Monday, March 29, 2010

Evolution of Eye

Señor Coyne explains that the bacterial flagellum, "is composed of dozens of separate proteins, all of which must work in concert for the hairlike 'propellor' to move" (137). The link below is a diagram of what it looks like. This is an apparatus that demonstrates such complexity that creationists use it as a "poster-flagellum" to demonstrate the case that evolution can't produce all things. Yet evolutionists have shown that this development is "feasible". (138) Research the leading theory on how this motor evolved to the complex structure it is today, and report your findings here. Also, can you think of any other structures that ID proponents would be eager to use as "proof" of intelligent design? Examples from the book included: the human brain, eyeball, and blood-clotting chain.


  1. To begin, we must establish the concept of "irreducible complexity". What is this you may ask? First proposed as a pseudoscience by Michael Behe, an intelligent design proponent, it is the notion that certain traits that are extremely complex in nature, much like the bacterial flagellum, involve "many parts that must cooperate for that trait to function at all" (Coyne 137), and thus "defy Darwinian explanation" (Coyne 137). As stated by Behe himself, when "the number of required parts increases, the difficulty of gradually putting the system together skyrockets" (Behe) and evolution over time seems less likely. All I have to say is that we're AP Bio students! We know better than that! But before we discuss the implications of evolution that prove Behe's claim erroneous, we must explore the relationship between structure and function in the bacterial flagellum itself.

    The bacterial flagellum, discussed in the first semester of our course during the cellular unit, specifically prokaryotes, is composed of many, many proteins. About 50 individual units! If you really feel like reading through all of the intricate relationships between each protein unit, go ahead and read the Wikipedia article in this link: but I don't think that it is absolutely necessary to get into the real nitty-gritty to understand the big idea: The flagellum is built just like a motor that humans designed, and all parts work in concert to make one big machine that does one job to move the bacterium around. Irreducible complexity implies that you can take this thing apart and nothing works, but evolution means that individual parts can still work together to do some work even though the entire machine is not fully assembled. Intelligent design is based on assumption of the first idea and no hard evidence, which is kind of obvious, but evolution does have evidence of its existence. That is, in the form of the type-III secretory system also used in bacteria. As stated in a Ken Miller presentation debunking irreducible complexity, the system "is a molecular syringe, in which some of the nastiest bacteria on this planet produce toxic proteins, grab onto one of our cells, and inject those proteins into our cells" (Miller), and is specifically seen in the bacteria that cause the bubonic plague. The cool thing is that the type-III secretory system is only made of 10 protein parts, much less complex than the bacterial flagellum, yet still uses all the same proteins in the same concert as in the flagellum, just for a different function! That is the evolutionary background and the proof debunking the pseudoscience of irreducible complexity, as it proves you CAN take a supposed "irreducibly complex" biological machine apart and parts of it still work, albeit for a different thing.

  2. To go a bit further into other examples of extremely complex systems, I actually talked with my veterinarian mom, who told me about the cilia in the lungs. Lung cilia are hairlike projections which move in concert, pushing debris out of the airways or into the digestive tract to avoid going into the respiratory organs. Intelligent design proponents argue that this reaction is too smart, too coordinated to have evolved in the "traditional sense", yet it is shown to have basis in peristalsis of smooth muscles. The evolutionary background in peristalsis is theorized to have morphed into these hairlike projections also moving together to perform the task of clearing debris, and proves that the intelligent design theory is not so intelligent after all. The implication is that if something has a precursor form, it is evolved, much in the case of cilia arising from peristalsis, and earlier, the bacterial flagellum from the type-III secretory system. After all, Darwin said himself that "ignorance more frequently begets confidence than does knowledge: it is those who know little, and not those who know much, who so positively assert that this or that problem will never be solved by science" (Coyne 140). Intelligent design is oft not thought through enough, made on hasty generalizations, and its biggest pieces of "evidence" (quotations used sarcastically in this case) proven wrong by the existence of precursor forms as shown in the examples above.

    Page 137 of the book
    Page 140 of the book

  3. The first issue at hand, as the above response mentioned, is understanding the criteria necessary to show that evolution is, indeed, able to produce complex systems in the context of natural selection. That is, (1) it needs to be shown that it is possible for the complex systems to have evolved, (2) there must have been enough time for this system to have come about, given that complex systems must have required numerous steps, and (3) each of these steps needed to be promoted by natural selection, and each “part” advantageous to the individual at that point in evolutionary time. Coyne states it more eloquently, that “there must be evolutionary precursors of each new trait, and that evolution of that trait does not violate the Darwinian requirement that each step in building an adaptation benefits its possessor” (138). The example that the response above gives is the type III secretory system in bacteria that is a part of the complex bacterial flagellum, yet has its own specific function, independent of that of the bacterial flagellum, meaning that it could have been a precursor that passed on as a result of natural selection, and one structure that became more complex throughout time to eventually become the bacterial flagellum. This example demonstrates the most crucial element towards proving the theory of evolution, that given evolutionary time has been so great, complex systems are quite possible to have been assembled, and these complex systems may be traced back to increasingly lesser complex systems that were able to stand alone to benefit the individual (or else they would not have been selected by natural selection). This is seen at all points when giving examples for natural selection driven evolution; it is a key element that natural selection allows for the most suitable genes for reproduction and survival to be passed onto the next generation.

  4. (cont'd)
    The main example that Coyne gives in the book is a proposed evolutionary timeline of the development of the eye, one of the more complex structures that intelligent design proponents believe could not possibly have been a product of evolution. Our textbook states that it is quite possible that these partial/degenerate versions of eyes were in fact useful to the organisms that possessed them, because it was [wrongly] assumed that “only complicated eyes are useful” (529), and that “'simple' eyes are quite adequate to support their survival and reproduction” (529). Both the textbook and the book go through the proposed pathways through which the eye may have evolved, the key being that as the eye did evolve into the complex structure it has today, as seen in humans and even squids and octopuses, it enabled “the eye to gather more light or form better images, both of which aid survival and reproduction” (Coyne 142). The eye may have began as a patch of pigmented cells, or photoreceptors, which were simply sensitive to crude intensity of light, and possibly direction, which might have been the only adaptation necessary for, for instance, limpets, who generally lead a sedentary lifestyle; they mainly rely on chemical signals for movement and feeding, including pheromones to guide their movement. The only time they would require these rudimentary photoreceptors is in case of an exposed shoreline, during which their photoreceptors would allow them to recognize increased intensity of light, and therefore clamp down on the rocks they inhabit to minimize water loss and prevent desiccation (Wikipedia). I do not see the need to explore each and every proposed step found in either the textbook or the book, as it will probably be seen to have this same kind of relationship so easily found yet so crucial to understanding evolution by natural selection. As the eye developed into the structure we use today to visually process our surrounding environment, it evolved as a tool, an adaptation that gave clearer images, more focused light, all crucial to more active lifestyles. What is more striking is the fact that squid and octopus eyes evolved independently of that of the human, yet were just as complex, and very similar; this in itself suggests that this mechanism for evolution of complex structures is not only possible, but it definitely has had enough time to manifest itself throughout evolutionary history.