Continuum Thinking in Speciation Research

Discussions about the continuous nature of speciation long preceded the first use of the term speciation continuum. Darwin 1859 discusses the relationship between varieties and species at length, arguing that the process of descent with modification might eventually convert intergrading varieties into well-defined species. Wallace 1865 also recognizes and discusses the obvious bridge between within- and between-species variation in the butterflies of the Malay region. Wallace recognizes six “phenomena” including polymorphisms, varieties, subspecies, and species, and highlights that the study of less distinct forms might be more informative about the processes that create variation than the study of well-fixed species because “ . . . nature is still at work, in the very act of producing those wonderful modifications of form” (p. 5). A year earlier, Walsh 1864 made a similar point in a paper about phytophagous insects. Using the transition from childhood to adulthood as a metaphor, it suggested that the comparative study of populations, host races, and species might be viewed as a series of snapshots during the otherwise long, gradual process of speciation. It states, “We cannot, from the shortness of human life, see the same identical species develop gradually from century to century, first into slight varieties, then into marked varieties, then into geographical or phytophagic races, then into new species; but in one and the same year we may see all the stages of development, with all the possible intermediate grades, in different species” (p. 249). Although these early works did not use the term “speciation continuum” explicitly, they clearly acknowledge that speciation is a continuous process, and suggest that we might learn something about the process as a whole if we combine knowledge from many examples of speciation in the present day.

  • Darwin, C. 1859. On the Origin of Species. London: John Murray.

    Darwin discusses the origin of species and varieties, arguing that they are generated by the same processes. He surmises that continued modification of varieties by natural selection may eventually cause them to become well-defined species.

  • Walsh, B. D. 1864. On phytophagic varieties and phytophagous species. Proceedings of the Entomological Society of Philadelphia 3: 403–430.

    Outlines thoughts and experiments on varieties and species of phytophagic insects. Like Darwin, Walsh recognizes that varieties and species are difficult to distinguish and surmised that differences between varieties gradually strengthen to give rise to species.

  • Wallace, A. R. 1865. I. On the phenomena of variation and geographical distribution as illustrated by the Papilionidæ of the Malayan region. Transactions of the Linnean Society of London 1:1–71.

    DOI: 10.1111/j.1096-3642.1865.tb00178.x

    Describes natural units of variation that he observed during his extensive study of butterflies in the Malay Archipelago. Argues that varieties and species can be further subdivided into six phenomena: (1) simple variability, (2) polymorphism, (3) local form, (4) coexisting varieties, (5) races or subspecies, and (6) true species.

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