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Some Problems and Patterns of Evolution Exemplified by Fossil Invertebrates

Preston E. Cloud, Jr.
Evolution
Vol. 2, No. 4 (Dec., 1948), pp. 322-350
DOI: 10.2307/2405523
Stable URL: http://www.jstor.org/stable/2405523
Page Count: 29
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Some Problems and Patterns of Evolution Exemplified by Fossil Invertebrates
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Abstract

This paper considers certain phases of homeomorphy, paedomorphosis, and eruptive evolution, with illustrations drawn from the fossil invertebrates. Homeomorphy. Homeomorphy is the phenomenon of general superficial resemblance, with dissimilarity in particular significant structural detail. The term is employed only to designate similarity between the external forms of whole organisms, reaching a degree or being of a nature such as may mislead even a trained naturalist. Homeomorphy involves differences in both time and mode, and may result either from convergence or from parallelism. The discussion is illustrated with examples from the brachiopods, pelecypods, cephalopods, and corals. Paedomorphosis. Paedomorphosis refers to the situation where phylogenetic progression has resulted from the onset of persistent phenotypic (thus genotypic) novelties relatively early in the ontogeny of ancestral radicles. In instances of paedomorphic evolution the adult representatives of descendant populations may resemble in important details the young stages found among ancestral populations. This results from persistence into the adult stages of the descendants of features that characterized youthful or early ontogenetic stages of the root stock. Paedomorphosis includes three interrelated things-intercalation, displacement, and phylogenetic neoteny. In theory the latter may go so far as to wipe out traces of its action. Paedomorphosis is a way of building new organisms from old foundations. It may well have produced or contributed to the production of the seemingly bridgeless gaps between some major systematic categories. The evolution of major morphologic grades among the trilobites and corals is considered from the standpoint of paedomorphosis, and it is concluded that this concept offers a way of comprehending certain anomalies in the sequence of these grades. Eruptive evolution. Eruptive evolution is the expression employed to refer to a relatively sudden breaking out of evolutionary diversification within any group of organisms. The inferred pattern and sequence of eruptive evolution are (1) relatively sudden appearance of marked variability; (2) probable availability and proximity of a variety of ecologic niches; and finally (3) increased selective pressure, resulting in the weeding out of inadaptive or poorly adaptive radicles and leading to a more regularly channeled evolutionary phase for the particular stock involved. The phenomenon is illustrated by the evolution of the earlier terebratuloid brachiopods, and the discussion then leads into the highly speculative problem of origin of the Cambrian faunas. It is argued that if diversification of multicelled animal life did not begin previous to the Cambrian or the latest pre-Cambrian there would have been essentially no competitive pressure at this time, and virtually all ecologic niches that multicelled animals could then occupy would have been available. It is proposed as an hypothesis that the diversification of the Early Cambrian faunas may be in large part a matter of eruptive evolution, most nearly comparable to the seemingly abrupt deployment of mammalian stocks in Cenozoic time.

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