Allergies and the evolutionary process
Date: Around 1993
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
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.
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.
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Update: June 2012