Species complex

In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear.

It may represent an early stage after speciation in which the species were separated for a long time period without evolving morphological differences.

[9] Some authors apply the term to a species with intraspecific variability, which might be a sign of ongoing or incipient speciation.

[19] In those cases, other characters, such as in the species' life history, behavior, physiology, and karyology, may be explored.

For example, territorial songs are indicative of species in the treecreepers, a bird genus with few morphological differences.

[27] Analysis of DNA sequences is becoming increasingly standard for species recognition and may, in many cases, be the only useful method.

[19] Different methods are used to analyse such genetic data, such as molecular phylogenetics or DNA barcoding.

[3] Species forming a complex have typically diverged very recently from each other, which sometimes allows the retracing of the process of speciation.

Nevertheless, similar but distinct species have sometimes been isolated for a long time without evolving differences, a phenomenon known as "morphological stasis".

[19] For example, the Amazonian frog Pristimantis ockendeni is actually at least three different species that diverged over 5 million years ago.

[19] Also, asexual reproduction, such as through apomixis in plants, may separate lineages without producing a great degree of morphological differentiation.

[39] Similar methods also found that the Amazonian frog Eleutherodactylus ockendeni is actually at least three different species that diverged over 5 million years ago.

[40] That suggestion came before the detailed analysis of many systems using DNA sequence data but has been proven to be correct.

In the marine bryozoan Celleporella hyalina,[42] detailed morphological analyses and mating compatibility tests between the isolates identified by DNA sequence analysis were used to confirm that these groups consisted of more than 10 ecologically distinct species, which had been diverging for many millions of years.

The butterfly genus Heliconius contains some species that are extremely difficult to tell apart.
The rubyspot damselfly Hetaerina americana is suspected to be a cryptic complex with at least one other species of rubyspot.
Schematic phylogram with nine species, five of which form a group with short branches, separated from the others by a long branch
A species complex typically forms a monophyletic group that has diversified rather recently, as is shown by the short branches between the species A–E (blue box) in this phylogenetic tree .
Scheme showing morphological stasis and hybrid speciation, with species presresented by circles, their color indicating morphological similarity or dissimilarity
Possible processes explaining similarity of species in a species complex:
a – morphological stasis
b – hybrid speciation
A mosquito sitting on the tip of a finger
The Anopheles gambiae mosquito complex contains both species that are a vector for malaria and species that are not. [ 43 ]