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Name: Caroline
Status: Educator
Grade:  9-12
Location: NV
Country: United States
Date: January 2005


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?



Replies:
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


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


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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