Saturday, April 11, 2009
Can evolution expain all traits?
Carroll frequently uses the concepts of "compound interest" and Evo Devo to prove evolution developed all traits we see in nature. However some traits like the flagellum have generated waves of controversy among the scientific community. The flagellum is an irreducibly complex structure such that if any components of the structure were to not have developed the flagellum would not function. However, Darwinian evolution acts on only working traits because traits evolve only in response to previous selection pressures. How would you resolve the issue of irreducibly complex structures using the knowledge gained from Carroll's work?
Subscribe to:
Post Comments (Atom)
This is a very good question Vikram Baruah, my good friend, and it hits at the supposed "weak point" of the theory of evolution (according to intelligent design supporters). Supposedly, it seems that evolution cannot account for irreducibly complex structures due to their non-functioning nature as parts, but this is a fallacy because evolutionary theory can in fact explain how these structures developed. I will focus in on the flagellum for this explanation.
ReplyDeleteFirstly, the creation of a gene that codes for a flagellum would take billions of years to develop due to its complicated nature. Throughout these billions of years, organisms that expressed these proto-flagellum genes were competing against organisms of the same species that did not. At first glance, it might seem like a selective disadvantage to express a gene that codes for only a portion of a working flagellum, since flagellum are irreducibly complex, but if you think about it, the selective pressure against it is very small. Since it is a non-functioning flagellum, it will not consume much, if any, ATP and therefore be an energy drain on the organism (a definite selective disadvantage). The only detrimental effect of expressing a proto-flagellum gene is that it sequesters a very small amount of alpha and beta tubulin into a non-working structure (the proto-flagellum). However, this detrimental effect is almost negligible since it is relatively easy for an organism to obtain more amino acids, so sequestration of proteins in nonessential structures does not have such an effect on an organism to the point that it will not be able to survive or reproduce because of it. Maybe there is a beneficial use for a proto-flagellum that none of us can easily recognize. Maybe it worked as a way to block a small section of the plasma membrane from bacteriophages so there was less of a likelihood that a bacteria expressing its proto-flagellum genes would be infected. It is quite possible that the selective advantage of having protection of a proto-flagellum outweighed the selective disadvantage or sequestration of amino acids in a proto-flagellum. This seems to be the case because non-functioning flagellum were kept in the population of early bacteria. As the proto-flagellum genes were passed on to offspring and new mutations arised, the proto-flagellum gene came to become closer to the working flagellum gene. Eventually, once a working flagellum was finally realized by a very lucky bacteria, the selective advantage of having a flagellum far outweighed the selective disadvantage of sequestration, and the bacteria with working flagellum were more "fit" and were therefore better able to survive and reproduce due to their new found mobility. This seems to me to be the most probably explanation for the origin of flagellum.
Though Goone puts forth a good argument, I think that complicated structures came about it a rather different manner. Rather than slowly building up genes of a nonfunctional flagellum into a fully functional one, I think that the first precursors of a flagellum were indeed functional, but they did not at all resemble the flagella we would recognize today. I think it is more likely that flagella were born out of another organelle, perhaps cilia. Let's say for example that a gene for cilia in a bacterial cell has become mutated, and as a result the productant cilia is longer than normal. Well, this would provide an obvious advantage from a physics standpoint. Having one longer cilia, this organism would be able to more efficiently move itself. Thus, this trait would provide a selective advantage and become prominent in the population. From this initial mutation, further mutations that lengthened and strengthed this particular cilia would be favorable, and over time, the contemporary flagellum would be born.
ReplyDeleteAnother prime example can be found in the human nervous system, specifically the brain, arguably the most complex structure ever developed. It is highly unlikely that parts of sensory systems developed to be non-functional before accumulating to make an entire working system. Rather, it is more likely that these complex systems developed from the more basic ones. Let's say for example that we have a cephalized organism with little more nervous tissue than basic sensory receptors. Rather than having neurons in the brain independently start forming their own systems, say for example a cardiovasculatory regulator system, it is more likely that these neurons slowly changed to perform different duties. There is evidence of this in the complex linkages of different systems across one another. Though the various segments of the brain are roughly distinguishable from one another in terms of location, all the systems are crossed and intermingle with one another, a result of each's outgrowth from a simple singular precursor.
Now, although we can show and reason out how complicated systems came from simple ones, the hole in the theory lies in how these simple structures came to be. How did the first cilia form, or the first neuron? It is this question of the origin of entirely new traits that promotes of intelligent design point out and use to poke holes in evolution.
Evolution doesn’t make traits, it only selects for traits. The creation of traits is solely depended upon the luck of the draw in the mutation lottery. There will always exist variations for one gene in any population of organism. The fate of these variations is depended upon the selection pressure of the environment. The variation of the gene that gives the organism a selective advantage will persist in the gene pool and be passed down to later generations. Although variations only cause slight changes in an organism, Carroll explains that “slight differences accumulated during many successive generations” (Carroll 66) will have a profound impact on the organism.
ReplyDeleteIn Chapter 4, Carroll also discusses how organisms often make the new from the old. For example, the enzyme critical to the ruminating in colobus monkeys is actually a modification of the ribonuclease found in non-ruminating monkeys. While non-ruminating monkeys only have one ribonuclease, the colobus have three different versions of this enzyme, and one of the enzymes is identical to the one found in non-ruminating monkeys. This provides strong evidence for that the gene for the ribonuclease was duplicated during the course of evolution and then modified so the new ribonuclease could “harvest the large amounts of nitrogen in the RNA of the fermenting bacteria” (Carroll 113). This mechanism of duplication and modification is also responsible for the third opsin that provided primates with a trichromatic color vision.
Eric mentioned that there seems to be “holes” in the theory of evolution: How did a simple structure like a neuron or cilia form? In my opinion, I believe that these holes occur because evolution jumps from one stage to the next. Contrary to what Aaron said, I don’t think that major evolutionary changes occurred in gradual steps. For example, molecules can not progress from being from non-life to life. Although its possible that protobionts is the precursor to the cell, protobionts would have to cross the border between life and non-life in a single step. A threshold barrier exists between life and non-life and a protobiont need one big mutation to push it over this threshold. No middle ground exists between life and no life. You can only be a cell or a protobiont, you can never be both.