structural:scholar_edition
Concept of species

The classification of archaea into species is also controversial. Ernst Mayr 's species definition — a reproductively isolated group of interbreeding organisms — does not apply, as archaea reproduce only asexually.

Archaea show high levels of horizontal gene transfer between lineages. Some researchers suggest that individuals can be grouped into species-like populations given highly similar genomes and infrequent gene transfer to/from cells with less-related genomes, as in the genus Ferroplasma. On the other hand, studies in Halorubrum found significant genetic transfer to/from less-related populations, limiting the criterion's applicability. Some researchers question whether such species designations have practical meaning.

Current knowledge on genetic diversity in archaeans is fragmentary, so the total number of species cannot be estimated with any accuracy. Estimates of the number of phyla range from 18 to 23, of which only 8 have representatives that have been cultured and studied directly. Many of these hypothesized groups are known from a single rRNA sequence, so the level of diversity remains obscure. This situation is also seen in the Bacteria; many uncultured microbes present similar issues with characterization.