speciesbut

Speciesbut is a term used in speculative biology and philosophy of biology to describe a proposed intermediate state in the process of speciation. In this sense, an organism or population designated as a speciesbut would be more closely related to two diverging lineages than to one fully defined species, yet would not meet a strict standard of complete reproductive isolation.

The term is not part of formal taxonomy. It is a neologism found primarily in theoretical discussions, thought experiments, and some science fiction writing to illustrate gray areas of species concepts, particularly where gene flow persists alongside adaptive divergence. Its usage varies and is not standardized across disciplines.

In practice, speciesbut is treated as a transitional or exploratory concept rather than a formal rank. Some writers use it to denote populations that are on the cusp of becoming distinct species, while others describe them as functionally two species in ecological terms but still taxonomically unified due to ongoing interbreeding. The idea highlights the continuum of divergence and the difficulties of applying strict species definitions in real-world populations.

The speciesbut concept is useful for examining debates about the biological species concept, the role of reproductive isolation, and how gradual genetic and ecological differentiation can produce a spectrum of evolutionary states rather than a simple split into separate species.

See also: incipient species, ring species, speciation, reproductive isolation, species concept, hybrid zone.