From page 39 to 47, Jerry Coyne talks about the evolution of birds from animals that live on land to animals that slowly adapted to the birds we know today. The evolution of birds shows the slow process of evolution at work to create a new species of organisms.
Question: How do the multiple "transition animals" from land animal to flightless bird show the adaptive process of evolution? What other adaptions besides wings do the birds evolve over time and how were they adpative? Provide examples of transition animals that exist today.
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The species Compsognathus was a theropod dinosaur, which later evolved into Archaeopteryx. Archaeopteryx is more reptilian; however, it shows the first adaptions due to natural selection that led to modern birds, such as the chicken. Archaeopteryx is a transitional animal between reptilian dinosaurs to birds. Theropods evolved into birds and shared many characteristics or similar ones that lead to the evolution of the bird. Besides wings, other characteristics of birds evolved too. These include, large feathers and opposable big toes used for perching. Asymmetrical feathers, like airplane wings, create the "airfoil" shape necessary for aerodynamic flight. As the fossils get younger, scientists observe reptilian tails shrinking, teeth disappearing, claws fusing together, and the appearance of large breastbones, which show the evolving characteristics of reptiles to birds. Tails were unnecessary because they got in the way and made the animal more prone to injury. Large breastbones help the animal anchor its flight muscles. "Many characteristics that typify birds were present in the theropods before birds evolved, including hollow bones, a wishbone, a backward-pointing pelvis, and a three-toed foot. In the course of theropod evolution, the forelimbs and hands became progressively longer. In some theropods, the bones of the wrist took on a shape that allowed the joint to flex sideways. This would have allowed these animals to whip their long hands forward in a swift snatching motion, perhaps to catch prey. The wishbone in theropods served to anchor the muscles that pulled the forelimb forward in this grabbing movement -- a motion that functional analysis shows to be almost identical to the flight stroke of modern birds." (http://www.pbs.org/wgbh/evolution/library/03/4/l_034_01.html) A few examples of different transitional forms between theropods and birds include, Ornithomimosaurs, Microraptor, Yandangornis, and Jixiangornis. "Ornithomimosaurshad a body covering of tufted feathers and had feathers with a central rachis on its wings and tail. Microrapto, the most primitive dromaeosaur, is also the most birdlike; specimens have been found with undisputed feathers on their wings, legs, and tail. Yandangornis, and Jixiangorniswere slightly more advanced than Archaeopteryx, especially in characters of the vertebrae, sternum, and wing bones." (http://www.talkorigins.org/indexcc/CC/CC214.html) These species, along with other animals, help emphasize the evolution and adaption of birds as they adapted the ability to fly and their other characteristics changed as well.
ReplyDeleteNice job, Lexi! However, I feel like some things weren't mentioned about the origin of birds, and I would like to add them in addition to what was already said.
ReplyDeleteIt is important to note that "essential feathers evolved before birds could fly" (46). While the selective advantage to this is not known for certain, it has been hypothesized that such feathers before flying were used as display to attract mates. Another hypothesis is that feathers were used for insulation; theropods, unlike modern reptiles, may have been partially warm blooded, so the feathers would aid in thermoregulation and homeostasis.
There are several more unknowns as to how natural selection fashioned modern birds. On page 46 Coyne says that early carnivorous dinosaurs evolved long forelimbs and hands in order to grab and handle prey more efficiently. This kind of grabbing would promote the evolution of muscles "that would quickly extend the front legs and pull them inward" (46). This is the exact motion used for the downward motion during the stroke of true flight. Given this, there are at least two ways that flight could then have evolved: the "trees down" scenario and the "ground up" theory.
The "trees down" theory uses the belief that many theropods lived in trees. The earliest birds began flying by gliding from tree to tree which could help the bird escape from predators and find food more easily (http://news.softpedia.com/news/First-Birds-Had-Four-Wings-36233.shtml).
On the other hand, the "ground up" theory sees flight evolving as an outgrowth of open-armed runs and leaps that could have been useful in catching prey. Longer wings would then have evolved as running aids like in the modern day chukar partridge (46). The partridges hardly ever fly; instead they flap their wings to help them run uphill. "The flapping gives them not only extra propulsion, but also more traction against the ground. Newborn chicks can run up 45-degree slopes, and adults can ascend 105-degree slopes" (46).
Following flying came other innovations to improve flight such as hollow bones for lightness and large breastbones.