Trends in Ecology & Evolution
ReviewCryptic species as a window on diversity and conservation
Section snippets
Cryptic species: the biodiversity wildcard
Increasing worldwide destruction and disturbance of natural ecosystems are precipitating catastrophic extinctions of species [1]. Given that most species remain undescribed, efforts to catalogue and explain biodiversity need to be prioritized. Research on cryptic species has increased exponentially over the past two decades (Figure 1), fuelled in large part by the increasing availability of DNA sequences. Identifying cryptic species (Box 1; see Glossary) challenged biologists and naturalists
What are cryptic species?
The literature is brimming with definitions of cryptic or hidden species. Most authors regard cryptic species to be synonymous with ‘sibling species’ [16], whereas others specify that ‘sibling’ connotes more recent common ancestry than does ‘cryptic’, implying a sister-species relationship [17]. Although not yet universally adopted, we feel that differentiating the term ‘sibling species’ in this way gives the phrase enhanced utility. Some advocate that sibling species graduate to the category
Description and discovery of undetected diversity
Large genetic distances within traditionally recognized species, often in combination with morphological, geographical and other subtle differences, have revealed cryptic species in most types of organism and habitat, from deep-sea clams [20] to freshwater fish [21], and from tropical butterflies [22] to arctic plants [23]. We surveyed literature referenced in ISI Web of Science (http://scientific.thomson.com/products/wos/) and Zoological Record Plus (//www.csa.com/factsheets/zooclust-set-c.php
Why are cryptic species cryptic?
Most sensory information processed by the human brain is visual, perhaps explaining why morphological characters feature more prominently than chemical and auditory characters in our classification of the natural world. Integrative taxonomists look for concordant changes in more than one characteristic of an organism, and corroboration from independent data (e.g. molecules, morphology or mating signals) is frequently deemed to be good evidence for separating species [4] (Box 1). However, there
Correcting cryptic assumptions
Several common but perhaps incorrect assumptions about cryptic species pervade the literature (Figure 1). One of the most common assumptions is that most cryptic species result from speciation that is so recent that morphological traits or other diagnosable features have not yet evolved. Although undoubtedly true for some taxa such as coccolithophores (see Ref [16]), the view that cryptic species are recent is challenged by studies of bonefish [40], amphipods [39] and copepods [41], which show
Paradigm shifts in ecology
Newly discovered cryptic species complexes have overturned traditional notions about ecological specialization in interspecific interactions. Investigations of many antagonistic interactions have found that some exploitative species tend to have more specialized diets than was previously thought. Studies of herbivorous beetles [47], Lepidoptera [22] and whole guilds of other herbivorous insects [48] have shown that presumed dietary generalists are complexes of dietary specialists. Similarly,
What we don’t know might hurt us
Several lines of evidence underscore the importance of recognizing cryptic species. Conservation planning, bioprospecting, biological control and the treatment of diseases and snakebites all hinge on the identification of species in cryptic complexes. An inability to identify biologically important species therefore hampers our efforts to conserve, study, contain or utilize them.
Summary
Molecular techniques (primarily DNA sequencing) have transformed the ability of scientists to describe and define biological diversity. Although they are not a panacea for species delimitation, molecular data are important and useful when combined with other types of data. Studies using non-morphological characters, such as chemical and auditory signals, to discriminate otherwise indistinguishable species are being published at an increasing rate. These newly discovered cryptic species present
Acknowledgements
We thank Glenn Adelson, Allen Herre, Dan Janzen and members of the Conservation Ecology Laboratory at the National University of Singapore for discussion and comments on earlier versions of this review, and the Singapore Ministry of Education for funding to pursue the study of cryptic species (R-154–000–270–112).
Glossary
- Allopatric
- populations or species with geographically separated ranges.
- Alpha taxonomy
- the description and naming of organisms.
- Clade
- a monophyletic group of organisms.
- Cladogenesis
- the evolutionary splitting of groups of organisms or, literally, the generation of a clade.
- Cryptic species
- two or more distinct species that are erroneously classified (and hidden) under one species name.
- Karst
- irregular (often limestone) formations often including structures such as caves, sink holes, underground water ways
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