Tempo of evolution in a Neogene bryozoan: rates of morphologic change within and across species boundaries

Abstract

Inadequacies in stratigraphic resolution or completeness can make true rates of morphologic change through geologic time impossible to estimate precisely. However, relative rates may be sufficient to test whether the tempo of change within species can account for morphologic differences across species boundaries, and hence to distinguish between gradual and punctuated patterns of evolution. The conditions under which these patterns can be distinguished statistically are explored by simulating varying degrees of within-species rate variability relative to across-species morphologic difference. The statistical methods are then applied to multiple-character morphologic data from closely spaced sequential populations of the Neogene bryozoan Metrarabdotos, using discriminant analysis to compare overall morphologies. In nine comparisons of ancestor-descendant species pairs, all show within-species rates of morphologic change that do not vary significantly from zero, hence accounting for none of the across-species difference. In all cases the ratio of within-species fluctuation to across-species difference is low enough to allow the punctuated pattern to be distinguished with virtual certainty. In at least seven of the cases, ancestor species persisted after giving rise to descendants, in conformity with the punctuated equilibrium mode of evolution.

Speciation and Macroevolution

“…Maynard Smith, in 1966, Kondrashov (1983a, 1983b, 1986 and others have developed a number of models of speciation in sympatry, due to habitat shift or behavioral isolation. The conditions necessary for speciation in sympatry to go forward are rather restrictive.…”

“…There are several studies of punctuated fossil sequences; Cheetham's (1986) work on the Miocene to Pliocene bryozoans is a well-worn example. Cheetham shows almost static lineages coexisting with lineages that appear, from polygenetic analysis, to be their descendants.…”

Effects of nonnormality on studies of morphological variation of a Rhabdomesine bryozoan, Streblotrypa (Streblascopora) prisca (Gabb and Horn)

“…It is noteworthy that Cheetham (1986) arrived at the same number through different criteria, using five observations per specimen in a study of cheilostome bryozoans.…”

Phylogeny of Genus Cupuladria (Bryozoa, Cheilostomata) in the Neogene of Tropical America

“…10 and 11). This is despite quantitative estimates of the probability of completeness of the fossil record of nearly 100 percent and near maximum probability of the occurrence of ancestors of PA¼0.5 (Foote, 1996;Cheetham andJackson, 1998, 2000). Resolution of this issue will require cladistic reexamination of eight or more Eocene to Recent principally eastern Atlantic species traditionally assigned to Cupuladria but intermediate in a number of characters between Discoporella and Cupuladria (Lagaaij, 1963;Cook, 1965aCook, , 1965bCook, , 1985Boardman et al, 1970;Cadée, 1975).…”