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Re: gack!

Posted by Adam on November 28, 1999, at 22:14:54

In reply to gack!, posted by Bob (check the email add. if you can't tell which) on November 26, 1999, at 9:30:25

> Adam, maybe you could fill me (and the rest of us) in on something else here.
> That was a great description of the traditional view of evolution*. The thing about it, tho, is that it suggests a rather gradual change in species over the course of millions of years. I guess there's evidence that sometimes evolution makes great leaps over the course of rather short periods (in the geoligical sense of terms ... thousands or years, perhaps only hundreds, but certainly not millions). How does that notion of "punctuated equilibrium" fit in theoretically?
> *One thing I would add is a point about what makes a "good" mutation. It's a very common lay-conception that nature somehow produces the mutations an organism needs to survive, meaning that "survival of the fittest" is somehow a purposeful process and ascribing to Nature some sort of will or intelligence. Traditionally, tho, this is horse hockey. Mutations are produced randomly and (over time and across an entire species) rather prolifically. Think of the high end of a bell curve--very few of those mutations will have a much better fit with the environment than what is ordinary. If it wasn't for time and numbers, if life remained a rather constrained phenomenon, then we wouldn't see much speciation. In a traditional sense, there is no such thing as a good mutation, only random ones that on quite rare occasions wind up conveying an ecological advantage.
> Then again, I guess there is evidence against a purely randomized process of mutation as well.
> bob


Hmm. Well, speaking teleologically, I was always under the impression that all mutations were random in
nature. I would be interested to know what the evidence is against a "purely randomized process". I
assume this doesn't imply a plan, but may refer to some environmental factors affecting specific loci?
One example of that I can think of off the top of my head is is dimethylbenzamine, a nasty carcinogen that
always causes mutations at codon 61 of the Ras oncogene, constituatively activating it.

As for punctated equilibrium vs. my example, as I implied, it was a simplification. There aren't many rules
in evolution, it would seem, and for every species, there is a different story. The main factors are mutation,
genetic drift, and selection. Mutation is a given. Drift and selection may play more or less important roles
depending on the circumstances. A good way to envision what goes on in "punctate equilibrium" is to look at a
population as a bell curve. Draw a line down the middle of the curve. Think of that curve as representing
the genetic makeup of the population, and the line as the environment. So, the average individual in the
population is ideally adapted to that environment. Remember, the breadth of the curve represents genetic
drift, the random deviation from the norm through mutation.

Now, move that line over near either end of the curve. The environment has changed (say a big meteorite hit
the Yucatan or something). If you move the line too far left or right you're either out of the curve (nothing
can cope or survive, extinction) or so far towards the edge that you're left with too few well-adapted individuals
(inbreeding leads eventually to extinction). But say your line isn't too far one way or the other. Selective
pressures now favor a new sub-population. The curve quickly shifts to center itself about that new line. You
haven't got a new species in strictest sense, but there is a new norm. Time passes, drift continues, speciation

To really mix things up, the shifting "environment line" might cut the curve in more than one place...the norm
not only shifts, but fractures. Different subpopulations compete. Which might prevail? One, both, neither?

Depending on the circumstances, almost anything could happen.




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