Taxonomic evolution in North American Neogene horses (subfamily Equinae): the rise and fall of an adaptive radiation

Abstract

The 18 m.y. history of the subfamily Equinae (exclusive of Archaeohippus and “Parahippus”) in North America consisted of a 3-m.y. radiation phase, a 9-m.y. steady-state diversity phase, and a 6-m.y. reduction phase. During the steady-state phase, species richness varied between 14 and 20, with two maxima at about 13.5 and 6.5 Ma. Species richness of the tribes Hipparionini and Equini was about equal through the middle Miocene, but hipparionines consistently had more species in the late Miocene and early Pliocene. Overall mean species duration was 3.2 m.y. (n = 50), or an average extinction rate of 0.31 m.y.-1 During the radiation phase, speciation rates were very high (0.5 to 1.4 m.y.-1), while extinction rates were low (<0.10 m.y.-1). Speciation and extinction rates both averaged about 0.15 m.y.-1 during the steady-state phase, with extinction rates having more variation. Extinction rates increased fourfold during the reduction phase, while speciation rates declined slightly. Late Hemphillian extinctions affected both tribes severely, not just the three-toed hipparionines, and were correlated with global climatic change.

Age and rate of diversification of the Hawaiian silversword alliance (Compositae)

“…Without a detailed fossil record or reliable clock-based divergence time estimates, detection of episodes of unusually high or low diversification is difficult. The case of Neogene horses is instructive in this regard because a sufficiently rich fossil record exists to permit estimates of speciation and extinction rates over time (43). Neogene horses underwent an early period of extremely rapid radiation, with Ŝϭ 0.5 Ϫ 1.4 (43), which is comparable to the rate inferred in Hawaiian silverswords, especially given the likelihood that our estimate is low.…”

“…The case of Neogene horses is instructive in this regard because a sufficiently rich fossil record exists to permit estimates of speciation and extinction rates over time (43). Neogene horses underwent an early period of extremely rapid radiation, with Ŝϭ 0.5 Ϫ 1.4 (43), which is comparable to the rate inferred in Hawaiian silverswords, especially given the likelihood that our estimate is low. Thus, the tempo of the silversword alliance radiation over the last 5 million years does not just exceed average rates for continental radiations, it may well be comparable to peak rates during these radiations.…”

Novelty and Innovation in the History of Life

“…Adaptive Radiations in the Fossil Record Many adaptive radiations have been described in deep time, ranging from the fusion of the mantle to produce a siphon and allow deep burrowing among bivalves [24], the Neogene diversification of horses [25], the extensive diversification of Mesozoic mammals [26] to possibly every other clade one could examine. Early bursts of morphological and taxonomic diversity should be associated with adaptive radiations [5] and many have been identified in the fossil record, ranging from the Cambrian radiation [27] to many invertebrate [28,29] and vertebrate [30,31] clades.…”

The Nature of Evolutionary Radiations: A Case Study Involving Devonian Trilobites

“…While an increase in taxonomic diversity can be caused by unusually high rates of speciation, normal rates of speciation coupled with exceptionally low rates of extinction could also cause such patterns. Thus far, however, it appears that evolutionary rates during evolutionary radiations are generally associated with high speciation rates, at least early in the radiation, and these subsequently decline, often quite rapidly (Eldredge and Cracraft 1980;Lieberman et al 1991;Hulbert 1993;Lieberman 2001). Eldredge and Cracraft (1980) identified a number of patterns expected in a clade undergoing an adaptive radiation: rapid appearance of numerous closely related species; monophyletic status of the clade; confinement to an endemic area; and high morphological diversity.…”