Idle Thoughts

I had to catch up on some yard work today. I also had several books to try and finish. I had started them a while ago and kept getting side tracked. I was starting to feel guilty about not posting anthing today – then I noticed a lot of blogs have posted little to nothing. Now I don’t feel so bad.

One of the books I am trying to get through is “New Directions in Ecological Physiology” by Feder et al. One of the chapters (chapter 8: The Importance of Genetics to Physiological Ecology, by Richard Koehn) mentioned something that got me to thinking. But first, a little background for the genetically impaired- um, that came out wrong, I should say for those who are not very familiar with the science of genetics. For those of you who are familiar with genetics I apologize in advance for the intro material.

Genes can come in more than one form (alleles). For example, a gene can have two alleles- call them A and a. From these two alleles you can derive three possible genotypes AA, Aa and aa. Genotypes AA and aa are called homozygous and genotype Aa is called heterozygous. In a system lacking selection the frequency of the heterozygous genotype is contingent on the frequencies of A and a. Using Hardy-Weinberg one can calculate the frequency of Aa fairly simply.
When selection is present things become a little more difficult. Take sickle cell anemia. Sickle cell anemia is a recessive disease which causes red blood ceels to collapse into a sickle shaped cell. Terminology for the gene varies and I will use the Hb terminology. Basically, Hb A HbA is normal Hb S Hb S is the homozygote recessive – which is fatal – and Hb A Hb S are heterzygotes. Because having Hb A Hb S confers a certian amount of immunity to malaria this genotype enjoys a slight selective advantage over Hb A HbA. Even though there is selection for the heterozygote, we can still calculate the frequency. We just have to know the amount of selection and include a term for it in our formula for estimating gene frequencies.
Another interesting question is how many genes in a given genome are heterozygotic. I don’t have figures at hand, but the answer is quite a few. So the next question is- leaving aside when heterozygotes have a selective advantage – are there any other advantages in having a large number of heterozygous genes.
Apparently, the answer is yes – at least in some species. Which brings us back to the above mentioned chapter in “New Directions in Ecological Physiology”. Koehn reviews several studies which demonstrate that heterozygosity (and here I am refering to heterozygosity at the enzyme level) increase metabolic efficiency, resulting in energy savings. These energy savings can be channeled into any number of different areas – growth or increased feeding rates to name a few. Or (this is afarensis extrapolating) perhaps into new adaptations that allow for expanded niches or moves into new niches? Or to provide the energy for co-opting other traits and making something new out of them?

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