Ask A Scientist Molecular Biology Archive

 name        Caroline
status       educator
grade        9-12
location     NV

Question -   I read your response to why do people have allergies- the
responses basically said that the gene/genes are not harmful enough to
interfere with survival and reproduction.  However, if genes are not
being selected for they will begin to encounter genetic drift
correct?  The frequency should decrease overtime but that is not what we
are seeing in humans- allergy rates are on the rise and in many cases
serious deleterious effects are coming with them.  What would be the
reason for this in an evolutionary sense?
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Dear Caroline,

I would suggest that although there is an evolutionary basis for 
allergies, the reason for the increased frequency of allergies is likely 
to be caused by changing environmental factors.  The time scale (2-3 
human generations) for the increased frequency is too short for natural 
selection and genetic drift.  Genetic drift occurs from one generation 
to another and .  The human environment has changed radically over the 
last 50 years and in this time frame you have essentially a genetically 
fixed immune system functioning within this altered environment.  People 
have suggested that man-made chemicals are stimulating the immune 
system, overly clean living environments leave the immune system with 
nothing else to do but react to minor irritants, or exposure to more 
chemicals--natural and man-made, are activating the immue system.  If 
allergies have no effect on reproduction, than genetic drift might occur 
but these changes would be observed over a longer time frame.  If 
allergies are linked to another process that is essential for modern 
living, they will continue to rise.  If allergies begin to affect the 
ability to produce offspring, then the frequencies may be reduced.

Dr. Jim Tokuhisa
Max Planck Institut fuer Chemische Oekologie
Germany
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I was not the original responder and am not sure what the entire question
originally was...but...first off, genes (and other gene-related factors) are
the primary determinants of the traits that organisms have that allow them
to respond to their environment, and are also the primary determinants in
the organism's ability to reproduce successfully in certain environments.
As far as allergies... evolution of mammalian immune systems has taken
millions of years.  The changes in our environment that are occurring are at
a much faster rate than our immune systems have so far been able to respond
and many of the putative causes of allergies are a product of changes that
have occurred over just the past few hundred years....some even less.  Our
species will evolve in one way or another in dealing with these
environmental hazards that precipitate allergies. There is an interesting
variable here in that the drugs and medical interventions that are used to
control diseases might very well help in the carrying of the seemingly
deleterious genes through the gene pool.  If the allergies are not severe
enough to interrupt successful reproduction, like many other human ailments,
they will remain quite common in the population. Most allergies, hay fever,
hives, etc. are not nearly so sever as to prevent reproduction.
Additionally, what seem to be deleterious genes can be beneficial in certain
environmental challenges...sickle cell anemia in its heterozygous form helps
prevent against death from malaria. Also remember, that even lethal
recessive alleles are exceedingly difficult to eliminate from a population.
As the occurrence of a recessive lethal allele becomes less common, it is
less likely for two mates that carry that allele to meet, and have offspring
that express the condition and die.

pf
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First of all, genetic drift can only occurr in very small populations (<100 
individuals) so it wouldn't apply to the vast majority of human 
populations. A gene that is harmful in one environment, may have been 
beneficial in a different environment. For example, the gene for sickle 
cell anemia confers partial resistance to malaria. People that are 
heterozygous for this gene have a selective advantage over people who are 
homozygous for the "normal" or wildtype gene. The same might apply to genes 
that promote allergies.

Ron Baker, Ph.D.
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