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A chronospecies is a species which changes physically, morphologically, genetically, and/or behaviorally over time on an evolutionary scale such that the originating species and the species it becomes could not be classified as the same species had they existed at the same point in time. Throughout this change, there is only one species in the lineage living at any point in time, as opposed to cases where one species branches off into many through divergent evolution. As opposed to paleospecies (see below), "chronospecies" is the general term for the elements of a sequential succession of species evolving into another, anywhere in time, for any length of time, with or without having extant descendants.
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If one has young fossil material - 100.000s to a few millions of years old or so - that is indistinguishable from a living species, it could be considered to represent a chronospecies that is the immediate ancestor of the living taxon. Similarly, if one has a morphologically unchanged fossil record that stretches for tens of millions of years, longer than distinct species are usually known to exist judging from the fossil record, this would obviously represent the remains of several species. Taxa described based on such material are termed pal(a)eospecies. These are often not universally accepted as distinct, as they are not unequivocally identifiable without robust stratigraphic information. See e.g. Howard (1947) for a discussion of this problem.
With a phyletic gradualism model of evolution, it can be difficult to separate chronospecies, since morphological changes accumulate over time and two very different organisms could be connected by a series of intermediaries. In practice, chronospecies are usually recognized because two species in the same lineage are named by palaeontologists without knowledge of any intermediate organisms. When organisms evolve by punctuated equilibrium, chronospecies can be readily seen in the fossil record when a lineage changes without splitting.
Prehistoric (but comparatively recent - usually Late Pleistocene) subspecies of extant taxa which evolved in a similar way as chronospecies are called pal(a)eosubspecies Many of those are known from subfossil material and most have changed in size adapting to the climatic changes during the last ice age (see Bergmann's Rule).
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Chronospecies". A list of authors is available in Wikipedia.|