Microevolutionary Patterns in Late Cenozoic Radiolaria

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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 prismatiumandPseudocubus 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. calvertensedecreased 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.