Tuesday, April 13, 2010

Evolutionary Pathways

In the section of Why Evolution is True titled "Can Selection Build Complexity?", Coyne delineates the many arguments in which ID proponents suggest that there is no evolutionary pathway that can construct certain traits, protein complexes, etc., and how there is often an answer if you think long enough. One of the most complex systems we have learned about so far is the immune system. What pathways could lead to having so many different antigen complement proteins and antibodies, able to be selected and proliferated as needed? What about the process of presenting an antigen on the cell surface? Perforin and the process of lysing antigen-presenting cells?


  1. While the immune system does seem like a complex and unevolvable system of reactions, some of the different parts of the immune system become much easier to envision the evolution of. The first, and one of the most important defenses of animals are the coverings of the animals. The evolution of these is easy to theorize. As animals became bigger and prey/predator relationships became more and more defined, natural selection probably chose for fish to become more scaly as to give them more protection. As animals moved onto land, different outer coverings, like the chitin exoskeleton in insects would have prevented the animal from drying out. At the same time, both these outer coverings protected the animal from many different infections. Our skin probably evolved to be more elastic than our reptilian ancestors and the mucus of vertebrates doesn't just keep pathogens from getting inside our body. It keeps our insides moist, providing a perfect environment for diffusion of materials across membranes.
    The acquired immune system is much more complicated, but can also be explained. Immunoglobulins (antibodies) , as explained by Evolving Immunity, could have developed by T Cell Receptors that became free floating. This was a selective advantage and natural selection could have chosen to turn some T Cells into cells that produce these simpler antibodies. The complement system has been theorized to have evolved independently from antibodies, as the alternative complement system doesn't even need, according to http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Complement.html. The complement system proteins also do not lead to the same results. Some proteins lead to inflammation. Some lead to the creation of the membrane attack complex. A slight alteration of these different pathways could have made them accessible from one starting reaction and more therefore more efficient. I think it becomes clear when we break apart the huge immune system as to how it could have evolved gradually, with only little changes at a time.

  2. While Eric did a splendid job explaining the physiological reasons for the possibility of the evolution of the immune system, I think that he primarily lacked the molecular selective pressures that pushed for the evolution of the immune system. The first of which you point out is the ability for the body to create so many antigen complement proteins and antibodies. Part of this evolution can be looked at from a gene expression perspective. For instance, the ability of alternative splicing has allowed for genes to be produced from the same segments of DNA to be used in various ways and have eventually lost purpose here and there, such as the Olfactory Receptor genes that Coyne discusses in his book (Coyne). Similarly, the Biology book describes the formation of epitope formation as almost a “random” process with certain regions cut out and others spliced to create a heave chain that is able to act as an antigenic receptor or antibody (Campbell, Reece). One of the selective pressures that maintained this variation that we see in various biochemical processes lies in the ability for pathogens to undergo antigenic variance, the changes in their antigens to evolve and become more resistant to natural and artificial immune responses (Campbell, Reece). So assuming a “null hypothesis” that pathogens could never antigenically vary, then the gene sequence that would produce the most effective immune response would become the primary, and possibly only, gene sequence that made antigenic receptors because that’s all that would ever be needed. However, this is not the case. The reality is that pathogens constantly evolve to the point that it is more advantageous for this “randomness” to continue, which provides some insight as to why there are so many types of antigenic receptors in our immune system.
    One journal article that described the evolution of the antigen presentation system pointed out that the body has always had a constant selective pressure to be able to differentiate between self and non-self (Groenenboom et. al). Thus, when a macrophage undergoes endocytosis or phagocytosis and is able to present an antigen, an inclination to think that this is a way of distinguishing between self and non-self is evident. The underlying reason is that although a the macrophage or other antigen-presenting cell will die because of the antigenic presentation, it will, through T cells, cytokines, and the like, send a signal throughout the body to focus on a given antigen. On the other hand, if the ability to present antigens on the surface was not an ability, then the macrophage may have had the ability to send the signal out to the body that it is infected, but both types of second line immune responses would lack the information to track down the pathogenic cells and would probably go on a rampage killing all cells in its way—self and non-self. On a side note, I’d like to point out why this also may contribute to the evolution-is-true side of the debate on evolution because while ID proponants would say this is a perfect design to protect the body, it wouldn’t be true. In fact, the fact that CD4 and CD8 are surface proteins that act as stabilizers for T Cells to activate indicates that evolutionary pressures had caused this protein to develop to make this response more effective for the overall immune system. Similarly, peforin and lysing may have been adaptations that also helped make the system as a whole more effective by actually equipping T Cells with “ammunition” to fight the cells they recognize as bad.

    Why Evolution is True