A point came up in the talk.origins newsgroup that I thought was worth mentioning here. That concerns the conceptual status of transitional fossil sequences linking species given the view that punctuated equilibria (PE) is the mode of evolutionary change seen in the fossil record. This post is an expansion of my text from talk.origins.
The objection is raised that since PE says that phyletic gradualism (PG) is rare, then PE conceptually disqualifies the idea that one can find transitional sequences linking species, where the transitional sequence is fine-grained or as one objector put it, finely graduated. The objection is based upon a misunderstanding of what PE implies.
Eldredge & Gould's brief is not against *finely (graduated | graded) transitional sequences* (FGTS), it is against FGTS which are interpreted as showing anagenetic change *without appropriate reference to geography and stratigraphy*. This further set of restrictions is important, but overlooked by those who believe that accepting PE means rejecting the idea of transitional sequences. E&G argue against "phyletic gradualism" (PG) as typifying the mode of evolutionary change seen in the fossil record. E&G (and other proponents of PE) often refer to PG confusingly as "gradualism". Gradualism, though, is a separate and distinguishable concept.
Some seem to think that E&G banished phyletic gradualism as *existing*. E&G did not do that. Gould and Eldredge 1977 takes critics to task for implying that they did so. (The critics may not agree that they were mistaken; it is an arguable point.) G&E 1977 specifically validate an example of phyletic gradualism in the fossil record.
Now, on to the main point: It *is* possible to have finely graded transitional sequences in the PE mode, and it *is* possible that FGTS showing PG may be found. E&G 1972 present two sequences showing PE-type FGTS from the authors' own fieldwork, and G&E 1977 point to several sequences showing PE as well. What distinguishes a PG FGTS from a PE FGTS? A PG FGTS samples will show the changes over the whole range of the parent species, and will show the same rate of change in characters throughout the transition. A mismatch in geographic range seen for the change and the parent species, or an indication that collected specimens had no per-specimen information on geographic siting is enough to disestablish a sequence as showing the *characteristics of* PG. A difference in rate of change of a character during the transition period is enough to disestablish a transitional sequence as showing the *characteristics of* PG. Disestablishing a sequence as showing the characteristics of PG does not necessarily disestablish the sequence as showing the characteristics of descent with modification linking two species. It is a necessary, but not sufficient, condition that a sequence show descent with modification between two species to be considered an example of PG or an example of an observed transition under PE.
In contrast to a PG FGTS, a PE FGTS will be restricted to some relatively small region of the entire range of the parent species, will take place in a small fraction of the total residence time of the parent species, and will result in at least some time of overlap between the parent species and the daughter species to indicate cladogenesis has occurred. As I have noted above, E&G 1972 discusses two fine-grained transitional sequences as examples of transitions in the PE mode on pp.98-108, and G&E 1977 cites other researchers' examples of transitionals in the PE mode approvingly.
What does all this mean for the conceptual status of transitional fossil sequences? First, accepting PE's conclusions means that one views PE as the mode of evolutionary change, and not that it is exclusive of PG. Fossil evidence may be found which comports more closely with PE than PG, or vice versa. PE's claim concerns the relative frequency with which one will find examples of PE as compared to PG. Second, any finely graded sequence linking two successive species *is* a transitional sequence; the only question is whether its geographic, morphological, and stratigraphic characteristics argue for *classification* as an example of PE or as an example of PG.
Eldredge, N., & Gould, S. J. 1972. Punctuated equilibria: an alternative to phyletic gradualism. In: Models In Paleobiology (Ed. by T. J. M. Schopf).
Gould, S. J., & Eldredge, N. 1977. Punctuated equilibria: the tempo and mode of evolution reconsidered. Paleobiology, 3, 115-151.