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Name: Ray
Status: Educator
Grade:  9-12
Location: IL
Country: United States
Date: July 2008


Question:
What would be the evolutionary purpose for allergies? It seems they are nothing but a weakness and if evolution is to allow better traits to dominate then the allergies must be a strengthened, the same as with colds and other problems.



Replies:
First, the frequency of individuals having a negative trait is often maintained in the population (this relates to population genetics). If the condition is only slightly harmful those frequencies may even increase. Also, there is or was a silver lining behind many negative traits (sickle cell anemia, cystic fibrosis, etc). Many had some advantage which kept the gene frequencies up despite their obvious drawbacks. Lastly, it is the interaction or total impact of all the genes in an individual that determines fitness. Genes for mildly negative traits may tag along with little negative selection pressure. It is only the severe autoimmune disease or immunodeficiency disorders that will prove lethal and have little opportunity for being kept. Allergies are quite mild by comparison. How allergies develop in an individual is still a very important part of this puzzle. If man-made compounds in any way enhance this process, it would further explain any increases in their frequency. Of course medication helps us cope and minimize the negative effects as well.

Lou Harnisch


This is sort of off the top of my head, but the basic idea is here: as Lou Harnisch wrote, selection happens at the level of the individual, not that of the gene (or in this case, or phenotype. It seems that evolution has often put up with useless and what we perceive as useless or negative features for other purposes. It is certainly the case that survival of organisms that immune function is required for our survival, and that of many other species.

After all, we provide a rich food and source and a controlled environment for all sorts of parasites. So the advantage ge of having and immune system is huge. In contrast, the cost of most allergies seems to be rather small - Lord knows I get nasty hayfever, but I hope that it will not ruin my chances of survival or reproductive success. The important thing to note here is that it is *not* just because a feature has evolved does not necessarily mean it has a purpose. This is fundamental to evolutionary theory. What for instance, is the purpose of male nipples? For the answer to this and for an excellent series of articles on evolutionary biology, I would like to recommend d any of the books by Stephen Jay Gould. You will probably find some in your local library. They are readable, accurate, and amusing.

Ethan Benatan



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