Feather evolution exemplifies sexually selected bridges across the adaptive landscape

Abstract

Over the last two decades, paleontologists have pieced together the early evolutionary history of feathers. Simple hair‐like feathers served as insulating pelage, but the first feathers with complex branching structures and a plainer form evolved for the purpose of sexual display. The evolution of these complex display feathers was essential to the later evolution of flight. Feathers illustrate how sexual selection can generate complex novel phenotypes, which are then available for natural selection to modify and direct toward novel functions. In the longstanding metaphor of the adaptive landscape, sexual selection is a means by which lineages resting on one adaptive peak may gradually bridge a gap to another peak, without the landscape itself being first altered by environmental changes.

Bird evolution by insulin resistance

“…One of the most striking outcomes of paleontology over the past 60 years has been the discovery of a very close relationship between birds and little theropods [73][74][75][76]. Up to the present time, it was widely accepted that birds evolved from certain little theropods [73][74][75][76].…”

“…Instead, theropods (a bird lineage) became dominant, associated with insulin resistance and gene loss [22][23][24]. In the late Jurassic period, theropods expanded their body size, associated with increasing oxygen concentrations [73][74][75][76]. Euornithes (a primitive bird) became separated from the little theropods during the Cretaceous period [73][74][75][76].…”

“…In the late Jurassic period, theropods expanded their body size, associated with increasing oxygen concentrations [73][74][75][76]. Euornithes (a primitive bird) became separated from the little theropods during the Cretaceous period [73][74][75][76]. Associated with a further increase in oxygen concentration [63], Neoaves (swallow and crow) and Palaeognathae (chicken and ostrich) became separated [63].…”

On the Multifunctionality of Feathers and the Evolution of BIrds

“…Research suggests Archaeopteryx was capable of at least some powered flight (e.g. Dyke 2010, Voeten et al 2018); however, feathers likely evolved for some purpose other than flight (Feo et al 2015, Lloyd et al 2016) and were co-opted to serve as integuments for powered flight later in the evolutionary history of birds (Persons and Currie 2019). Despite the evolution of feathers predating that of flight, birds subsequently evolved numerous physiological adaptations for flight, and all crown birds descended from an ancestor that was highly adapted for flight (Padian et al 1998).…”

“…If a trait occupies a fitness peak and can only evolve through gradual, stepwise changes, selection would not favor the evolution of this trait to higher fitness peaks if it requires the traversal through troughs of lower fitness. However, Persons and Currie (2019) point out that feather evolution provides an example of how multiple separate axes of selection (e.g., sexual vs. natural selection) can help organisms "jump" across gaps between peaks in selective landscapes. However, this applies more broadly than with sexual vs. natural selection.…”