Re: Species diversity through time

John Harshman wrote:
dkomo wrote:


John Harshman wrote:



dkomo wrote:


<snip for brevity>


Consider the state space of life. Evolution follows a trajectory through this space as a function of time. At a given t the "step" to the next point is random just as in an ordinary random walk, and it is a memoryless, Markov process because the next point depends only on the current point, again like an ordinary random walk. Every time Gould's "tape of life" is rewound and played again, a different trajectory through the state space results. That's why I say evolution is a random walk in Gould's view. Please keep in mind, this is not a view I agree with!


Please keep in mind that it's not a view Gould would have agreed with
either. Gould didn't think evolution was a random walk. He didn't think
that the next point on the typical evolutionary trajectory was random.
He knew that there was such a thing as natural selection, for example.

I was going to address the selection issue but ran out of steam. But I have just enough steam left to do it here.

Point being, selection itself *fluctuates*. That is, the environment is continually changing. Gould believed strongly in historical contingency did he not? He believed it was a major factor in evolution. If a huge comet hits the earth, that's one hell of a selection event! If there is a sudden increase in volcanic activity, which is one theory of what caused the Permian extinction, that also is a major change in selection.

Given that throughout deep time there have been many such unpredictable enviromental upheavals, evolution can't help but to be a random walk, expect maybe on very short time scales, where you could claim that if selection is fairly constant, evolution will then be approximately deterministic.


I would say, rather, that a great complexity of causes can result in
something quite like a random walk. (So can quite simple causes, if
they're chaotic -- I believe someone brought up the example of a
pseudo-random number generator.) But it's not really a random walk.

My point is that what you said above was a distortion of Gould's views.

And anyway, your apparent notion is that there is some general principle
of increasing complexity, which is nowhere in evidence.


Complexity, increasing or not, is *everywhere* in evidence. It is a characteristic of every living thing from the simplest bacterium to the most elaborate animals. At root, what I'm saying is that you have *no* good explanation for how it came about it other than the vague "variation and selection".

I don't deny that variation and selection are real. What I object to is that they leave a huge explanatory gap. Between the DNA being changed by mutation and selection operating on the organism's traits, there is the whole extremely complex hierarchical realm of that organism's structure and function, with tens of thousands of coordinated biochemical reactions occuring in a coordinated fashion. How did it *really* come about? I predict that the detailed explanations will increasingly come through the ideas of self-organization and emergence. Just to say, "oh, obviously it came about through variation and selection" is no longer acceptable.


Now, this part I'm not sure of, but in Gould's replay of the tape of life metaphor, didn't he write that the historical contingencies would also be different? So it wasn't just play the tape through the same set of historical contingencies and depend on mutation to produce different results.


I wouldn't go examining the metaphor all that closely. He merely means
that life is not some grand progression toward some predetermined goal.
If the world is chaotic (in the mathematical sense), then all you need
would be an infinitesimal change in initial conditions, or a little bit
of randomness here and there, to make the end result vary wildly. Where
are you going with this? Do you think that the history of life is indeed
a series of inevitable events?


Gould is wrong. There are basins of attraction in the state space of life. Replay the tape of life and the end result will be close to what it was before, as long as you don't vary the major historical contingencies. A little bit of randomness here and there won't perturb evolution's trajectory all that much.


--dkomo@xxxxxxxx




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