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Contrasting the Underlying Patterns of Active Trends in Morphologic Evolution

Peter J. Wagner
Evolution
Vol. 50, No. 3 (Jun., 1996), pp. 990-1007
DOI: 10.2307/2410641
Stable URL: http://www.jstor.org/stable/2410641
Page Count: 18
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Contrasting the Underlying Patterns of Active Trends in Morphologic Evolution
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Abstract

Gastropod evolution during the early Paleozoic featured active trends (i.e., differential replacement of morphologies) for at least three shell characters. Selective sorting, either of individual organisms or of whole species, is an obvious mechanism for such active trends. Sorting of individuals should result in a disproportionate number of ancestor to descendant transitions being in the same direction as the trend, whereas sorting of species should result in species with particular morphologies producing more daughter species. Sorting of species can occur over long periods of time or it can be concentrated over a particular interval, such as an extinction event. Constraints on morphologic evolution also can drive trends, especially in cases where it is easier to produce a particular morphology than it is to change it. Finally, active trends can be artifacts of unrelated differential diversification within a clade (i.e., species hitchhiking), which might result from sorting of species based on phylogenetically associated characters or simply by chance. Unlike other active trends, trends attributable to species hitchhiking do not support hypotheses about selection or evolutionary constraints. From the latest Cambrian through the Silurian, gastropods show significant increases in the proportions of species with high shell torques (~= high spire height), inclined apertures, and narrow sinuses. Ancestor-descendant contrasts indicate a significant bias towards daughter species with narrower sinuses than their ancestors. This is not true for shell torque or apertural inclination. However, species with high shell torques appear constrained to produce daughter species with high shell torques. Analogous constraints do not exist for sinus width or apertural inclination. To separate the predictions of species hitchhiking from those of differential speciation and differential extinction, a bootstrapping procedure was used to simulate the range of trends that phylogenetic effects alone could generate. The simulations indicate that the trend towards species with strongly inclined apertures is indistinguishable from a phylogenetic effect. However, the simulations indicate that differential clade evolution affected the evolution of both shell torque and sinus width. The deviations between observed and expected distributions are sudden and coincide with extinction events: species with low shell torques were significantly less likely to survive the Llandeilo/Caradoc (Middle Ordovician) turnover, and species with wide sinuses were significantly less likely to survive the end-Ordovician mass extinction. However, there is not a significant association between diversification and morphology for any of the characters studied here. These results suggest that selection on individuals, evolutionary constraints, and differential survivorship over extinction events all drove trends among early gastropods.

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