Thursday, March 31, 2011
Germ Layers
On page 98-99, Shubin talks about how chickens, frogs, and fish limbs all look very similar at the start of development. Shubin also states that three germ layers are all the same in these animals. Yet later in development, trillions of cells form very different animals. Describe how this can be used against the creationist argument and describe the similarities and differences of the general process of development using the three germ layers in frogs, chicks, and humans.
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As according to "The Five Very Best Reasons for Creationism" as according to The Telegraph in September of 2009 the first reason as to why evolution is wrong is simply that "There is no evidence that evolution has occurred because no transitional forms exist in fossils i.e. scientists cannot prove with fossils that fish evolved into amphibians or that amphibians evolved into reptiles, or that reptiles evolved into birds and mammals" (http://www.telegraph.co.uk/culture/film/film-news/6163313/Creation-film-sparks-evolution-arguments.html). Clearly, this is a very outlandish statement from a fairly respectable newspaper (in the UK), seeing as Shubin's book was copyrighted in 2008, after already published journal articles announcing the discovery of Tiktaalik the fossil that bridges the gap between reptile and fish. As Shubin describes on page 98 chickens, frogs, and fish “all look generally alike” in development as embryos. This, too, helps disprove one of creationism’s arguments for validity.
ReplyDeleteSome of the similarities between development and the three germ layers between frogs, chicks, and humans is that all animals, regardless of phyla have this same body plan resulting from the endoderm, ectoderm, and mesoderm. As a brief recap from an earlier unit we learned, an endoderm regardless of the animal will form parts such as the digestive tract and numerous glands, the ectoderm forms the outer part of the body and the nervous system, and the mesoderm forms the muscles, tissue, and a lot of the skeleton. The differences in development between these three organisms, and any others for that matter, occur much later in development when cells begin to differentiate and take on characteristics unique to its genome. For example, though the mesoderm forms all sorts of limbs, in fish it forms fins, in chicks wings, and humans arms.
Adding to Ben’s question, how do other things such as age of the universe and fossils help disprove creationism?
(Jackie Edelson, jedelson92@hotmail.com)
It’s such an interesting point that Jackie brought up about the UK Telegraph article, because the situation of staunch creationists that are outspoken about their disbelief in evolution has become such a media-hyped and mainstream issue in society today. After reading her response, I was strongly reminded of the controversy surrounding former Republican Senate candidate Christine O’Donnell for her similarly enthusiastic support of creationist theory (Source 1); she once said in an interview that she wanted creationism to be taught because she believed “that the world began as the Bible in Genesis says, that God created the Earth in six days, six 24-hour periods. And there is just as much, if not more, evidence supporting that” (March 30, 1996, CNN). Even though that politician had an infamous reputation for erroneously referencing constitutional policies on teaching creationism in schools (it is illegal), I feel the fact that high-profile society figures and publications outwardly deny proof of evolutionism in fossil history when there IS proof can definitely detract from the credibility of their position. Jackie is correct in pointing out that research on Tiktaalik and the similiarity of frog/chicken/fish embryos contributes to evidence of evolution of these organisms from a primitive ancestor over natural selection due to environmental changes and preferences, but Neil Shubin also notes that “it is more difficult to find our basic design in more primitive animals – jellyfish, for example” (Shubin 97). Since most mammals share that “basic design” of major cephalization, nerve development in the head region, bilateral symmetry in addition to the triploblastic tissue structure with ectoderm, mesoderm and endoderm that Jackie explained, it can be deduced that there exists enough genetic variation between organisms sharing a same general body plan that they could have evolved gradually over natural selection from a smaller group of related ancestral organisms (such as a Tiktaalik) to better suit the selective advantages pertaining to their environments (Campbell 659). In addition, bilateral symmetry also promotes the deuterosomic development (where cleavage of cells will cause the embryo to develop anus to mouth along a lateral line running across the blastospore) and protostomic development (cleavage in the blastospore that causes mouth to anus development) in such embryos (Campbell 660).
ReplyDelete(continued) In response to Jackie’s question , fossils are often used to contradict evolutionism because skeptics will argue that the strictly separated fossil layers prove a counterpoint to traditional evolution theories, but Shubin mentions very early on in his book that the original layers of fossils in a richly preserved area like the Grand Canyon may have shifted over time due to environmental disturbances and shifts, but that the order can be pieced to together to follow a predictable pattern of “everything with heads/limbs/hair/breasts that walks on two legs” on the youngest rock layers, “everything with heads/limbs/hair/breasts” on the secondary level, “everything with heads/limbs” on the third level, and “everything with heads” on the most primitive layer (Shubin 9). Since you will never find two organisms of significant evolutionary difference close together in the fossilization layers, those fossil records are a fairly predictable and reliable indicator of the gradual progression of genetic variation and physical development of animals over time. Also, he states that the research of fossils like Tiktaalik support the theory that fish were the first primitive marine organisms to advance to land habitats, which was a major evolutionary and environmental change that inevitably led to the genetic variation we see between marine and terrestrial organisms today (Shubin 5-6). Thus, the combination of Earth’s history as evidenced by known fossil records and organization and age of the universe do support the themes of gradual change over natural selection and differentiation between increasingly complex organisms and adaptations in evolution.
ReplyDeleteSources:
1. http://scienceblogs.com/dispatches/2010/10/odonnell_and_creationism_--_ag.php
2. Neil Shubin
3. Campbell, Chapter 32
Christine Lin
choco_cat11@comcast.net