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Feather evolution exemplifies sexually selected bridges across the adaptive landscape

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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.

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The widespread view that Archaeopteryx was a primitive (basal) bird has been recently challenged by a comprehensive phylogenetic analysis that placed Archaeopteryx with deinonychosaurian theropods. The new phylogeny suggested that typical bird flight (powered by the front limbs only) either evolved at least twice, or was lost/modified in some deinonychosaurs. However, this parsimony-based result was acknowledged to be weakly supported. Maximum-likelihood and related Bayesian methods applied to the same dataset yield a different and more orthodox result: Archaeopteryx is restored as a basal bird with bootstrap frequency of 73 per cent and posterior probability of 1. These results are consistent with a single origin of typical (forelimb-powered) bird flight. The Archaeopteryx-deinonychosaur clade retrieved by parsimony is supported by more characters (which are on average more homoplasious), whereas the Archaeopteryx-bird clade retrieved by likelihood-based methods is supported by fewer characters (but on average less homoplasious). Both positions for Archaeopteryx remain plausible, highlighting the hazy boundary between birds and advanced theropods. These results also suggest that likelihood-based methods (in addition to parsimony) can be useful in morphological phylogenetics.
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The fossil record of early feathers has relied on carbonized compressions that lack fine structural detail. Specimens in amber are preserved in greater detail, but they are rare. Late Cretaceous coal-rich strata from western Canada provide the richest and most diverse Mesozoic feather assemblage yet reported from amber. The fossils include primitive structures closely matching the protofeathers of nonavian dinosaurs, offering new insights into their structure and function. Additional derived morphologies confirm that plumage specialized for flight and underwater diving had evolved in Late Cretaceous birds. Because amber preserves feather structure and pigmentation in unmatched detail, these fossils provide novel insights regarding feather evolution.
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Unlike a wing made of feathers, one that evolved by extending a lateral patagium, like that of flying squirrels, has to be tensioned between two or more skeletal members. The diversity of bats is much less than that of birds, because of their less versatile wings, whose structure also constrains the evolution of the legs. The wings of pterosaurs also involved the legs, but may have had an elastic membrane that allowed control of span and area, with a degree of versatility nearer to birds than to bats.
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The recent discovery of small paravian theropod dinosaurs with well-preserved feathers in the Middle-Late Jurassic Tiaojishan Formation of Liaoning Province (northeastern China) has challenged the pivotal position of Archaeopteryx, regarded from its discovery to be the most basal bird. Removing Archaeopteryx from the base of Avialae to nest within Deinonychosauria implies that typical bird flight, powered by the forelimbs only, either evolved at least twice, or was subsequently lost or modified in some deinonychosaurians. Here we describe the complete skeleton of a new paravian from the Tiaojishan Formation of Liaoning Province, China. Including this new taxon in a comprehensive phylogenetic analysis for basal Paraves does the following: (1) it recovers it as the basal-most avialan; (2) it confirms the avialan status of Archaeopteryx; (3) it places Troodontidae as the sister-group to Avialae; (4) it supports a single origin of powered flight within Paraves; and (5) it implies that the early diversification of Paraves and Avialae took place in the Middle-Late Jurassic period.
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Fossil data indicate that feathers and their precursors may have evolved over a much longer span than previously thought.
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If females prefer traits that deviate from the population mean, they usually prefer traits of greater quantity, because such traits elicit greater sensory stimulation. Fisher's theory of "runaway' sexual selection and the "good genes' hypothesis, predict that traits and preferences coevolve, whereas the "sensory exploitation' hypothesis predicts that males evolve traits to exploit preexisting female biases. Some studies of frogs and fish support the sensory exploitation hypothesis, although this does not exclude the role of other factors in establishing the preexisting bias or in the subsequent elaboration of the preference. Studies of mate choice will benefit by a more integrative approach, especially one that combines knowledge of sensory mechanisms with appropriate phylogenetic comparisons. -from Authors
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The joint evolution of female mating preferences and secondary sexual characters of males is modeled for polygamous species in which males provide only genetic material to the next generation and females have many potential mates to choose among. Despite stabilizing natural selection on males, various types of mating preferences may create a runaway process in which the outcome of phenotypic evolution depends critically on the genetic variation parameters and initial conditions of a population. Even in the absence of genetic instability, rapid evolution can result from an interaction of natural and sexual selection with random genetic drift along lines of equilibria. The models elucidate genetic mechanisms that can initiate or contribute to rapid speciation by sexual isolation and divergence of secondary sexual characters.
