Feather evolution exemplifies sexually selected bridges across the adaptive landscape (original) (raw)
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Filament to Protofeather: An Intricate History of Feather Evolution
Feathers are stunning embellishments that stand out in avian history, a curious detail that has sparked great debate regarding origin and evolution, and as per Prum (1999, p. 291) a “complex evolutionary novelty”. They are necessary for flight, warmth, camouflage, sexual selection (Prum, 1999) and even communication (Li et al., 2010). They are essential to overall survival of a species: one which has evolved over hundreds of millions of years, but how similar are these integumentary structures of modern birds (Neornithes) to those that covered ancestral relatives? Protofeathers, or “dino-fuzz”, have been observed in several theropod fossils, such as Archaeopteryx lithographica (Longrich, Vinther, Meng, Li & Russell, 2012), Sinosauropteryx (Sloan, 2010) and Anchiornis huxleyi (Longrich et al., 2012), but the specifics of their evolution is still a major point of contention for some researchers. Because research is constantly suggesting new scenarios, hypotheses and discoveries about the evolution of feathers and flight, the range of thought varies widely. Amazingly, it’s only in the last three decades that so much new data has come to light, which means that theories are dynamic and researchers don’t always agree.
The geometry of feather barbs (barb length and barb angle) determines feather vane asymmetry and vane rigidity, which are both critical to a feath- er’s aerodynamic performance. Here, we describe the relationship between barb geometry and aerodynamic function across the evolutionary history of asymmetrical flight feathers, from Mesozoic taxa outside of modern avian diversity (Microraptor, Archaeopteryx, Sapeornis, Confuciusornis and the enantiornithine Eopengornis) to an extensive sample of modern birds. Contrary to previous assumptions, we find that barb angle is not related to vane-width asymmetry; instead barb angle varies with vane function, whereas barb length variation determines vane asymmetry. We demonstrate that barb geometry significantly differs among functionally distinct portions of flight feather vanes, and that cutting-edge leading vanes occupy a distinct region of morphospace characterized by small barb angles. This cutting-edge vane morphology is ubiquitous across a phylogenetically and functionally diverse sample of modern birds and Mesozoic stem birds, revealing a funda- mental aerodynamic adaptation that has persisted from the Late Jurassic. However, in Mesozoic taxa stemward of Ornithurae and Enantiornithes, trailing vane barb geometry is distinctly different from that of modern birds. In both modern birds and enantiornithines, trailing vanes have larger barb angles than in comparatively stemward taxa like Archaeopteryx, which exhibit small trailing vane barb angles. This discovery reveals a previously unrecognized evolutionary transition in flight feather morphology, which has important implications for the flight capacity of early feathered theropods such as Archaeopteryx and Microraptor. Our findings suggest that the fully modern avian flight feather, and possibly a modern capacity for powered flight, evolved crownward of Confuciusornis, long after the origin of asymmetrical flight feathers, and much later than previously recognized.
The molecular evolution of feathers with direct evidence from fossils
Proceedings of the National Academy of Sciences
Dinosaur fossils possessing integumentary appendages of various morphologies, interpreted as feathers, have greatly enhanced our understanding of the evolutionary link between birds and dinosaurs, as well as the origins of feathers and avian flight. In extant birds, the unique expression and amino acid composition of proteins in mature feathers have been shown to determine their biomechanical properties, such as hardness, resilience, and plasticity. Here, we provide molecular and ultrastructural evidence that the pennaceous feathers of the Jurassic nonavian dinosaur Anchiornis were composed of both feather β-keratins and α-keratins. This is significant, because mature feathers in extant birds are dominated by β-keratins, particularly in the barbs and barbules forming the vane. We confirm here that feathers were modified at both molecular and morphological levels to obtain the biomechanical properties for flight during the dinosaur–bird transition, and we show that the patterns and t...
bioRxiv, 2023
Feathers are complex integumentary structures with high diversity across species and within plumage and have varied functions (e.g., thermoregulation, flight). Flight is lost in many crown lineages, and frequently occurs in island founding or semiaquatic context. Different extant lineages lost flight across at least three orders of magnitude of time (~79.58 Ma-15 Ka). Flight loss effect on sensory capacity, brain size, and skeletomusculature have been studied, but less work exists on relations between flightlessness and feathers. To understand how flight loss affects feather anatomy, we measured 11 feather metrics (e.g., barb length, barb angle) from primaries, tertials, rectrices, and contour feathers on skins of 30 flightless taxa and their phylogenetically closest volant taxa, supplemented with broader sampling of primaries across all orders of volant crown birds. Our sample includes 27 independent losses of flight; the sample contains nearly half the extant flightless species count and matches its ~3:2 terrestrial:semiaquatic ratio. Vane symmetry increases in flightless lineages, and these patterns are strongest in flight feathers and weakest in coverts. Greatest changes in feathers are in the oldest flightless lineages like penguins, which show robust filaments (rachis, barbs, and barbules) on small feathers, and ratites, which show high interspecific diversity with plumulaceous filaments and/or filament loss. Phylogenetic comparative methods show that some of these microscopic feather traits, such as barb/barbule length and rachis width, are not as dramatically modified upon flight loss as are body mass increase and relative wing and tail fan reduction, whereas the effect on vane symmetry is more easily detected. Upon relaxing selection for flight, feathers do not soon significantly modify many of their flight adaptations, although increased vane symmetry is likely the most detectable shift. Feathers of recently flightless lineages are in many ways like those of their volant relatives. Feather microstructure evolution is often subtle in flightless taxa, except when flight loss is ancient, perhaps because developmental constraints act upon feathers and/or selection for novel feather morphologies is not strong. Changes in skeletomusculature of the flight apparatus are likely more evident in recently flightless taxa and may be a more reliable way to detect flight loss in fossils, with increased vane symmetry as potentially a microscopic signal. Finally, we see an intriguing, reversed pattern in feather evolution after flight loss from the pattern proposed in popular developmental models of feathers, with the later stages of feather development (asymmetric displacement of barb loci) being lost more readily, while early stages of development (e.g., differentiated barb ridges on follicle collar) are only lost after many millions of years of flightlessness.
Development, Regeneration, and Evolution of Feathers
Annual Review of Animal Biosciences, 2015
The feather is a complex ectodermal organ with hierarchical branching patterns. It provides functions in endothermy, communication, and flight. Studies of feather growth, cycling, and health are of fundamental importance to avian biology and poultry science. In addition, feathers are an excellent model for morphogenesis studies because of their accessibility, and their distinct patterns can be used to assay the roles of specific molecular pathways. Here we review the progress in aspects of development, regeneration, and evolution during the past three decades. We cover the development of feather buds in chicken embryos, regenerative cycling of feather follicle stem cells, formation of barb branching patterns, emergence of intrafeather pigmentation patterns, interplay of hormones and feather growth, and the genetic identification of several feather variants. The discovery of feathered dinosaurs redefines the relationship between feathers and birds. Inspiration from biomaterials and f...