Sep 03 2009

What’s Your Mutant Power

The unavoidable truth is that we are all mutants. This doesn’t mean that some of us can control magnetic fields at will while others have claws protruding from their knuckles (although it is fun to fantasize about what you would like your mutant power to be).

Rather, it means that mutations – random changes in the base pair sequence of our DNA brought about by copying errors in the reproductive process – are common. This is old news, but scientists have been trying to nail down exactly how common for years.

Now a new study, using the latest genetic sequencing techniques, gives us a more accurate estimate of the mutation rate in humans – 3 x10 -8 mutations per nucleotide per generation. This translates to 100-200 mutations per person. That means that you have  100-200 new mutations not present in either of your parents.

The paper concludes:

Twelve mutations were confirmed in not, vert, similar10.15 Mb; eight of these had occurred in vitro and four in vivo. The latter could be placed in different positions on the pedigree and led to a mutation-rate measurement of 3.0 × 10−8 mutations/nucleotide/generation (95% CI: 8.9 × 10−9–7.0 × 10−8), consistent with estimates of 2.3 × 10−8–6.3 × 10−8 mutations/nucleotide/generation for the same Y-chromosomal region from published human-chimpanzee comparisons [5] depending on the generation and split times assumed.

What this means, as Larry Moran explains quite nicely, is that there is nice agreement between the theoretical mutation rate based upon the rate of copy error, the mutation rate we calculate when we compare two people separated by 13 generations, and the rate we calculate when we compare humans to chimpanzees.

What this further means is that most mutations are neutral.

For some quick background, each strand of DNA contains a sequence of four different base pairs (GTCA). Each triplet of three base pairs (in a coding region) codes for either an amino acid or a basic command, like “stop coding here.” There is redundancy in this code in that there are more triplet combinations (4^3 = 64) than amino acids (20). So each amino acid on average has about three triplet combinations that code for it.

Therefore, some mutations are completely silent in that they shift from one triplet code to another for the same amino acid – therefore the amino acid sequence and the resultant protien are identical. Actually, recent research shows that silent mutations, while they do not affect the resulting protein, can affect the transcription rate and therefore the amount of that protein cells make. There’s always another layer of complexity to biology.

A mutation may also change one amino acid to another, but the new amino acid may be chemically interchangeable with the old one, and again the three dimensional structure of the resulting protein may be unchanged. Or, there may be a slight change in the protein structure, but not one that affects function. Or, function may be altered but in a way that does not affect health and survival (let’s say it has a small effect on hair color).

These are all neutral mutations – they do not make the organism more or less fit, but they do add genetic diversity to the population.

Mutations may also be harmful or beneficial if they result in a change of protein structure that alters its function, or changes signals that alter the developmental pathways. Also, not all mutations swap one amino acid for another. A point mutation may change a triplet code to a stop codon, telling the cell’s machinery to stop the process of making the protein prematurely and resulting in a truncated protein. Or, there may be a frame-shift mutation – a base pair can be added or deleted which will shift the three letter code, essentially scrambling the amino acid sequence from that point forward.

If most mutations were harmful then many of these mutations would be lost to spontaneous abortions – embryos that are just not viable and therefore die in the womb. If the measured rate of mutations were far less than the theoretical rate, we could infer that most mutations may be harmful. However, this new study and other evidence supports the conclusion that most mutations are neutral, and only a few that are harmful, and also a few that are beneficial.

Creationists, as they often do, habitually misunderstand the nature of “mutations.” They see them as deviations from a perfect “normal” state. But in reality, all gene variations are just that – variations. No one variation is any more perfect or normal than another. There is likely, for most proteins, a large variety of base pair and amino acid sequences that function just fine, or may have only subtle differences. There is also context – one sequence may have a survival advantage in one environment but be a detriment in another.

We can actually infer the degree to which protein variations are tolerated by how well they are conserved in evolution. If peas and humans have the same version of the same protein, than that protein is highly conserved, which means that evolutionary pressures select against change. The most highly conserved proteins are the histones (also the nerdy name of my medical school acapella group) – proteins that wrap up and control the long DNA strand. This makes sense as this is a very basic function to the machinery of cells.

This is similar to the creationist misunderstanding of transitional species – all species are transitional just as all individuals are mutants. Life is a continuum of variation, although an uneven one, and much of the variation extinct.

Understanding the nature of variation, mutation, and diversity is critical to conceptualizing evolution, and a misunderstanding of these things is often a part of creationist denial of evolution.

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