Cryptic Species Complex

In biology, a cryptic species complex is a group of species which satisfy the biological definition of species—that is, they are reproductively isolated from each other—but whose morphology is very similar (in some cases virtually identical). More and more such groups are being discovered as DNA analysis is applied more widely.

The species in a cryptic complex are typically very close relatives and in many cases cannot be easily distinguished by molecular phylogenetic studies. If lineage sorting has not yet been completed, members of a cryptic species complex widely share plesiomorphic haplotypes, while individual species might not have evolved distinctive autapomorphic mutations yet. But usually, individual species within the complex can be separated by analysing data from multiple sources, such as by comparing DNA sequence analyses, bioacoustics and thorough life history studies.

They may be parapatric, are frequently sympatric, and are sometimes allopatric. Cryptic species complexes are not the same as populations undergoing speciation: they typically represent a situation where speciation has already broken the gene flow between populations, but where evolution has not progressed to a point where easily-recognizable adaptations have taken place.

Cryptic species that do not form a complex may be somewhat more distantly related and simply represent lineages that have been so successful as to require little evolutionary change, possibly coupled with parallel evolution. A famous example are the Eurasian and Short-toed Treecreepers, perhaps the first cryptic species to be recognized as such (by Christian Ludwig Brehm in 1820). Other ornithologists refused to accept that more than one species was involved until Brehm presented his bioacoustic studies, which left no room for doubt. The European Treecreeper has since been found to be a very close relative of the Himalayan Hodgson's Treecreeper, while the Short-toed Treecreeper is probably the sister species of the North American Brown Creeper.

A related concept is the superspecies. This is a clade of at least two more or less distinctive species with approximately parapatric distributions. Not all cryptic species complexes are superspecies, and vice versa, but many are.