, the evidence for species
comes mainly from the comparative anatomy
. A chronospecies is defined in a single lineage (solid line) whose morphology changes with time. At some point, palaeontologists judge that enough change has occurred that two forms (A and B), separated in time and anatomy, once existed. If only sporadic examples of each survive in the fossil record, then the forms will appear sharply distinct.
A chronospecies is a species derived from a sequential development pattern which involves continual and uniform changes from an extinct ancestral form on an evolutionary scale. This sequence of alterations eventually produces a population which is physically, morphologically, and/or genetically distinct from the original ancestors. Throughout this change, there is only one species in the lineage at any point in time, as opposed to cases where divergent evolution produces contemporary species with a common ancestor. The related term paleospecies (or palaeospecies) indicates an extinct species only identified with fossil material. This identification relies on distinct similarities between the earlier fossil specimens and some proposed descendant, although the exact relationship to the later species is not always defined. In particular, the range of variation within all the early fossil specimens does not exceed the observed range which exists in the later species.
A paleosubspecies (or palaeosubspecies) identifies an extinct subspecies which evolved into the currently existing form. This connection with relatively recent variations, usually from the Late Pleistocene, often relies on the additional information available in subfossil material. Most of the current species have changed in size adapting to the climatic changes during the last ice age (see Bergmann's Rule).
The further identification of fossil specimens as part of a "chronospecies" relies on additional similarities which more strongly indicate a specific relationship with a known species. For example, relatively recent specimens – hundreds of thousands to a few million years old – with consistent variations (e.g. always smaller but with the same proportions) as a living species might represent the final step in a chronospecies. This possible identification of the immediate ancestor of the living taxon may also rely on stratigraphic information to establish the age of the specimens.
The concept of chronospecies is related to the phyletic gradualism model of evolution, and also relies on an extensive fossil record, since morphological changes accumulate over time and two very different organisms could be connected by a series of intermediaries.