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Microevolutionary Patterns in Late Cenozoic Radiolaria

Published online by Cambridge University Press:  25 May 2016

Davida E. Kellogg  and
James D. Hays
Davida E. Kellogg
Affiliation:
Department of Geological Sciences, Brown University, Providence, Rhode Island 02912; Lamont-Doherty Geological Observatory, Palisades, New York 10964.
James D. Hays
Affiliation:
Department of Geological Sciences, Brown University, Providence, Rhode Island 02912; Lamont-Doherty Geological Observatory, Palisades, New York 10964.
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Abstract

In this paper, we present detailed quantitative studies of evolutionary changes over all or part of the stratigraphic ranges of five fossil radiolarian species from Pacific deep-sea sediment cores. Each of these species shows some variation of a distinctive evolutionary pattern: increase in size of measured morphologic characters, preceded and/or followed by an interval during which little or no significant change occurred.

One of the species studied (Eucyrtidium matuyamai) was allopatrically differentiated from another (Eucyrtidium calvertense). The others (Calocycletta caepa, Pterocanium prismatium and Pseudocubus vema) underwent phyletic change within a single lineage. Those species undergoing phyletic change towards larger size maintained almost constant variability of shell size over long periods of time, including periods of both rapid and extremely slow evolution. This constancy of variability suggests that diminution of selection against larger size may have acted as a stimulus to size increase. In contrast, E. calvertense decreased in variability during evolution towards smaller shell size. We believe this decrease may be interpreted as the result of two factors: (1) strong selection against larger size apparently exerted on this species by its direct descendant, E. matuyamai, during the neosympatric phase of speciation and (2) continuation of previous selection against very much smaller size.

Information

Type
Research Article
Information
Paleobiology , Volume 1 , Issue 2 , Spring 1975 , pp. 150 - 160
Copyright
Copyright © The Paleontological Society 

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References

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