Cryptic Species and Their Evolutionary Significance

Abstract

Cryptic species are detected at an ever‐increasing rate, mainly due to the application of molecular data. While the impact of this hidden diversity on macro‐ecology and conversation biology is widely recognized, its evolutionary significance is rarely. In recent years, it became apparent that definitions of cryptic species are too ambiguous to allow the differentiation between natural phenomena from human‐made artefacts. Hence, recently, a unifying conceptual framework has been proposed highlighting the necessity to test the degree of reduced phenotypic disparity in cryptic species. Within this reduced disparity also lies the evolutionary significance, as cryptic species can be regarded as the opposite of adaptive radiations. Specifically, studies on evolutionary stasis can substantially benefit from including these by addressing both patterns of reduced disparity and processes resulting in the lack of phenotypic evolution. In addition, this will allow connecting macro‐evolutionary and paleontological studies with micro‐evolutionary investigations of genotype‐phenotype linkage.

Key Concepts

  • Cryptic species are phenotypically highly similar species.
  • Species complexes resulting from taxonomic artefacts should not be considered as cryptic species.
  • To identify cryptic species, first one should establish species boundaries and only then study processes resulting in phenotypic similarity.
  • Cryptic species could result from recent speciation, parallelism, convergence or stasis.
  • Evolution of cryptic species through stasis holds the potential to enlighten us about deceleration of phenotypic evolution.
  • Cryptic species (shallow morphological differences, pronounced genetic divergence) could be considered as the opposite of adaptive radiation (pronounced morphological differences, shallow genetic divergence).

Keywords: taxonomy; speciation; recent divergence; parallelism; convergence; stasis; paleontology; morphology; phenotype; adaptive radiations

Figure 1. Unifying conceptual framework based on Struck et al. . The x‐axis represent time since divergence from the last common ancestor. Often genetic divergence is used as proxy for this. The y‐axis represents the degree of phenotypic disparity. The dark blue area is the area of ongoing speciation and hence no species boundaries have been established yet (white circles). The light blue area indicates evolution between pairs of species (orange circles) as it is intuitively assumed. That is phenotypic disparity more or less increases linear with time. The orange area reflects cases (yellow circles) in which phenotypic evolution occurs at a much higher rate than anticipated such as in adaptive radiations. The green area represents cases (red circles) of significantly reduced phenotypic disparity given time as it is the case in cryptic species.
Figure 2. Schematic representation of the Anopheles example for recent divergence (based on the results of Reidenbach et al., ). The left panel exemplifies the recent divergence of the two cryptic species. In the middle, the genomic inversion at chromosome 2 is shown and the right one lists the ecological differences observed between the two species.
Figure 3. Schematic representation of the Mastigias example of parallelism and convergence based on the results by Swift et al. . The phylogenetic relationship between the two morphotypes (oceanic and lake; indicated by the two icons) is shown to the left. Swift et al. regarded the origin of the two lake morphotypes within the Chinese Sea as well as two within the Pacific Islands & South Philippinean Seas as examples of parallelism as they originated from the same oceanic species. In contrast, they concluded that the other lake morphotypes to each other as well as to these previous ones evolved by convergence as they originated from different oceanic species.
Figure 4. Schematic representation of the Cavernacmella example for stasis given the results of Wada et al. . Wada et al. recognized a total five cryptic species as well as five morphologically distinct, noncryptic species within them (indicated by the different forms and colours; icons are relative in size to each other). The occurrence of these ten species is indicated as well as their life‐history and habitat (epigenean and cave‐dwelling; indicated by the two icons).
Figure 5. Schematic representation how different biological disciplines can contribute to the research of cryptic species and thereby increase our understanding of macro‐ecological, evolutionary and genomic patterns and processes.
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Struck, Torsten H, and Cerca, José(Jan 2019) Cryptic Species and Their Evolutionary Significance. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0028292]