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Article Additions: https://en.wikipedia.org/wiki/Flying_and_gliding_animals

Elongated feathers may have arisen as a result of sexual selection, and this drive towards longer feathers allowed for gliding and eventually flight. In the purview of the ground-up model, this would allow for those originally flightless theropods to gain flight by running or hopping. The same holds for the trees down model, where the elongated feathers would permit for more sustained gliding.

Archaeopteryx is a well-known intermediate species between birds and theropods; its rudimentary feathers and hollow bones are the foundation of the movement towards what would eventually become modern birds.

Selective pressures driving wing formation were such things as competition for resources, energy conservation, and necessity of escape. In early gliders, it would have been advantageous be able to travel farther for less energy; from this traits arose that moved incrementally towards powered flight. Limb position, digit length, and body size were all important traits in early selection. Genetic histories can facilitate investigation into the timing of the evolution of such traits, and how subsequent dispersal of the trait occurred.

The genetic basis of wing development can also not be overstated, as it is through evolution at the genetic level that such traits can be passed on. Links have been made to regulatory genes controlling the expression of wing length in bats, which would have been profoundly important in the development of true wings.

Bats and birds often share similar ecological niches, in large part due to their shared ability of flight. However, birds have had the capacity of flight far longer and are far more prevalent in such roles the world over.

FINAL PAPER

Convergent Evolution of Flight in Vertebrates Powered flight has existed in various forms since the time of the dinosaurs, but only in more recent years have we discovered the mechanisms by which flight evolved in subsequent vertebrates. Flight is a tremendous evolutionary advantage to those organisms that have use of the ability, and it is no surprise that it has arisen multiple times throughout history. That said, there are difficulties in painting a complete picture of how exactly it came about in its current forms; this is due mostly to an incomplete fossil record and contention among evolutionary biologists as to what model of flight is correct. What is known, however, is that there exist ancestral forms that modern wings can trace their line back to and that there is at least some idea as to what pressures would have driven the evolution of powered flight among early organisms. Convergent evolution of advantageous traits is nothing new in evolutionary biology, but instances as well documented and studied as flight are rare. It is for this reason that it is important to study the convergence of traits like flight in order to gain a better understanding of how traits may arise independently among distinct groups. From the fossil record, we know that certain dinosaurs were capable of winged flight (e.g. pterosaurs) and that as they evolved the trait dispersed among many different species. However, where our view grows hazy is in finding ancestral links between dinosaurs and what would eventually become the birds of today. What few links we have are promising though, as they offer clues into certain stages in the progression towards modern birds. Archaeopteryx is the classic example of the so-called “missing link”, having been discovered to be one of the principal ancestors of modern birds; its rudimentary feathers and hollow bones allowed for some controlled gliding, and further modifications stemming from its form yielded more ancestral birds (Brush, 1998). Dig sites in China show another ancestral bird species capable of perching and folding their wings, recognizable traits present also in modern birds (Sereno, 1992). With these traits exhibited in ancestral states, we can extrapolate upon this model and glean further from the traits exhibited and how they may have been passed on. From an evolutionary perspective this is intriguing because it shows that such traits have remained intact in some capacity for many millions of years, or have arisen multiple times (which would not be out of the question). Further examination of the development of avian characteristics has also yielded interesting information as to how birds differentiated from their ancestral state: things like feather lengthening, wing placement, and body weight are all crucial stages in the process of ‘perfecting’ flight (Seymour, 1985). The pressures that drove evolutionary development of wings were the same then as they are today; that is, competition for resources, conservation of energy, and the necessity to escape predators are as old as evolution itself. In evolutionary thinking there exist two models that attempt to explain the development of powered flight, each invoking these pressures as the driving forces (Norberg, 1990). These two are the trees-down and the ground-up model, each seeking to explain how powered flight arose but with very different mechanisms. The ground-up model posits that flight developed from the practice of early, feathered ancestors running along the ground to catch prey, eventually resulting in gliding and later flight. The ground-up model addresses well the pressures of competing for resources like food and mating, in fact, as it is believed that larger feathers were more attractive to potential mates (Brush, 1998). New research into development of modern birds has yielded information in support of this theory, finding that as birds grow they have the capacity to increase the degree to which their wings stretch and thereby increase their ability to take flight from the ground by simply running. This gives major support to the theory, as it offers a modern link to the ancestral model (Jackson, 2008). The trees down model takes the opposite tack, stating that flight arose after gliding organisms gradually developed fuller wings and flight. This model emphasizes conservation of energy and evasion of predators, as the treetops offer a good vantage point for resources as well as shelter (Dial, 2003). This is the favored model for bats in particular, in part because of the precedent set in extant gliding species (Norberg, 1990). Powered flight in the state is in today took a considerable amount of evolutionary fine-tuning, as it started from rougher forms of gliding or soaring. In bats it was a longer road, as there was no ancestral state to derive from as there was in birds (e.g. pterosaurs). If we were to apply the trees-down model for flight we would have a rough starting point for the progression towards wings, and there would be a real-world example still today in the form of gliding animals that use their ‘proto-wings’ to glide from treetop to treetop (Rayner, 1988). That said, there was still much modification to be done; things like wing placement, digit elongation, and body structure all underwent changes (Garner et al. 1999). Since bats oftentimes occupy environments or fill niches similar to birds, it is to be expected that they have similar demands from their wings. Where they differ is in these critical evolutionary steps, where the ancestor of the bats took a decidedly different step towards the formation of wings (Norberg, 1990). This is unsurprising, as different ancestral morphologies would rarely be assumed to create the exact same configuration, especially in so complex a structure as wings. However, the result is the same, and both birds and bats can take wing and fly unaided. Taking these things into account, it is also important to understand the genetic factor involved in wing convergence. Evolution, as we know, takes place in genotypic space; as such, genetics are very important in the derivation of traits and their transmission. For instance, it has been found that forelimb modification is tied to regulatory modifications of specific genes. In particular, correlations have been made between gene modification and increased wing length in bats (Xu et al., 2013). Genes such as these would have been instrumental in the formation of true wings and would be integral in the development of true flight. Tracing genes like this can also give clues as to when the trait may have arisen and how it branched among species. We know that flight has existed in birds for a longer period of time, but it would assist evolutionary science to know precisely when the trait arose in mammals and under what circumstances (Xu et al., 2013). In short, genetics help to paint a clearer picture of how and when the traits arose; with this knowledge, it is easier to understand the mechanisms that shape such developments and could one day even help to find a link between the genes controlling flight traits in birds and bats, respectively. These findings pertain to the broader field of evolutionary biology because of how well defined the trait of wings is and how its convergence is more easily mapped. By understanding the processes that shaped the convergence of multiple organisms on flight, evolutionary science as a whole can benefit from the wealth of knowledge generated by its study. Analogy is the occurrence of similar structures with the same function derived not from an evolutionary root but from similarity of use. Wings are similar in many ways to fins, which are also modified limbs used to conduct an organism through a fluid. Because of this, wings can be said to be homologous to the fins or forelimbs of other organisms with which they share a common ancestor. To evolutionary scientists, these cannot be overlooked, as they are of significance in defining characteristics like homoplasy and analogy, which are applied to the broader field of biology to help assort the varied species and their defining traits. The convergence of wings and flight is to many the model form of convergent evolution, and while it may seem intuitive or simple even that wings evolved to fit organisms with similar pressures, this sentiment undermines the incredible amount of selection and mutation that had to occur for the same trait to arise in two independent populations. In studying such events in evolutionary history, we learn much about the evolutionary process itself and the world at large through examination of specific organisms and their descent. Convergence of flight in vertebrates was indeed a long process, and we are still uncovering details to help solve its final mysteries.

