Punctuated Equilibrium and the Validation of Macroevolutionary Theory 845
davidis following local and allopatric origins of derived taxa that then become
extinct at diastems, with the ancestor reappearing in strata just above.
Similarly, Ager (1983) traced the allopatric origin of late Pliensbachian brachiopod
species from the central stock of Homoeorhynchia acuta, and the later Toarcian
migration of the descendant H. meridionalis into the ancestral region. Williamson's
(1981) celebrated and controverted study (see pp. 769-771) of punctuational origin
for several pulmonate snail species in African Pleistocene lakes invokes the same
kind of evidence—as the ancestral species migrates back (in several separate
episodes, moreover) after a coalescence of lakes and the extinction of descendant
species that had originated in previous times of isolation.
When all evidence derives from a restricted region, the separation of
punctuation in situ from migrational incursion (with origin elsewhere at an
unspecified tempo) becomes more difficult, but some criteria of admittedly
uncertain inference may still be useful. For example, Smith and Paul (1985) argue
that the sudden appearance of the descendant echinoid Discoides favrina in strata
still holding ancestral D. subucula may represent an event of punctuational
speciation on morphological grounds—for the descendant species, though visually
distinct in many features, can be easily derived, given allometric patterns shared by
both forms, through a simple heterochronic process of hypermorphosis.
In graptolites, the pattern of ancestral survival after cladogenetic origin of a
descendant taxon has been noted frequently enough to inspire its own terminology
as the concept of "dithyrial populations" (Finney, 1986), or samples from the same
stratum containing two directly filiated and noninter-grading species.
The widespread geographic distribution of many late Tertiary and Quaternary
mammalian lineages provides several examples of geographically resolvable
allopatric origin followed by later survival with the ancestral species. For example,
Mammuthus trogontherii, the presumed ancestor of the woolly mammoth M.
primigenius, first appears in northeastern Siberia while its presumed ancestor, M.
meridionalis, continued to survive in Europe (Lister, 1993a, p. 209).
Other forms of evidence can lead to strong inferences from data of ancestral
survival to origin of descendants by punctuated equilibrium, even in the absence of
such firm geographic data. I previously mentioned the growing evidence for rapid
cladogenesis as the primary pattern in hominid evolution (see p. 833), based on
several criteria, including the high relative frequency of observed overlap, the
limited time available for cladogenetic origin (even when place and geological
moment have not been clearly specified), and our confidence that all events (at
least preceding the origin of Homo erectus) occurred in Africa. In his review,
McHenry (1994, p. 6785) stated "ancestral species overlap in time with
descendants in most cases in hominid evolution, which is not what would be
expected from gradual transformations by anagenesis." McHenry's summary
diagram (reproduced here as Fig. 9-19) shows a clear pattern of dominant relative
frequency for rapid cladogenesis—