Quantitative Genetics of Bryozoan Phenotypic Evolution. Ii. Analysis of Selection and Random Change in Fossil Species Using Reconstructed Genetic Parameters

Abstract

Abstract. -The roles of natural selection and random genetic change in the punctuated phenotypic evolution of eight Miocene-Pliocene tropical American species of the cheilostome bryozoan Metrarabdotos are analyzed by quantitative genetic methods. Trait heritabilities and genetic covariances reconstructed by partitioning within-and among-colony phenotypic variance are similar to those previously obtained for living species of the cheilostome Stylopoma using breeding data. The hypothesis that differences in skeletal morphology between species ofMetrarabdotos are entirely due to mutation and genetic drift cannot be rejected for reasonable rates of mutation maintained for periods brief enough to account for the geologically abrupt appearances of these species in the fossil record. Except for one pair of species, separated by the largest morphologic distance, directional selection acting alone would require unrealistically high rates of selective mortality to be maintained for these periods. Thus, directional selection is not strongly implicated in the divergence ofMetrarabdotos species. Within species, rates ofnet phenotypic change are slow enough to require stabilizing selection, but mask large, relatively rapid fluctuations, all of which, however, can be attributed to chance departures from the mean phenotype by mutation and genetic drift, rather than to tracking environmental fluctuation by directional selection. The results are consistent with genetic models involving shifts between multiple adaptive peaks on which phenotypes remain more or less static through long-term stabilizing selection. Regardless of the degree to which directional selection may be involved in peak shifts, phenotypic differentiation is thus related to processes different than the pervasive stabilizing selection acting within species.

A macroevolutionary expansion of the modern synthesis and the importance of extrinsic abiotic factors

“…The fossil record provides abundant evidence that many species persist relatively unchanged for millions of years (Eldredge and Gould ; Barnosky ; Stanley and Yang ; Cheetham et al . ; Jackson and Cheetham ; Webster et al . ; Eldredge et al .…”

“…The principal explanation of punctuated equilibrium is that the majority of phenotypic change occurs during speciation, with subsequent dynamic morphological stasis maintained across the remainder of the species' lifetime (Barnosky ; Stanley and Yang ; Cheetham et al . ; Jackson and Cheetham ; Webster et al . ; Eldredge et al .…”

Revisiting a Landmark Study System: No Evidence for a Punctuated Mode of Evolution inMetrarabdotos

“…A series of seminal articles by Cheetham and colleagues consolidated Metrarabdotos as a textbook example of a punctuated mode of evolution during the 1980s and 1990s (Cheetham 1986(Cheetham , 1987Cheetham et al 1994;Jackson and Cheetham 1994). A phylogenetic hypothesis of the genus was created on the basis of large-scale analyses of morphology and stratigraphic positions of fossils (Cheetham 1986;Cheetham and Hayek 1988;Jackson and Cheetham 1994).…”

“…A series of seminal articles by Cheetham and colleagues consolidated Metrarabdotos as a textbook example of a punctuated mode of evolution during the 1980s and 1990s (Cheetham 1986(Cheetham , 1987Cheetham et al 1994;Jackson and Cheetham 1994). A phylogenetic hypothesis of the genus was created on the basis of large-scale analyses of morphology and stratigraphic positions of fossils (Cheetham 1986;Cheetham and Hayek 1988;Jackson and Cheetham 1994). Using this phylogeny as a backbone, several articles with detailed analyses of morphological evolution within lineages and at speciation events found mostly strong support for minimal within-species evolution and large-scale changes during speciation (Cheetham 1986;Cheetham et al 1993Cheetham et al , 1994Jackson and Cheetham 1994).…”

How colonial animals evolve

“…These colonies where originally studied by Jackson and Cheetham (28) to investigate whether or not skeletal characteristics are sufficient to rigorously identify biological species in the fossil record. They were subsequently used in a quantitative genetics analysis where the among-zooid variability was used to infer the magnitude of heritable genetic variation (29)(30)(31). However, that approach neglected the hierarchical structure that nests zooids within colonies, effectively conflating patterns of variability among zooids for heritability between colonies.…”