That is where Darwin’s (1859, 1871) concept begins and ends. We do not need to redefine or expand it (Clutton-Brock, 2007; Doxorubicin Carranza, 2009); we need

to return to its original meaning. To do so eliminates confusion related to consequent questions such as the function of bizarre structures in dinosaurs, to which we now turn in responding to some of Knell and Sampson’s specific points. 1. With few exceptions, sample sizes for individual dinosaur species are too small to conduct statistical tests for the presence of sexual dimorphism. We agree, but as we showed, this has not stopped many paleontologists from arguing in its favor, without testing for other causes of variation, such as ontogeny Selleckchem Sorafenib or phyletic (anagenetic) change in evolution. On the other hand, we do have many well-represented dinosaur species, including those that bore bizarre structures,

and these are tractable to statistical analysis. 2. Bovid males use their horns predominantly in competition for mates. Yes, and in these bovids sexual dimorphism is generally high; females usually lack horns, except in small species, in which both sexes typically have small horns. There are really no living animals related to or comparable with the Mesozoic dinosaurs that we discussed in these respects. Analogies to living animals and their patterns of behavior must therefore be tested stringently. 3. Species recognition has not been documented as a key factor in the evolution of exaggerated traits among any extant animals. We think it has generally not been examined, and the hypothesis certainly cannot be rejected. Very few living animals have exaggerated structures comparable with those of Mesozoic dinosaurs, and to our knowledge extensive studies of species recognition have not been carried out on those that do, although this would not be prohibitively difficult. The present is sometimes a key to the past, but it is not its universal arbiter. 4. We assume that traits under directional

selection evolve slowly enough for directional change to be evident on phylogenies of extinct clades. We do not pretend to know how rapidly N-acetylglucosamine-1-phosphate transferase these changes occurred, or even what triggered them in these dinosaurs. There is increasing evidence of anagenetic change between species that previously were considered sister taxa (e.g. Evans, 2010; Scannella & Horner, 2010), and these changes may have taken thousands to tens of thousands of years or less, judging by biostratigraphic distributions. However, we are making a rather different point that we would not expect directional trends within clades in which species are simply evolving to be recognizably different from each other. This is a quite different process, and on a quite different scale, than for example the directional ‘runaway selection’ of sexual characters seen in living populations (e.g. Kirkpatrick, 1982; Andersson, 1994).