Tuesday, May 08, 2007

More on Haldane's Dilemna

A Bradford comment at Telic Thoughts serves as a useful device around which to frame this post. I had previously written this about Haldane's Dilemna and this about genetic load.

What does a deleterious mutation "look" like? It must impair without being lethal. The point has often been made that such mutations entail recessive alleles. But the implications of this are often misleading. When two recessive alleles are present and a deleterious mutation is expressed, impairment is observed. But death prior to reproduction is not a logical imperative. Let's get specific and examine a hearing impairment. Although the onset of the disease usually occurs before adulthood and hearing loss is progressive, one can survive and pass on his genes.

Mutations that impair but are not lethal are like nature's gene knockout experiments. Disable gene x and an organism loses the function correlating to it. However, such loss merely hinders an organism with respect to a function that may not be a vital one. But if there are non-vital functions then how do we know selection favored the gene in the first place? Because it confers an advantage, that when nullified by loss of function, results in no observable loss of reproductive capacity?

Mutations that compromise fitness, without being fatal, are common. They involve color blindness, speech defects, and a mutation that makes one more susceptible to addiction. There are also many, many other examples.

These types of mutations are well known. Also well known is the potential danger of a nuclear armed Iran. The common denominator of the two disparate facts may very well be a failure to draw correct conclusions based on data. Nature itself supplies case studies that are problematic for the idea that Haldane's Dilemna has been overcome. At least with respect to the concept that many theoretical beneficial mutations become fixed throughout a population.

Why are we to believe that genomic changes rendering slight advantages become fixed in populations? Why believe that specific identifiable genes, whose functions afford slight advantages, became fixed in accordance with standard theory, when affected organisms reproduce despite mutations causing loss of their function. Oh but the traits descendents of other more fit individuals will eventually become predominant right? But where are such perfect specimens in nature? In reality individual members of populations have slightly impaired functions of varying sorts. However solutions to Haldane's dilemna envision degrees of reproductive fitness centered on slight reproductive advantages which in reality are theoretical advantages that can become lost in the much bigger picture of many unaccounted for genomic pluses and minuses.



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