Tempo and mode of morphologic evolution near the origin of the radiolarian lineage Pterocanium prismatium

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Abstract
Morphometric examination of cladogenesis and phyletic evolution in two late Neogene sister lineages of marine microfossils (Pterocanium prismatium and P. charybdeum, Radiolaria) from two equatorial Pacific sediment cores was undertaken to better understand the rate of cladogenesis and its relation to subsequent phyletic change. The origin of P. prismatium from P. charybdeum ∼4 ma ago has been estimated to take place over an interval of ∼500,000 yr. Results show that the speciation event consists of two distinct phases. The first phase, cladogenesis, occurred relatively rapidly (on the order of 50,000 yr). A second phase of relatively rapid divergent phyletic evolution away from the common ancestral state followed in both descendant branches and continued for at least 500,000 yr after completion of the cladogenetic event. Net evolutionary rates over the next 2 ma appear to be much lower. Individual characters change by as much as 2 population standard deviations during cladogenesis, and by a total of approximately 3 standard deviations over 2.5 ma of phyletic evolution. Up to 5 population standard deviations of change during ≦ 50,000 yr of cladogenesis, and 7 additional standard deviations of phyletic change over 500,000 yr are observed in multivariate (discriminant function) indices of morphologic difference. The measured pattern does not appear to be either strictly “punctuated” or strictly “gradual,” but instead shows features of both hypotheses.