References

Brush A. 1998. Taking Wing: Archaeopteryx and the Evolution of Bird Flight. The Auk  [Internet]. 115(3): 806-809. Available from: http://www.icr.org/article/321/ Dial K. 2003. Wing-Assisted Incline Running and the Evolution of Flight. Science [Internet]. [2007 Feb 17] 299(5605): pp.402-404. Available from: http://www.sciencemag.org/content/299/5605/402.short

Garner P, Taylor G, Thomas A. 1999. On the origins of birds: the sequence of character acquisition in the evolution of avian flight. Proceedings of the Royal Society [Internet]. [1999 Jun 22] 266. Available from: http://rspb.royalsocietypublishing.org/content/266/1425/1259.full.pdf+html

Jackson B, Segre P. 2008. A fundamental avian wing-stroke provides a new perspective on the evolution of flight. Nature [Internet]. [2009 Jan 23] 451: 985-989. Available from: http://www.nature.com/nature/journal/v451/n7181/abs/nature06517.html

Norberg U. 1990. Zoophysiology: Vertebrate Flight [Internet]. 1st.; pp. 257-266. [cited 2014 Sept 9] Available from: http://link.springer.com/chapter/10.1007/978-3-642-83848-4_13

Rayner J. 1988. The Evolution of Vertebrate Flight. Biological Journal of the Linnaen Society [Internet]. [2008 Jun 28, cited 2014 Sept 9] 34(3): 269-287. Available from: http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.1988.tb01963.x/

Sereno P, Chenggang R. 1992. Early Evolution of Avian Flight and Perching: New Evidence from the Lower Cretaceous of China. Science (14) [Internet]. [cited 2014 Sept 9] Vol(255): 845-848. Available from: http://www.sciencemag.org/content/255/5046/845.short

Seymour K. 1985. An Aerodynamic Model for the Transition From Gliding to Active Flight. The American Naturalist [Internet]. [1994] 126(3): 303-327. Available from: http://www.jstor.org/discover/10.2307/2461357?uid=3739840&uid=2&uid=4&uid=3739256&sid=21104890402767

