Draft:Cryptic species complex

A cryptic species complex occurs when two or more groups of organisms are morphologically identical, but are still classified as distinct species.

There is not a consensus on the precise criteria for a cryptic species complex, but it is often prerequisite that cryptic species were previously classified as the same species. These are also sometimes referred to as “sister” or “sibling” species.

Cryptic speciation challenges conventional understandings of species boundaries and biodiversity. It has implications for conservation efforts, ecological studies, and the comprehension of the mechanisms driving evolution.

History
Early taxonomists relied on morphological traits to distinguish species. The biological species concept by Ernst Mayr in the mid twentieth century emphasized reproductive isolation as opposed to just morphological differences. Allowing species distinctions to be made based on non-morphological criteria created conceptual space for cryptic species.

Before this, as early as the eighteenth century, European naturalists were perplexed when drawing species boundaries between morphologically similar species. William Derham, F.R.S., is credited with being the first to identify cryptic species; in 1718, Derham claimed to have discovered three distinct species of goldcrest (Regulus non cristatus). In 1940, C.D. Darlington used the term “cryptic species” to describe species which do not change their appearance despite being genetically isolated due to mate choice.

Since the 1990s, the growing prominence of utilizing molecular data in making species delineations has increased awareness of the incidence and prevalence of cryptic species.

Contestation
While cryptic species can be broadly described as multiple distinct species which are morphologically indistinguishable, the exact criteria for what qualifies a cryptic species complex is contested among biologists. Because the “cryptic” status of cryptic species complexes is determined vis-a-vis a lack of coherence with applied taxonomic criteria, this contestation reflects the larger taxonomic indeterminacy created by the application of multiple species concepts which delineate species boundaries based on different parameters. The issues with species delineation also bring into question whether cryptic species complexes should be understood as a representation of the limitations of current taxonomic methods and resources, or as a real natural phenomena which provide their own insight on the history and mechanisms of evolution.

Incidence
In general, cryptic species are recently diverged sister taxa, such that morphological differences have not yet had time to develop. Cryptic speciation events often occur in sympatry.

The use of molecular species delimitation methods (such as karyotype or DNA sequence analysis) has expanded scientific understanding of the incidence and distribution of cryptic species. Cryptic species are understood to be prevalent, especially across animal phyla. However, even with the expansion of reporting on cryptic species, the reported rates of incidence across different phyla is variable, and the accuracy of these rates is limited due to both natural variability of cryptic species across different taxonomic groups and methodological concerns with sampling and species delineation methods.

The identification of a cryptic species complex between species which are not closely related, or between sister taxa which diverged long ago, has potential implications for understanding evolutionary phenomena such as parallelism, convergence, and stasis.

The incidence and distribution of cryptic species also has implications for the evaluation of extant biodiversity. Moreover, the prevalence of cryptic speciation poses challenges for conservation efforts, as cryptic species may remain undetected and thus overlooked in conservation planning.

Examples
The Anopheles gambiae complex is a malarial vector consisting of several virtually morphologically indistinguishable species of mosquitoes.

Molecular techniques have recently revealed that Orbicella annularis, a widespread reef-building coral in the Caribbean, is actually a complex of three distinct species.

The speciation of cichlids has led to debate over taxonomic classification within this family, which is exacerbated by vagueness in literature describing the many cichlid species and differences in the thoroughness of research depending on the geographic or taxonomic areas that are surveyed.