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The Allopatric Model and Phylogeny in Paleozoic Invertebrates

Niles Eldredge
Evolution
Vol. 25, No. 1 (Mar., 1971), pp. 156-167
DOI: 10.2307/2406508
Stable URL: http://www.jstor.org/stable/2406508
Page Count: 12
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The Allopatric Model and Phylogeny in Paleozoic Invertebrates
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Abstract

Emphasis on time and gradualistic transformation has led to the dominance of a strictly phyletic model of species transformation in most paleontological thought. Even documented cases of lineage-splitting are often interpreted by recourse to a gradual (morphological) divergence model in which a strong element of phyletic thinking is incorporated. Allopatric speciation, not a strict alternative to gradual divergence, seems to fit the common pattern of non-intergrading species within a lineage as typically preserved in Paleozoic epeiric sediments. The majority of species preserved in epeiric sediments show no change in species-specific characters throughout the interval of their stratigraphic occurrence, and the phyletic model is inapplicable to most of these elements of the fossil record. Instead, change in, or development of, species-specific characters are envisioned as occurring relatively rapidly in peripheral isolates. Morphological stability of epeiric species is attributed to stabilizing selection. The Devonian trilobite Phacops rana illustrates, at two different times in its history, the origin of a new character state in peripheral isolates. Most new Paleozoic invertebrate taxa probably arose in geosynclines (marginal seas) bordering the cratons; though these origination events themselves were probably not related to marine regressions, invasion of new taxa into the epeiric seas seems directly related to periods of transgression.

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