Xu X, Mackem S. 2013. Tracing the Evolution of Avian Wing Digits. Current Biology [Internet]. [2013 Jun 17] 23(12): 538-544. Available from: http://www.cell.com/current-biology/abstract/S0960-9822(13)00512-5

https://en.wikipedia.org/wiki/Flying_and_gliding_animals


 * There is no information on the models of how powered flight evolved in mammals, simply that they presumably evolved from gliders; more attention should be given to how the transition to powered flight was made. Little mention is made of the selective pressures that led to the development of flight, perhaps more information could be added on the subject. While there is mention of ancestral forms of flight (pterosaurs), the article could benefit from more information on ancestral flight. Wolfer.13 (talk) 21:12, 24 September 2014 (UTC)


 * This "trees-down" model is one of the primary theories for the origins of powered flight, as opposed to the "ground-up" model; its use among gliding animals today is strong evidence in support of this model. [5]

5. ^ Feduccia A. 1996. The Origin and Evolution of Birds [Internet]. 2.Yale University; [1999, cited 2014 Sept 9] Available from: http://books.google.com/books?hl=en&lr=&id=8QRKV7eSqmIC&oi=fnd&pg=PR8&dq=convergent+evolution+of+wings+flight&ots=fqR1hMcCBg&sig=_6JfZas7XmuIkQ9vqudNBVGXQMQ#v=onepage&q=convergent%20evolution%20of%20wings%20flight&f=false

Annotated Bibliography:

'''-How were the analogous structures of wings formed in vertebrates? What selective pressures directed the adaptation of wings for powered flight?'''

Altringham J. 1996. Bats: From Evolution to Conservation [Internet]. 1st. Oxford University Press; [cited 2014 Sept 9] Available from: http://books.google.com/books?hl=en&lr=&id=zuM2Uu-jFucC&oi=fnd&pg=PP2&dq=vertebrate+flight+convergent+evolution&ots=mv5t9dK85e&sig=_s5AfSAD50EKWA_hHx1XHKO3ww4#v=onepage&q=vertebrate%20flight%20convergent%20evolution&f=false

Altringham’s focus is entirely on the development of mammalian flight, specifically bats. From the ancestral roots, Altringham details the advance of powered flight in bats and how selective pressures shaped them to what they are today. The varied niches that bats fill are a direct result of their flight adaptation, and the evolutionary factors leading to their perfection are explained in depth. This is a fine resource for exploring the source of mammalian flight and how it compares to avian flight.

Feduccia A. 1996. The Origin and Evolution of Birds [Internet]. 2.Yale University; [1999, cited 2014 Sept 9] Available from: http://books.google.com/books?hl=en&lr=&id=8QRKV7eSqmIC&oi=fnd&pg=PR8&dq=convergent+evolution+of+wings+flight&ots=fqR1hMcCBg&sig=_6JfZas7XmuIkQ9vqudNBVGXQMQ#v=onepage&q=convergent%20evolution%20of%20wings%20flight&f=false Feduccia covers in depth the evolutionary origins of birds, and how the wing has shaped the role they play in environments the world over. This source is particularly useful in understanding the roots of avian flight, and how birds developed flight over many millennia, from the time of the dinosaurs to now. Feduccia’s work is exhaustive in that respect, but it (expectedly) does not address the other forms of flight or how analogous wings came to be in other species.

Norberg U. 1990. Zoophysiology: Vertebrate Flight [Internet]. 1st.; pp. 257-266. [cited 2014 Sept 9] Available from: http://link.springer.com/chapter/10.1007/978-3-642-83848-4_13 From Norberg comes information on the current theorized mechanisms of early flight in vertebrates; Norberg also gives detailed accounts of why each may have been advantageous to the organisms in question, very useful when examining how natural selection came to favor such a phenomenon. There is only information pertaining to vertebrate flight and the evolutionary processes that shaped the wings of today in vertebrates, but the information presented is exhaustive.

Rayner J. 1988. The Evolution of Vertebrate Flight. Biological Journal of the Linnaen Society [Internet]. [2008 Jun 28, cited 2014 Sept 9] 34(3): pp. 269-287. Available from: http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.1988.tb01963.x/ Rayner examines the relationship between the different forms of vertebrate flight, and how the roots of flight can be traced. The article explains that there is much controversy over the true origin of the adaptation, but that its independent formation in different organisms shows its evolutionary worth. This information is very useful in showing how the adaptations are rooted and allow for similar benefits, but ultimately are derived independently and as such are unique.

Sereno P, Chenggang R. 1992. Early Evolution of Avian Flight and Perching: New Evidence from the Lower Cretaceous of China. Science (14) [Internet]. [cited 2014 Sept 9] Vol(255): pp. 845-848. Available from: http://www.sciencemag.org/content/255/5046/845.short The research of Sereno et al. provides insight into the ancestral form of wings, and how flight was shaped through the selection of specific traits in such things as body size, wing length, and bone structure. The background given in this article is important for understanding how flight and wings in particular were shaped through successive generations and in a number of different species. This article provides the groundwork and specific examples necessary to begin understanding the ancestral formation of winged flight.