Can anyone tell me why Genetic linkage and hitchhiking does NOT invalidate Natural Selection?

Question

Genetic Linkage, along with antagonistic epistasis and

pleiotropy ,is known to be a major limitation on the

efficacy of natural selection.

It causes the following:

1) Reduces genetic diversity in a population

2) It increases the genetic "load"

3) Interference from background loci can prevent

fixation of advantageous alleles.

All of which should render natural selection pretty

much useless. Yet scientists are reluctant to

admit this. Why?

Answer 1

The problem with your argument is that you assume this reduction is absolute. While linkage can inhibit the selection of favorable mutations, if those mutations are linked to less favorable alleles,  you must remember that:

1) Not all genes are linked. Most eukaryotes have their genome arranged across multiple chromosomes and even genes on the same chromosome are unlinked if they are located 50 centimorgans apart.

2) Recombination. In areas of the genome where recombination is more likely to occur, there is less of a supression of selection due to linkage. In regions of the genome where recombination occurs less frequently, you see more effects of linkage on selection (see  PNAS October 15, 2002 vol. 99 no. 21 13616-13620). This may also help (in part) to explain the strong conservation of recombination. In most eukaryotes, failure to undergo crossover leads to sterility.

3) You must consider the genome as a whole. It's not what any one gene does, but the net effect of all genes.

In the end linkage will reduce selection in certain regions of the genome, but it's not eliminate it. Other factors, especially recombination compensate for this reduction.

Answer 2

Because only someone desperate for a counter-argument to natural selection would read these as absolutes:

>"1)Reduces genetic diversity in a population"

This does not say *ELIMINATES* genetic diversity.

>"2) It increases the genetic "load"

This does not say that it increases the genetic "load" until natural selection is impossible.

>"3) Interference from background loci can prevent fixation of advantageous alleles."

It says it *CAN* prevent fixation ... doesn't say it *MUST* prevent fixation of advantageous alleles.

In other words, geneticists have long recognized that genetic linkage puts some constraints and limits on natural selection.  

But it is an unjustified leap to read this as genetic linkage "invalidating" natural selection completely, or "rendering natural selection pretty much useless."

You are reaching.