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The evolution of sexual selection is studied using a two-locus model of a polygynous population that follows both a male trait causing loss of viability and a female mating preference for that trait. The major conclusion is that such a mating preference is selected neither for nor against, but the mating advantage it confers on its preferred male type can maintain the less viable trait in the population. The equilibrium frequency of the preference is not uniquely determined by the forces of sexual selection, but this frequency determines the prevalence of the male trait in the population. If a mutant showing a mating preference for a male trait with reduced viability reaches high frequency, the trait can be taken to high frequency and consequently cause average male survivorship to deteriorate severely. Because the frequencies of the preference and hence the trait are indeterminate, it is possible that weak forces such as pleiotropy or genetic drift may control the direction and rate of their evolution. These conclusions still hold if a second, competing female mating preference that favors a more viable male type is introduced: neither that preference nor the more viable male trait will necessarily spread through the population.
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Fancy Feathers In the past few decades, an increasing number of dinosaurs have been shown to have possessed feathers. While it seems likely that feathers themselves may have evolved for thermoregulation, the original function of wings has been less clear and remained a matter of debate. Based on examination of three Theropod specimens from the genus Ornothomimus , Zelenitsky et al. (p. 510 ) conclude that the feathered wing may have evolved not for locomotion or prey capture (the animals were herbivores), but rather as a courtship display. All of the individuals examined had a covering consisting of short filamentous feathers, but the adult specimens, which would have reached sexual maturity, also had long shafted feathers on their forelimbs.
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Hone, D.W.E., Naish, D. & Cuthill, I.C. 2011: Does mutual sexual selection explain the evolution of head crests in pterosaurs and dinosaurs? Lethaia, Vol. 45, pp. 139–156. Cranial ornamentation is widespread throughout the extinct non-avialian Ornithodira, being present throughout Pterosauria, Ornithischia and Saurischia. Ornaments take many forms, and can be composed of at least a dozen different skull bones, indicating multiple origins. Many of these crests serve no clear survival function and it has been suggested that their primary use was for species recognition or sexual display. The distribution within Ornithodira and the form and position of these crests suggest sexual selection as a key factor, although the role of the latter has often been rejected on the grounds of an apparent lack of sexual dimorphism in many species. Surprisingly, the phenomenon of mutual sexual selection – where both males and females are ornamented and both select mates – has been ignored in research on fossil ornithodirans, despite a rich history of research and frequent expression in modern birds. Here, we review the available evidence for the functions of ornithodiran cranial crests and conclude that mutual sexual selection presents a valid hypothesis for their presence and distribution. The integration of mutual sexual selection into future studies is critical to our understanding of ornithodiran ecology, evolution and particularly questions regarding sexual dimorphism. □Behaviour, Dinosauria, ornaments, Pterosauria, sexual selection.
