We discussed the idea of organisms "using old parts in new ways" in class a few times, but I think the discovery of ZPA seems to contribute to the idea very nicely. At first, I thought the idea of using old parts in new ways seemed oversimplistic because there didn't seem to be enough "old parts" to use. However, ZPA was revealed to "[make] a molecule that then spread across the limb to instruct cells to make different fingers."(Shubin, 50). It is amazing how one tissue can have so much impact on the development of something as vital as fingers. What is more amazing is how similar structures as this one exist in both flies and chickens, and changes as small as an increase in vitamin A can cause enormous impacts as great as fingers being duplicated. The existence of ZPA seems to provide a basis of how animal diversity has occurred, or how the "old parts" came to be, but it does not give light to what "new ways" may exist due to the slight changes that may be not so slight.
Consider the theme of Evolution. Do random changes in the ZPA and similar genes in numerous organisms show that small changes over the course of thousands of years can create organisms as different as flies, chickens, humans, and even Tiktaalik? Also, do the differences in the common plan for animal limbs, "one bone, followed by two bones, then little blobs, then fingers or toes" (Shubin, 31), seem all that big because they may have come about as corruptions of the exact same structure? Finally, does genetics seem to have all the influence, or does the incidence of selective pressure have an even bigger role as to why some organizations of the limbs generated by ZPA exist as they do now?
Troy Glickstern
(cleverstar8@comcast.net)
Troy, you bring up an interesting question as to whether or not it is genes that control our development and evolution or selective pressure, and to answer it, I say both. On a large-scale look of all the organisms on Earth, it is clear that humans differ from snails due to our genetic codes being vastly different from each other. However, in the case of Darwin’s finches, “the ecological niches exert the selection pressures that push the populations in various directions. On various islands, finch species have become adapted for different diets: seeds, insects, flowers, the blood of seabirds, and leaves.
ReplyDeleteThe ancestral finch was a ground-dwelling, seed-eating finch” (http://www.pbs.org/wgbh/evolution/library/01/6/l_016_02.html). So if external, environmental factors can give rise to new species, it is no surprise that internal changes during the course of development can, too. To use the example of ZPA, “recall that when you treat the limb with retinoic acid, a form of vitamin A, you get a ZPA active of the opposite side” (Shubin, 53). Now, though we know having a two thumbs or a paddle for hand (two possible effects of a non-advantageous change in ZPA use) won’t give rise to a new species of human, enough of these mutations in the body in the right place at the right time most certainly could. Furthermore, if society chose to shun paddle-handed people and refused to mate with them, causing them only to be able to mate with each other, this external situation could possibly speed up the time it takes to create a new species that is un-mate-able with Homo sapiens because the amount of gene diversity present in the paddle-hand population would be significantly lessened.
Going back to Troy’s original question and title, I do not think the jump from wings to feathers really was that big of a deal. This is because both these limbs have the same structure "one bone, followed by two bones, then little blobs, then fingers or toes" (Shubin, 31) and more or less serve the same function—as extension of the organism’s body for transportation and other purposes. The idea of old parts in new ways becomes even more interesting the further one goes on an evolution time line. I found the example of alligators’ jaws and human inner ears Shubin discusses on pg. 164 to be a particularly interesting example because though the structure of the bones in terms of shape are more or less the same, their function is so different in setting the two organisms apart! Additionally, as I discussed in a different blog post, the microbes choanoflagellates possess “78 pieces of proteins, many of which in animals are involved in making cells adhere to one another” yet in these unicellular organisms “the molecules [maybe] are used in capturing prey” http://www.nytimes.com/2010/12/14/science/14creatures.html?_r=3. These small creatures give a perfect example as to evolution uses old parts in new ways.
-Jackie Edelson, jedelson92@gmail.com