396 Evolution, Theory of Ecdysozoa 80 Arthropoda Nematoda Priapula Orders Mollusca Classes Annelida Crown group Body fossils Lophotrochozoa Platyhelminthes Crown group Trace fossils Brachiopoda Deuterostoma Chordata 40 Stem or early Crown group ? Echinodermata 60 20 Cnidaria ? (Biomarkers) Porifera Ecdysozoa L M B/T T/A N-D Late Neoproterozoic Early Bilateria Cambrian (a) 490 500 510 +10 Burgess Chengjiang Sirius passet 543 Diverse ediacarans Nama CaCO3 610 (Ma) Oldest radially Doushantuo symmetric impressions PO4 (b) Figure Animal diversity across the Proterozoic–Cambrian transition By 510 Ma, all higher-level taxa of animals indicated had diverged Bilaterians are characterized by triploblasty, a rostral–caudal axis, a dorsoventral polarity, and an extensive cluster of Hox genes Reprinted from Knoll AH and Carroll SB (1999) Early animal evolution: emerging views from comparative biology and geology Science 284: 2129–2137 The third is gradualism, the idea that evolutionary divergence accrues incrementally by small steps For Darwin, large changes that could produce or maintain adaptation would be akin to miracles Although there is still debate (Stern, 2011), the modern consensus agrees with Darwin in rejecting saltational evolution, for a variety of reasons Prominent among these are that mutations of large effect are usually accompanied by deleterious pleiotropic effects, which would make such mutations selected against, and the many fossil sequences documenting the origin of higher taxa via numerous intermediate steps For example, that tetrapods evolved from lobe-finned fishes has long been known, and the discovery of fossils near the transition such as Panderichthys and Tiktaalik is filling in the structural differences between fish and amphibian, so that one can see exactly how, among other changes, legs gradually arose from fins From the evidence of at least vertebrate paleontology (Prothero, 2007; Shubin, 2008; Laurin, 2010), one can say that most of evolution is the gradual, adaptive modification of preexisting structures or, better, their underlying developmental programs The fourth is that evolutionary change is populational or variational Lewontin (1983) has argued that this is the most distinctive aspect of Darwin’s thought Rather than consisting of the transformation of individuals, Darwin proposed that individuals in a population have varying heritable properties and that evolution consists of changes in proportions of these heritable properties within the population Some properties persist or increase in frequency, whereas others disappear, so that the hereditary constitution of the population as a whole changes Finally, there is natural selection, which Darwin considered the chief, though not the only, means of changing the frequency of the hereditary properties in populations and the evolutionary force most able to produce that adaptation to the conditions of existence, the absence of a compelling explanation for which had been a chief stumbling block for evolutionary hypotheses Darwin proposed that, if the hereditary properties of individuals conferred on those bearing them varying propensities to succeed in the struggle for existence and reproduction, then those properties that conferred higher survival and/or reproduction (i.e., higher fitness) would increase in frequency in subsequent generations Evolutionary Mechanisms Variation Because the theory of evolution is variational, asking how much variation there is in natural populations is one of the key questions of evolutionary biology (Lewontin, 1974; Charlesworth and Charlesworth, 2010) And the answer is that