Wednesday, April 7, 2010


In ecology, one of the more fascinating topics is symbiotic relationships. 3 types of symbiotic relationships exist: mutualism, commentialism, and parasitism (1). Mutualistic symbiosis is a type where 2 organism may coexist to better each other's chances of survival, so natural selection would proliferate this type of relationship. For example, the stinging ant/hollow tree relationship Coyne discusses on page 121 is mutualistic. A commentialistic relationship is one where both organisms coexist, but neither receives any benefit (sometimes very little benefit) or harm this existence. The last type of relationship is parasitic; this means that one organism is harmed, while the other benefits. Usually, a parasitic relationship ends with the host organism dying, taking the parasite along with it.

How would killing of the parasite's host be beneficial to the parasite, if it will die as well? Would it not seem logical that genes would be selected that stop the parasite from growing too large or too harmful, to maximize its own life span, and thus its reproductive success?

1: Campbell, P. 1202-1203


  1. Quick note before answering the question: I think you meant to say a commensalistic relationship instead of a “commentialistic” relationship, and a commensalistic relationship is where one organism benefits from an interaction and the other organism is neither helped nor harmed.

    According to the Campbell book, “many parasites have complex life cycles involving multiple hosts”. When infecting a host, a parasite gradually leads to the declining health of the organism, but when the organism is near death or dying, the nutrition that it is able to extract from the host will be considerably lessening. Therefore, most parasites have certain tactics of changing hosts so that they do not die along with their hosts. For example, ticks can live as ectoparasites on moose, weakening the moose by withdrawing blood and causing hair breakage and loss, increasing the chance that the moose will die from cold stress or predation by wolves. If the moose then dies from the cold, scavengers will likely consume it, whereupon the parasite will be able to extract nutrients from another host, and if wolves consume the moose, then the parasite will be able to infect another host and survive.

    You bring up a valid point when you say that evolution of parasites would likely lead to the maximization of their lifespans, and it is seen through their ability to change hosts that their traits are advantageous for survival.

  2. Natural selection most likely occurs to benefit the individual. Therefore, when a parasite dies along with the host, most likely it has attained maximum living capacity.

    Mutualism, such as with roots and fungi (mycorrhizae) is successful for both species where one takes advantage of another and vice versa. It appears that not all life forms are able and willing to experience the most logical form of behavior, like with commensalisms where one benefits and the other is not harmed or benefited, parasitism is inevitable.
    Like with humans, the immune systems of vertebrates have responses that defend against various pathogens, but evolution calls for mutation and natural selection.
    With respect to parasites that use the organism as a host to feed off of, the parasite actually does NOT want to kill the host, but it may be inevitable. Research at Caltech (Source indicates parasite may reside in the brain of animals. After fending off the attack of the immune system, the parasite does not want to be detected by the immune system and hopes to lodge itself in a certain place to feed off of the brain tissues that are a source of food. Sometime, in humans, when the human body experiences symptoms such as seizures, etc, some of these symptoms are actually caused by the immune system itself. Therefore the parasite actually does not wish to kill the host and therefore, some people may have parasites for years and not show symptoms. But overtime, the parasitic larval defenses do break down, and the host immune system is able to launch its attacks.
    For amoebas, the amoebas do not need the host to survive therefore when the host dies, it is not too important. For tapeworms on the other hand, the symptoms are not noticeable for many years because when its host dies, the tapeworm also dies.
    As adaptations, some parasites also use vectors that transport a parasite from host to another. This helps increase the probability of the parasite surviving when one host dies off. (Source:

  3. As a clarification, I don’t believe all parasites kill their host. Like you have mentioned, it wouldn’t seem logical, if the objective is to live within the host, to kill it. The host is the parasites lifeline, so unless it is able to find a new host like Murad and Mary addressed, it wouldn’t be beneficial at all. If anything, the parasite would go extinct because it wouldn’t be able to survive and reproduce successfully. A smart parasite only takes what it needs and does it without being detected. Let’s use the tapeworm for example. Tape worm eggs can be found in meat human eat, and once in the body they attach their heads to the mucous of the intestinal wall and obtain nutrients by absorbing their hosts partly digested food through the tapeworm’s skin. There the head grows segments called proglottids that contain eggs and detach from the head where they are later found in feces of the animal. This parasite doesn’t hurt the human host severely at all. If anything the only evidence of its presence is an upset stomach and constipation that could be the results of any stomach ailment (Buzzle). This would be evidence of the traits you were asking about. It might range from a size of 6 inches to 26 ft, but it diet and focus on not causing attention to itself is an adaptation that developed over time. If it caused pain or began killing the person, the human would find out, go to a doctor, and get it removed. Like mentioned before, that could lead to its extinction. Still there is the question of if the human dies, the tapeworm dies. Why rely on the human so much? Adaptations don’t always evolve for survival. According to Coyne, “an adaptation must evolve by increasing the reproductive output of its possesssor” (120). In this case the adaptation is the ability to live inside a human’s stomach, get free meals, and release a ton of eggs. Tapeworms go on for long periods of time unnoticed, so in that time it can pass on it’ genes too many offspring. Even Coyne said it was reproduction and not survival “that determines which genes make it to the next generation and cause evolution” (120). Of course, the tape worm would have to survive to an age where it can reproduce, as otherwise it wouldn’t be an evolutionary advantage. Having the ability to live undetected increases those chances. Though the tapeworm risks dying if the host ever suddenly dies from other causes, it still would have enough time beforehand to make sure it’s children would carry on it’ genes.

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