Wednesday, March 31, 2010
Dead Genes or Hibernating Genes?
Jerry Coyne states on page 66 that "Atavisms and vestigial traits show us that when a trait is no longer used, or becomes reduced, the genes that make it don't instantly disappear...evolution stops their action by inactivating them, not snipping them out of the DNA". If this is true, it sparks the debate between nature and nurture. If genes are really just inactivated, is it possible for them to be reactivated provided that the species are placed in a replication of the environment where those genes would be necessary to survive? How does this connect to what we have learned about genes and the evolutionary advantage of adaptation?
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ReplyDeleteCoyne specifically mentioned that the genes of atavisms were “silenced by natural selection when they were no longer needed” (2009, p. 65). Therefore, the reason why the genes of atavisms are no longer expressed is because the survival rate of individuals that did not express the genes far exceeded the survival rate of individuals that did express the genes, and the individuals that did express the genes eventually died out. Generally, the mutations that allow a species to adapt occur by chance, hence the adaptations (including meiosis, conjugation, transformation, and transduction) that facilitate mutation.
ReplyDeleteIt is always possible for a dormant gene to be activated once again. Coyne points out that whales are capable of expressing “some genetic information for making legs” (2009, p. 65), genes that were passed down from the ancestors of the whales. However, since it was natural selection that deactivated the gene, and not individuals responding to an external stimulus (i.e. change in environment), I believe that placing individuals of a species in a replication of the environment where the genes would be necessary would not cause the said individuals to express the gene. However, the offspring of those individuals that, by chance, do express the genes will proliferate expression of the gene until, once again, expression of the gene becomes commonplace among the whales.
I agree with Yiding. The traits that were once expressed in ancestral species are no longer needed, due to some environmental change or other factor over time, but since they are still there, they can of course be expressed over yet more time. His reasoning that "genes of atavisms are no longer expressed is because the survival rate of individuals that did not express the genes far exceeded the survival rate of individuals that did express the genes" is completely accurate. Structure and function are intimately related in evolution - if a structure's function is no longer beneficial to the organism, such as the atavistic tail of humans or the genetic coding for legs in whales (p.62-65), then the organisms that do not possess that structure or possess a smaller, less detrimental version of that structure are better suited to survive.
ReplyDeleteThese genes could not, then, be reactivated over immediately given a replicated environment - instead, the individuals that are best suited to the new environment would survive. A whale-like scenario, where the organism develops from water to land and back to the water, returning to a similar shape with anatomical differences, is about a chance of one in who-knows-how-many. The re-placed organism may die out in the new environment or develop a completely different form in order to survive. The "dead genes" would need to rely on a mutation, usually "as an accident during cell division," then the brain would need to be rewired, different genes diverging from the original in order to make a functional organ or form (p. 70). These changes happen over generations, not days or weeks. The inactivated genes would take a long time to become re-activated.