Not-so-early bursts and the dynamic nature of morphological diversification

Abstract

In Wonderful Life, Stephen Jay Gould attempted to explain how the bizarre animal fossils from the 500 million-y-old Burgess Shale fauna teach a valuable lesson about the nature of morphological evolution (1). Finding such a remarkable diversity of body plans so early in the history of metazoan life should not be surprising, he argued, for this is an all-too-common pattern. Rather than gradually accumulating, as earlier iconography tended to illustrate, morphological diversity has often generated rapidly during the early phases of clade history. After an initial and rapid exploration of possibilities, successful forms proliferate while those less fortunate are quickly erased by extinction. Gould’s inverted iconography is, in fact, quite consistent with long-standing ideas in paleontology regarding early rapid morphological diversification in higher-level clades (2, 3), and this concept has since become entrenched in modern macroevolutionary biology as the early burst model of adaptive radiation (4). In PNAS, Hopkins and Smith (5) investigate rates of morphological evolution in the post-Paleozoic fossil record of echinoids (sea urchins, sand dollars, and their kin). The authors find that there is no evidence for an early burst of morphological innovation in echinoids as a whole. Instead, Hopkins and Smith document a third way: echinoid evolution is characterized by slow rates and constraints in stem lineages, but later pulses of evolutionary bursts are associated with shifts in feeding morphology.