Averageness is purportedly the result of stabilizing selection maintaining the population mean, whereas facial paedomorphosis is a product of directional selection driving the population mean towards an increasingly juvenile appearance. If selection is predominantly stabilizing, intermediate phenotypes reflect high genetic quality and mathematically average faces should be found attractive. If, on the other hand, directional selection is strong enough, extreme phenotypes reflect high genetic quality and juvenilized faces will be found attractive. To compare the effects of stabilizing and directional (...) selection on facial paedomorphosis (juvenilization), graphic morphing and editing techniques were used to alter the appearance of composite faces to make them appear more or less juvenile. Both facial models and judges of attractiveness were from the CSU-Long Beach campus. Although effect sizes for both preferences were large, the effect for averageness was nearly twice that found for juvenilization, an indication that stabilizing selection influences preferences for facial paedomorphosis more so than directional selection in contemporary humans. (shrink)

The evolutionary embryologist Gavin Rylands de Beer can be viewed as one of the forerunners of modern evolutionary developmental biology in that he posed crucial questions and proposed relevant answers about the causal relationship between ontogeny and phylogeny. In his developmental approach to the phylogenetic phenomenon of homology, he emphasized that homology of morphological structures is to be identified neither with the sameness of the underlying developmental processes nor with the homology of the genes that are in involved in the (...) development of the structures. De Beer’s work on developmental evolution focused on the notion of heterochrony, arguing that paedomorphosis increases morphological evolvability and is thereby an important mode of evolution that accounts for the origin of many taxa, including higher taxa. (shrink)

Transitional forms of the recent classes of vertebrates are only known in paleontology. The well described examples are:Eusthenopteron foordi,Ichthyostega andAcanthostega between Osteichthyes and Amphibia,Seymouria baylorensis between Amphibia and Reptilia,Archaeopteryx lithographica between Reptilia and Aves, and the mammal-like reptiles Pelycosauria, Therapsida and Cynodontia between Reptilia and Mammalia. The description of their phylogenetical heterochronies in terms of peramorphosis and paedomorphosis shows the progressive role of the motorial, especially the locomotorial organ systems and their functions in comparison with the retarded evolution of (...) the axial system, especially the skull and central nervous system. The evolution of the Hominidae shows the same rule. — The evaluation of these transitional forms in their fossil context reveals them as inhabitants of biotopes situated in the border areas of coastal and shore landscapes of marine, brackish or fresh water. These biotopes have obviously favoured the innovations on the high taxonomic level of macro-evolutionary characteristics. (shrink)