Monday, March 8, 2010
The Engine of Evolution
In Coyne's book Why Evolution is True, he describes the evolution of one parasitic species (Central American roundworm) to prey on native ants and changes not only their physical traits (change color of abdomen)--but also their behavior (crawl up tree to be more visible for birds). (pg. 112-3) In our behavior unit we learned how deeply ingrained, and even instinctive, animal behaviors can be. So how is it possible for a parasite to slowly evolve the multiple tools required to successfully control these ants (How did this process begin? Was each adaptation simultaneous?)? Doesn't this go against the very things we learned about in evolution? Provide another example of advanced symbiotic relationship between two species (parasitism, mutualism, or commensalism) between species and how it could have evolved/why it would be advantageous to survival of the species/'s.
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The driving force behind evolution is natural selection. SInce nature is in charge of natural selection, there are no limits on how many selective advantages an organism can come across through random mutations/variance in a lifetime. Just like anything else in nature, whatever variances or mutations creating diversity in a single species allows an organism to survive and reproduce throughout many generations will be passed on over and over for millions of years until speciation occurs, whereas the organism without the selective advantage is most likely extinct. This is apparent in the Central American roundworm, which over an immense period of time, went through many genetic changes to make it the successful parasite it is today. Jerry Coyne on page 113 comments, "All of these changes are caused by the genes of the parasitic worm as an ingenious ploy to reproduce themselves... [and] It is staggering adaptions like this- the many ways that parasites control their carriers, just to pass on the parasites' genes- that gets an evolutionist's juices flowing." Evolution has no bounds and anything is possible, and Coyne describes this perfectly on page 4 of his book, "The theory of evolution does not predict that species will constantly be evolving, or how fast they'll change when they do. That depends on the evolutionary pressures they experience."
ReplyDeleteAlong with the amazing adaptations of the Central American roundworm and their parasitic relationship with ants and birds, acacia trees and stinging ants (genus Pseudomyrmex) have a mutualistic relationship, which is "An interspecific interaction that benefits both species (+/+)" (Campbell et al., 2008). The ants feed on the nectar produced by the tree and on the protein-rich swellings at the tips of the leaflets, while the acacia benefits since the ants attack anything that touches the tree allowing it to survive and reproduce into the future. Along with attacking any organism that comes in contact with the tree, the ants also remove fungal spores, small herbivores, and debris, and clip vegetation that grows close to the acacia. Mutualistic relationships come over time as there are benefits (selective advantages) for both sides. This type of evolution involves much more than one adaptation, but instead multiple tools.
The question of a parasite evolving multiple tools to successfully become the parasite it is today is similar to the other questions of multi-step processes that occur in nature, like the “bacterial flagellum (a small hairlike apparatus with a complex molecular motor, used by some bacteria to propel themselves) and the mechanism of blood clotting” (Coyne 137). Such complex pathways seem to be an impossibility for natural selection to produce, because they work only as a full unit, and if one of the parts is missing, they fail. However, one of the central tenets of natural selection is that it does not produce new traits, it simply modifies old ones. This can explain both of the prior examples, because certain structures could start to come together to create the new, seemingly too-complex-for-evolution structures in later generations. Thus, it is proven possible that an organism can slowly evolve multiple tools that work together towards a certain goal. In the case of the roundworms, they almost undoubtedly evolved the mechanisms by which they flourish in a sequential manner. Most likely the first adaptation to occur was the advent of an ability to live within the digestive tract of the ant. When the ants would eat the bird droppings with the worms in them, the worms would probably perish due to the ants’ digestive system. However, one specimen at one time had some sort of genetic mutation that allowed it to survive within the abdomen of the ant. It may have produced some sort of protective protein. This worm then reproduced, spreading its protective gene to the next generation. Over time, as the ants would destroy the worms that did not have this adaptation, it would grow more prevalent, until the majority of the species was able to live in the digestive tract of the ants. Over the course of time, more mutations occurred in these worms. The mutations would allow the worms to produce the chemicals necessary to alter the ants’ behavior. The mutations that created the chemicals that caused the berry-like appearance of the ants, the sluggish movement of the ants, and the weakness in the junction between the abdomen and thorax could all have developed at different times, or at similar times, because one is not required for the formation of the other ones. However, each of these mutations provided a selective advantage for the worms that had them, as it made it more likely that they would be ingested by their second hosts, the birds. Eventually, it led to the behavior that we see today. This pathway is not counterintuitive to the evolution that we have learned; in fact, it is a solidifying argument, because the organisms that were best adapted to survive and reproduce had their genes survive.
ReplyDelete(continued)
ReplyDeleteOne example of a symbiotic relationship between two species is the parasitism that goes on between Plasmodium, the protist that causes malaria, and humans. The basic pathway that the parasite follows is: an infected mosquito bites a human, sporozoites from the parasite go to the liver and infect liver cells, the liver cells burst and produce merozoites, which travel to red blood cells, digest the hemoglobin for nutrition, then produce gametocytes of both genders, which are then eaten by the mosquito, where they fertilize and become a zygote, which releases sporozoites, which starts the cycle over again. Since Plasmodium has been in existence for a long time, much longer than humans have been extant, they must have developed traits that allowed them to utilize the human body as their host. One possible way that this could happen is that ancient Leukocytozoons, themselves evolved from a parasite that developed a way to enter the intestinal wall of an organism and hijack liver and blood cells, was taken in by a parasitic fly feasting on a meal of blood. It survived within the gut of the fly, where it began to spread through the infected organism’s bite. It continued to evolve manners to avoid organisms’ defense systems, and eventually evolved into Plasmodium, unable to mate with the Leukocytozoons. The new organisms traveled via mosquitoes rather than flies, but the method of transport was the same, by blood. Mosquitoes would feed on the blood of animals, and as the animals evolved, they would continue to feed. Eventually, through the millions of years, humans evolved, and became a food source for the mosquitoes. The Plasmodium parasites had already developed their means of reproduction, and they adapted to the new environment of the humans. Throughout this time of evolution, they developed ways of utilizing hemoglobin for their own food source, and ways of evading the body’s immune system o that they could infect the liver cells.
Sources:
Why Evolution is True. Jerry Coyne.
8th Edition AP Edition Biology. Campbell, Reece, Urry, Cain, Wasserman, Minorsky, Jackson.
Wikipedia- http://en.wikipedia.org/wiki/Plasmodium -Plasmodium