Heterochrony and phylogenetic trends

A model is proposed, based on examples that have been interpreted as phylogenetic trends, to explain how directional morphological evolution at the species level can arise by heterochrony. The examples illustrated are of Tertiary to Recent rhynchonellide brachiopods, Cambrian olenellid trilobites, living spatangoid echinoids, Tertiary to Recent schizasterid echinoids, Cenomanian ammonites and Si- lurian monograptids. Morphological discontinuities between species along morphological gradients (which can be recognised both spatially and/or temporally), and temporal morphological stasis within species, are both consistent with the punctuated equilibria model of macroevolution. It is argued that morpho- logical discontinuities have arisen by selection of morphological novelties produced by heterochronic processes. These novelties are preadaptations which allow ecological and, consequently, genetic isolation from ancestral species. Establishment of a heterochronic morphological gradient is only possible given a suitable environmental gradient. The terms "paedomorphocline" and "peramorphocline" are proposed for these heterochronic morphological gradients. Paedomorphoclines and peramorphoclines each comprise a number of species occupying a series of adaptive peaks, which have evolved sequentially through time by selection along an environmental gradient