Evolutionary Biology最新文献

The Late Cretaceous dinosaur Tyrannosaurus rex was recently split into three species based on the premise that variation in the T. rex hypodigm is exceptional, indicating cryptic species and “robust” and “gracile” morphs. The morphs are based on proportional ratios throughout the skeleton. The species are claimed to be stratigraphically separate, with an early robust species followed by robust and gracile descendants. There are problems with the hypothesis: the taxon diagnoses are based on two features that overlap between the species; several skulls cannot be identified based on the diagnoses; proportional comparisons between Tyrannosaurus and other theropods are based on incomparable samples; the tooth data are problematic; the stratigraphic framework divides the Hell Creek Formation into thirds, without the stratigraphic position of each specimen, or independent age control showing the subdivisions are coeval over the entire geographic area; previous work found variation in T. rex, but it cannot be parsed into discrete categories. We tested for “gracile” and “robust” morphs by analyzing the femoral and tooth ratios that were published in the multiple species study using agglomerative hierarchical clustering. The results found that each set of ratios are explained by one cluster, showing that dimorphism is not supported. We tested for exceptional variation of the femoral ratio of Tyrannosaurus; we calculated the mean intraspecific robusticity for 112 species of living birds and 4 nonavian theropods. The results showed that the absolute variation in Tyrannosaurus is unexceptional and it does not indicate cryptic diversity. We conclude that “T. regina” and “T. imperator” are subjective junior synonyms of T. rex.

Empirical studies have shown that several taxa exhibit a decoupled relationship between lineage species diversification (the balance between speciation and extinction) and phenotypic diversification. This has been recognized by some authors as fundamental evidence for non-adaptive radiation. In the leaf-toed geckos Phyllodactylus of North America, there is a wide inter-specific overlap of phenotypic traits and high intra-specific morphological variation, despite predominantly allopatric distributions and the colonization of both insular and continental habitats. Here we demonstrate two outstanding aspects of the evolution of Phyllodactylus—first, that the radiation occurred with rate decoupling (body size and diversification), and second, that the contributions of island colonizations to the genus’ diversification (phenotypic and species) have been of the same magnitude as those of continental habitats. Phyllodactylus diversification has proceeded with minimal ecological influence, as suggested by the identification of limited phenotypic diversity, evidenced by the wide representation of one morphotype (shared in island and mainland environments) and limited disparity (body size and shape) for long periods of time. Conversely, some head traits like snout length have increased in disparity in recent times. Most likely, snout length is being shaped by selective pressures associated with the differential exploitation of insular and continental trophic niches. The decoupling of rates (diversification and body size evolution), long periods of morphological stasis (body size and shape), overlapping of traits in the morphological space, and minimal ecological influence on the evolution of body size suggest that Phyllodactylus has proliferated following the tempo and mode of a non-adaptive radiation.