The niche-variation hypothesis: Its potential role in the evolution of the species (Neochonetes).

摘要

The genus Neochonetes represents the only large, costellate member of the Chonetacea. to occur in the Upper Pennsylvanian-Permian beds of the Midcontinent. Since it was first described, some of its members have always been in a constant state of taxonomic flux, as the species N. transversalis and N. meekanus have been regarded as ecological variants of N. granulifer. Taxonomic evaluation of the clade Neochonetes, which concentrated on interal rather than external morphology, shows that there are 12 valid species, and 2 ecological variants in the North American Midcontinent. A cladistic analysis using 32 internal and external characters supports the systematics and produced a single most parsimonious tree. The cladistics highlight a number of important factors: (1) it supports monophyly of the Neochonetes clade; (2) it separates N. transversalis and N. meekanus into distinct species, with N. granulifer as sister taxon to N. transversalis; (3) the cladogram matches a genealogical reconstruction using stratophenetics; and (4) tree support is primarily based on internal characters.; A morphometric analysis was performed to determine the validity of the cladogram. The analysis used 24 internal and external variables measured from 1097 adult-sized dorsal valves of N. granulifer and N. transversalis. A discriminant function analysis produced an eight-variable model, having a 99.69% correct classification when using a canonical discriminant function. The degree of correspondence is high between predictor variables for group classification and those characters designated as dorsal valve synapomorphies of the Neochonetes clade. Results from a principal component analysis on the eight variables produced a graphical representation of multivariate heterochrony and suggests that N. transversalis evolved by allometric acceleration.; The niche-variation hypothesis, which infers a direct relationship between niche dimensions and phenotypic variability, was also analyzed. All eight tests of the model support the contention that niche dimension is an important source of phenotypic variability. This hypothesis, coupled with Wrightian models of population genetics, becomes an important mechanism in driving evolutionary change; it explains the macroevolutionary patterns of onshore origination and offshore migration of higher order taxa; and it partly explains mass extinction resistance of generalists and the rapid evolutionary expansion of surviving clades during the extinction aftermath.