On page 46 of the book, Coyne describes two different theories for how birds might have evolved from reptiles. One theory is the "trees down" theory, and the other is the "ground up" theory.
Which theory seems more likely to you (ignore Coyne's preference)? Provide an explanation for why you chose that particular theory and back it up with information from the book, the textbook, and/or outside sources. Don't forget to tie in a theme.
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I believe that the tree down scenario would most likely be the cause for the evolution of birds. As Jerry Coyne wrote that “There is evidence that some theropods lived at least partly in trees. Feathery forelimbs would help these reptiles glide from tree to tree or from tree to ground, which would help them escape predators, find food more readily, or cushion their fall” (46). With an existence already in the trees, it wouldn’t be difficult to generate arboreal traits that benefit the organism through the evasion of predators and the acquisition of food.
ReplyDeleteI believe that Theropods initially used trees as a safe guard from predators. Utilizing jumps, they would be able to catch airborne prey such as numerous insects. Evolution would favor those individuals who could maintain air time and control direction. Extension of skin flaps would increase air time and maneuverability. There is evidence today of animals gliding across trees either to escape predators. There are the flying squirrels, paradise tree snakes, and Draco lizards. The hollowing of bones would also occur to decrease the overall weight of the organism and would allow for limited flight. An example would be a bat. Bats have large extensions of skin between the “fingers” that can be flapped to maintain altitude do to the bats relatively light weight. The development of feathers may have originated from fur. In time fur might have evolved into feathers to increase the surface area of the organism and decrease decent of the organism. The end result might have been the precursor to the modern bird containing feathers, hollow bones and large broad wings for flight.
This progression ties into the concept of evolution because of the gradual change in the organism. Evolution would favor the organism that would be lightest and most maneuverable in the air. Those organisms would acquire more food and would reproduce passing those traits onto the next generation. Then that generation would develop their own beneficial traits and pass them to the next generation. The process would continue indefinitely until the species gets wiped out. In the instance of the development of flight, the evolutionary traits would be the elongation of skin flaps, mass reduction from the hollowing of bone, and the development of feathers.
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ReplyDeleteI have to respectfully disagree with Andrew. Sure, logic would have it that flight originated through the tree down model. That would be the easiest, simply evolving from gliding to flying. Despite this, as we have seen many times, evolution is not perfect; Most of the evidence points to the ground up model.
ReplyDeleteThe problem with this theory is that neither Archaeopteryx nor theropods had the ability to climb trees and fly. Some scientists cite microraptors as arboreal theropods, but there are far more examples of theropods without this ability. (http://www.geologyrocks.co.uk/tutorials/origin_and_early_evolution_birds)
I agree with Coyne, not just because he wrote the book though, when he asserts on page 46 that the ground up theory is more likely. The commonly-ridiculed "half-wings" were not useless. The stepping stones to wings would be helpful in assisting dinosaurs' running as well as leaping to catch prey. This would apply to the structure relating to function theme. If the theropods used wing-like structures to improve hunting efficiency, the next reasonable step would be fully developed wings to allow flight. True wings would definitely increase the predators' (dinosaurs)ability to hunt down prey. Predator-prey interactions help drive evolution, and the origin of flight is another example.
I too disagree with Andrew L, the theory that seems more likely to me is the ground up theory. If you look at the skeletal structures of modern flightless birds such as emus, ostriches, or even chickens, you will find that their skeletal structures are remarkably similar to those of theropods like the Velociraptor. As I mentioned in a more recent post, birds like the emu have had genes turned off or repressed over time that weren't of great importance to the species. The genes that are "turned off" would have given emus and other birds longer tails, teeth, and other features similar to theropods. This is proven in an experiment on chicken embryos lead by Jack Horner. One feature of modern flightless birds that has changed very little over time is their legs. The legs of emus and ostriches are long, muscular, scaly, and the feet have three toes that leave footprints similar to those of fossil dinosaur footprints. These legs are built for speed. According to Emu Facts “They are flightless and strong runners, reaching ground speeds of up to 40 miles per hour in short bursts and covering about nine feet in stride” (http://www.emufacts.com/the-emu-bird.shtm). In order to increase speed further Coyne suggests that “Longer wings could also have evolved as running aids” (46). Perhaps, over time, birds that were lighter in weight were able to acquire longer air time. Then these animals would develop better suited wing structures. I believe that bird feathers weren't originally developed for use in flight. Instead I think that feathers were developed for thermoregulation. The feathers initially created a lot of drag, so the birds that were more aerodynamic had a selective advantage. Over time, in the struggle to become more aerodynamic, wings evolved. After wings were developed, the birds evolved even further into the thousands of species we see today.
ReplyDeleteThe theme that this topic most relates to is Structure and Function. The structure of the hollow bones and longer wings and feathers of birds ensure successful flight. The large surface and and low mass (light as a feather) of feathers generate relatively large amounts of lift, which allow birds to fly.