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Author Topic:   Genetic 'Bottlenecks' and the Flood
Member (Idle past 256 days)
Posts: 6349
From: San Diego, CA, USA
Joined: 05-03-2003

Message 39 of 59 (46847)
07-22-2003 6:50 AM
Reply to: Message 35 by Alec
07-21-2003 8:13 PM

Alec writes:

According to one of my newer textbooks, "most mutations are deleterious, but the neutral theory asserts that the selective advantages or disadvantages of most molecular mutations are so small that selection on them is too weak to offset the influences of genetic drift (Purves et al. 2001)."

Um, I have this book (though it's the second edition from 1987). And I was taught biology by Dr. Purves at Harvey Mudd.

I can state with certainty that he would be quite dismayed that you are using his text as some sort of indication that evolution has some fundamental problems that indicate a global flood.

If you like, I can email him and ask him for his opinion. Here's a more complete view:

The origin of genetic variability is mutation (Chapter 12). Most mutations are known to be harmful or neutral, but if the environment changes, previously neutral or deleterious alleles may become advantageous. Therefore, the amount of 'nonadaptive' variation in populations may have important biological consequences.

From Chapter 12, "Nucleic Acids as the Genetic Material":

Some mutations do involve extensive chemcial changes in the structure of DNA. Others, called point mutations, change only a single nucleotide in the genetic information. Point mutations can generally revert, that is, they can mutate back to the original form. Extensive mutations may be rearrangements (which change the position or direction of a DNA segment without actually removing any genetic information) or deletions (in which a segment of DNA is irretrievably lost).

All mutations are rare events. The observed frequencies of mutations are different for different organisms and for different genes within a given organism. Usually the frequency of mutation is lower than 1 mutation per 104 genes per DNA duplication, and someties the frequency is as low as 1 mutation per 109 genes per duplication. The majority are point mutations resulting from the substitution of one nucleotide during the synthesis of a new DNA strand.

The text then goes on to discuss the various types of point mutation (transition, transversion, frame-shift), and then compares the effects:

It is small wonder that a frame-shift mutation is so disruptive. An organism carrying such a mutant gene can only survive only if the gene product affected is not an essential part of the cellular machinery or if the organism also carries another copy of the gene in its normal form. This situation is different from that applying to base-substitution mutations. Often base-substitution mutations change the genetic message so that one amino acid is substituted for another in the protein. Such a missense mutation may sometimes cause the protein to be completely nonfunctional, but often the effect is only to reduce its functional efficiency. on rare occasions the functional efficiency is even improved. Individuals carrying issense mutations may survive even though the affected protein is absolutely essential to life.

In short, Purves is not saying what you are making him out to say.


This message is a reply to:
 Message 35 by Alec, posted 07-21-2003 8:13 PM Alec has not yet responded

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