) Some bizarre structures in extinct dinosaurs may have threatened rivals, but this is difficult Selleckchem Fulvestrant to test without direct knowledge of behaviors that are not preserved in the fossil record. (ii) Intersexual: The principal means of intersexual display is display for

mates, traditionally called sexual display. Sexual display usually implies sexual selection, and explanations of sexual selection must be evaluated much like those for mechanical adaptations. In contemporary populations, sexual selection often acts on minor features and elaborates them (Mendelson & Shaw, 2005); intense sexual selection can result in runaway selection (Futuyma, 2009) and (or) divergent selection (Kroodsma et al., 1985; Price, 1998). Evolutionary theory holds that this kind of divergence can result in speciation (Futuyma, 2009), and that like natural selection, sexual selection can be responsible for patterns of sorting in clades (Vrba, 1984; Sampson, 1999). This could be shown if the characters subject to sexual selection show non-random trends in clades (though the variation of the trends themselves does not have to be directional or trendlike). A problem with invoking sexual display Selleckchem RO4929097 as the explanation of bizarre structures can be traced to Darwin’s (1871) original formation of the problem of sexual selection. Darwin emphasized that sexual selection could only apply when one sex bears structures used in

intersexual display (or agonistic behavior in intrasexual interaction). In other words, sexual selection cannot be invoked without discrete, qualitative features of sexual dimorphism. (We acknowledge that many neobiologists [apparently originating with West-Eberhard's, 1983 conflation

of the concepts] feel that sexual dimorphism is not necessary 4-Aminobutyrate aminotransferase for sexual selection, but Darwin defined the concept in this way and by definition he cannot be wrong. This does not deny that various other phenomena associated with competition for mates and reproductive success are interesting and important; but they are not strictly part of sexual selection.) Unfortunately, this degree of sexual dimorphism, typical of birds and some mammals, has not been sufficiently established for dinosaurs. (iii) Social selection: This concept (West-Eberhard, 1983) was recently applied to dinosaurs by Hieronymus et al. (2009), who argued persuasively that the nasal cornifications of centrosaurine ceratopsians were progressively selected for larger size and broader display. According to them, ‘social selection occurs when there is differential success in within-species competition for any limited resource.’ Two problems with this definition, as applied to fossils, are that within-species phenomena can almost never be observed, and competition is particularly difficult to establish in extinct forms (Benton, 1996). On the other hand, it is possible to identify structures that can plausibly have functioned only in social interaction (as opposed to food gathering, thermoregulation, etc.