John Hawks has been generating some graphs for his class in order to illustrate random genetic drift. See how he does it at Some genetic drift graphs with Mathematica.
The important point, which most students don't get, is that eventually one allele will become fixed in the population and the other will be eliminated for ever.1
1. In a simple two allele situation in a sexual population.
8 comments :
Excel might be slower at it, but it functions equally well. A major difficulty is that students don't understand what is modelled,and working with a program does not improve that. The old fashioned approach with 9 bins of differently coloured beads in known frequencies, draw 10 beads, count frequency, go to next bin, works as well.
Here is a nice java applet from Kent Holsinger's web page :
http://darwin.eeb.uconn.edu/simulations/drift.html
An allele can become fixed in a population through either natural selection or genetic drift. Correct? If so, is there a way to determine which, of these two, was responsible?
Don asks,
An allele can become fixed in a population through either natural selection or genetic drift. Correct? If so, is there a way to determine which, of these two, was responsible?
Yes, but it's not always easy.
If the allele is neutral or deleterious then we know it ain't fixed by natural selection. If we can determine that it's beneficial then natural selection is a good candidate, depending on the selective coefficient.
The only rule is that one mustn't assume without evidence that all fixed alleles are beneficial. That's the adaptationist fallacy.
If you examine nucleotide and amino acid sequences from various organisms it looks like the vast majority of fixed variants are neutral or nearly neutral. This suggests that random genetic drift is the predominant mechanism of evolution.
The fact that we see an approximate molecular clock when we construct phylogenetic trees from sequence data is further evidence of the importance of random genetic drift.
Larry stated: The only rule is that one mustn't assume without evidence that all fixed alleles are beneficial. That's the adaptationist fallacy.
Actually, that is the militant anti-adaptationist strawman.
A Militant Anti-Adaptationist (MAA)believes that there is a substantial number of evolutionary biologists who think that if they can construct a plausible hypothesis to account for some feature of evolution (e.g., a "just so story") then they will immediately declare the problem solved without further testing, and move on to other things.
MAAs are ever vigilant.
divalent says,
A Militant Anti-Adaptationist (MAA)believes that there is a substantial number of evolutionary biologists who think that if they can construct a plausible hypothesis to account for some feature of evolution (e.g., a "just so story") then they will immediately declare the problem solved without further testing, and move on to other things.
Interesting. Do you know any militant anti-adaptationists who think that way? I don't.
For the record, my position is similar to that of Gould and Lewontin who claim the main problem is not that adaptationists consider their just-so stories to be the final solution, it's that they only consider adaptationist explanations.
Here's the common styles of argumentation for the adaptationist program according to Gould and Lewontin. (They give examples in the Spandrels paper.)
1. If one adaptationist argument fails, try another.
2. If one adaptive arguments fails, assume that another must exist.
3. In the absence of a good adaptive argument in the first place, attribute failure to imperfect understanding of where an organism lives and what it does.
4. Emphasize immediate utility and exclude other attributes of form.
If you examine nucleotide and amino acid sequences from various organisms it looks like the vast majority of fixed variants are neutral or nearly neutral. This suggests that random genetic drift is the predominant mechanism of evolution.
If you examine phenotypess from various organisms it looks like the vast majority of fixed variants are (or were recently) adaptive. This suggests that natural selection is the predominant mechanism of evolution.
But that "worldview" is "wrong."
*shrug*
Sven DiMilo says,
If you examine phenotypess from various organisms it looks like the vast majority of fixed variants are (or were recently) adaptive. This suggests that natural selection is the predominant mechanism of evolution.
First, I don't think your statement is correct. Lots and lots of phenotypic differences appear to be neutral with respect to survival.
Second, by looking only at phenotypes you are selecting a small subset of all variants that have become fixed in a lineage. When you look at sequences, on the other hand, you are looking at all variants that have become fixed. If most of them are neutral then you can safely say that the majority of changes are due to drift and not to selection. That's not a conclusion you can make if you limit yourself to "phenotypic" changes.
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