Speciesion

Speciesion is a theoretical concept in evolutionary biology used to describe a nascent or incipient lineage arising during the process of speciation. The term combines “species” with the suffix “-ion” to emphasize a transitional stage that lies between a single population and a fully recognized species. It is not a formal taxonomic rank and is mainly employed in discussions about how speciation progresses along a continuum rather than as a discrete category.

In use, speciesion serves as a way to frame questions about when differentiation becomes biologically meaningful for classification. Proponents argue that it helps researchers talk about partial reproductive isolation, accumulating genetic divergence, or ecological differentiation without prematurely assigning a full species status. Critics contend that without clear, universally accepted criteria, the concept can blur boundaries between populations and complicate taxonomy rather than clarify it.

Commonly discussed attributes associated with a speciesion include some degree of reproductive isolation, measurable genetic or genomic differentiation, and ecological distinctiveness, though none of these are required or universally agreed upon. Because it is a conceptual tool rather than a formal rank, the presence or absence of a speciesion depends on the framework and criteria used by researchers in a given study.

See also: speciation, species concept, incipient species, evolutionary biology.