Punctuated Equilibrium and the Validation of Macroevolutionary Theory 843
(p. 60) that "since speciation occurred, ancestral species and their descendants have
coexisted, in some cases sympatrically." In a study of Miocene deep-sea ostracodes
from the southwest Pacific, Whatley (1985, p. 109) documented two cases of
allopatric and punctuational origin for new species followed by migration back to
the parental range and subsequent coexistence with the ancestral species.
Alan Cheetham's work on American Cenozoic clades of the cheilostome
bryozoan genera Metrarabdotos and Stylopoma (Cheetham, 1986, 1987; Jackson
and Cheetham, 1994; Cheetham and Jackson, 1995) merits citation at several
points in this chapter for its unparalleled documentation of all major tenets of
punctuated equilibrium—both in clarity of conclusions and richness of empirical
evidence. I present a general summary in the section on relative frequency (p. 868),
but Cheetham's fruitful use of ancestral persistence should be noted here. Jackson
and Cheetham (1995, p. 204) cite three primary empirical sources for documenting
punctuated equilibrium from paleontological data: "The geologically abrupt
appearance of species in the record, the static morphologies of species for millions
of years, and the extensive temporal overlap between apparent ancestor-descendant
species pairs."
Their summary of overwhelming support for punctuated equilibrium from the
last source (Jackson and Cheetham, 1994, p. 407) states that "most well-sampled
Metrarabdotos and Stylopoma species originated fully differentiated
morphologically and persisted unchanged for > 1 to > 16 million years, typically
alongside their putative ancestors."
On Cheetham's celebrated and frequently reprinted diagram of evolution and
cladogenesis in the Metrarabdotos clade (Fig. 9-18, and redundant in citing
"evolution and cladogenesis" because all phyletic change occurs by cladogenesis in
this lineage), ancestors persist after the origin of descendants in 7 of the 9 cases
where Cheetham felt confident enough to assert a phylogenetic claim for direct
filiation. (Marshall's important challenge (1995) to assessments of stratigraphic
range in several cases does not counter Cheetham's hypotheses about filiation, and
certainly does not challenge the assertion of overlap, a claim based on direct
observation of joint occurrence, not on inference.) The two cases where ancestral
persistence has not been directly observed (see Fig. 9-18), but may well have
occurred (the derivation of M. tenue from sp. 10, and of M. unguiculatum from M.
lacrymosum), both fall "outside the interval of dense sampling" (Cheetham, 1986,
p. 201), where Cheetham achieved a stratigraphic resolution by Sadler's (1981)
criteria of 0.63. For Stylopoma, "eleven of the nineteen species originate fully
formed at p > 0.9, with no evidence of morphologically intermediate forms, and all
ancestral species but one survived unchanged along with their descendants"
(Jackson and Cheetham, 1994, p. 420).
By dense sampling in both vertical sequence and geographic spread, Nehm
and Geary (1994) demonstrated the punctuational origin of the gastropod Prunum
christineladdae from its ancestor P. coniforme in a small part of its