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Adult male Irish elk (Megaloceros giganteus) grew the largest antlers of any extinct or extant cervid. These antlers have often been implicated in the extinction of the Irish elk, although the effects of antler growth on Irish elk physiology have not been analysed quantitatively. We used a simulation model of energy and mineral metabolism to compare nutritional requirements for antler growth in Irish elk and moose (Alces alces), the largest extant cervid. The model simulates intake, metabolism, deposition, and excretion of energy, nitrogen, ash, calcium and phosphorus with mass balance for each of these nutrients on a daily time step. Predicted energy requirements for antler growth by moose are half as large as energy requirements for summer fat and protein deposition. In contrast, the predicted energy requirements for antler growth by Irish elk were about 75% as large as energy requirements for summer fat and protein deposition. Irish elk antlers weighing 40 kg at the end of velvet shedding would have contained 2.1 kg nitrogen, 7.6 kg calcium and 3.8 kg phosphorus. The nitrogen requirements for antler growth were met by forage intake. The model predicts that, to grow 40 kg antlers in a 150 day period, more than 60 g of calcium and more than 30 g of phosphorus were deposited in antlers daily for 60 consecutive days when antler mineralization rate was highest in mid-summer. Simulated Irish elk depleted skeletal mineral reserves to support antler growth more than extant moose, even when hypothesized adaptations to reduce skeletal mineral resorption were implemented. Even though Irish elk fit the allometric relationship between antler size and body size in extant cervids, mineral metabolism does not scale allometrically in the same manner. About 6% of the calcium and 10% of the phosphorus in the antler were resorbed from the skeleton because dietary intake of minerals was insufficient to meet requirements for antler mineralization. The minerals resorbed from the skeleton in summer would have to be replenished by dietary intake over the following winter. Pollen records document a shift in plant species composition from a tall willow–spruce community during the Allerod interstadial to a tundra during the Younger Dryas cold episode with reduced forage density coincident with the extinction of the Irish elk about 10,600 years before present (..). The reduction in forage density would have made replenishing calcium and phosphorus in the skeleton even more difficult, as well as making it more difficult for male Irish elk to replenish fat reserves depleted during the rut. Sexual selection pressures for larger antlers and larger body size were opposed by selection pressures for smaller antlers and smaller body size imposed by environmental change. We suggest that the inability to balance these opposing selection pressures in the face of rapid environmental change contributed to extinction of the Irish elk 10,600 years ..
Article
We introduce models for the runaway coevolution of female mating preferences and male display traits. The models generalize earlier results by allowing for direct natural selection on the preference, arbitrary forms of mate choice, and fairly general assumptions about the underlying genetics. Results show that a runaway is less likely when there is direct selection on the preference, but that it is still possible if there is a sufficiently large phenotypic correlation between the female's preference and the male's trait among mated pairs. Comparison of three preference functions introduced by Lande (1981) shows that open-ended preferences are particularly prone to a runaway, and that absolute preferences require very large differences between females in their preferences. We analyze the causes of the runaway seen in a model developed by Iwasa and Pomiankowski (1995).
Article
Recent discoveries of feathered dinosaurs from Early Cretaceous deposits in Liaoning, China, have not only lent strongest support for the dinosaurian hypothesis of bird origins, but have also provided much-needed information about the origins of feathers and avian flight. Preliminary analysis of character evolution suggests that the major avian osteological characters were acquired during the early evolution of maniraptoran dinosaurs. The available evidence also suggests that the first feathers with a filamentous morphology probably evolved in basal coelurosaurs and pennaceous feathers (including those with aerodynamic features) were developed in non-avian maniraptorans, indicating that feathers evolved before the origin of birds and their flight. An evolutionary model is proposed here to describe the major stages of feather evolution, a process characterized by a combination of both transformational and innovative modifications. This model is different from some recent developmental models, which suggest that feathers are evolutionary novelties without a homologous relationship to reptilian scales. Although non-avian theropods are traditionally regarded as distinctly cursorial animals, recent discoveries suggest that the closest relatives of birds might be arboreal theropods. Many bird features, such as the furcula and pennaceous feathers, evolved in a terrestrial context, whereas others, such as some pedal modifications, may have evolved in an arboreal context. Consequently, arboreality may have also contributed to the origin of avian flight.
Article
Two spectacular fossilized dinosaur skeletons were recently discovered in Liaoning in northeastern China. Here we describe the two nearly complete skeletons of a small theropod that represent a species closely related to Compsognathus. Sinosauropteryx has the longest tail of any known theropod, and a three-fingered hand dominated by the first finger, which is longer and thicker than either of the bones of the forearm. Both specimens have interesting integumentary structures that could provide information about the origin of feathers. The larger individual also has stomach contents, and a pair of eggs in the abdomen.