Skip to comments.Stephen Wolfram on Natural Selection
Posted on 09/04/2002 11:23:46 AM PDT by betty boop
Stephen Wolfram on Natural Selection
Excerpts from A New Kind of Science, ©2002, Stephen Wolfram, LLC
The basic notion that organisms tend to evolve to achieve a maximum fitness has certainly in the past been very useful in providing a general framework for understanding the historical progression of species, and in yielding specific explanations for various fairly simple properties of particular species.
But in present-day thinking about biology the notion has tended to be taken to an extreme, so that especially among those not in daily contact with detailed data on biological systems it has come to be assumed that essentially every feature of every organism can be explained on the basis of it somehow maximizing the fitness of the organism.
It is certainly recognized that some aspects of current organisms are in effect holdovers from earlier stages in biological evolution. And there is also increasing awareness that the actual process of growth and development within an individual organism can make it easier or more difficult for particular kinds of structures to occur.
But beyond this there is a surprisingly universal conviction that any significant property that one sees in any organism must be there because it in essence serves a purpose in maximizing the fitness of the organism.
Often it is at first quite unclear what this purpose might be, but at least in fairly simple cases, some kind of hypothesis can usually be constructed. And having settled on a supposed purpose it often seems quite marvelous how ingenious biology has been in finding a solution that achieves that purpose .
But it is my strong suspicion that such purposes in fact have very little to do with the real reasons that these particular features exist. For instead what I believe is that these features actually arise in essence just because they are easy to produce with fairly simple programs. And indeed as one looks at more and more complex features of biological organisms ¯ notably texture and pigmentation patterns ¯ it becomes increasingly difficult to find any credible purpose at all that would be served by the details of what one sees.
In the past, the idea of optimization for some sophisticated purpose seemed to be the only conceivable explanation for the level of complexity that is seen in many biological systems. But with the discovery that it takes only a simple program to produce behavior of great complexity [for example, Wolframs Rule 110 cellular automaton ¯ a very simple program with two-color, nearest neighbor rules], a quite different ¯ and ultimately much more predictive ¯ kind of explanation immediately becomes possible.
In the course of biological evolution random mutations will in effect cause a whole sequence of programs to be tried . Some programs will presumably lead to organisms that are more successful than others, and natural selection will cause these programs eventually to dominate. But in most cases I strongly suspect that it is comparatively coarse features that tend to determine the success of an organism ¯ not all the details of any complex behavior that may occur .
On the basis of traditional biological thinking one would tend to assume that whatever complexity one saw must in the end be carefully crafted to satisfy some elaborate set of constraints. But what I believe instead is that the vast majority of the complexity we see in biological systems actually has its origin in the purely abstract fact that among randomly chosen programs many give rise to complex behavior .
So how can one tell if this is really the case?
One circumstantial piece of evidence is that one already sees considerable complexity even in very early fossil organisms. Over the course of the past billion or so years, more and more organs and other devices have appeared. But the most obvious outward signs of complexity, manifest for example in textures and other morphological features, seem to have already been present even from very early times.
And indeed there is every indication that the level of complexity of individual parts of organisms has not changed much in at least several hundred million years. So this suggests that somehow the complexity we see must arise from some straightforward and general mechanism ¯ and not, for example, from a mechanism that relies on elaborate refinement through a long process of biological evolution .
[W]hile natural selection is often touted as a force of almost arbitrary power, I have increasingly come to believe that in fact its power is remarkably limited. And indeed, what I suspect is that in the end natural selection can only operate in a meaningful way on systems or parts of systems whose behavior is in some sense quite simple.
If a particular part of an organism always grows, say, in a simple straight line, then it is fairly easy to imagine that natural selection could succeed in picking out the optimal length for any given environment. But what if an organism can grow in a more complex way ? My strong suspicion is that in such a case natural selection will normally be able to achieve very little.
There are several reasons for this, all somewhat related.
First, with more complex behavior, there are typically a huge number of possible variations, and in a realistic population of organisms it becomes infeasible for any significant fraction of these variations to be explored.
Second, complex behavior inevitably involves many elaborate details, and since different ones of these details may happen to be the deciding factors in the fates of individual organisms, it becomes very difficult for natural selection to act in a consistent and definitive way.
Third, whenever the overall behavior of a system is more complex than its underlying program, almost any mutation in the program will lead to a whole collection of detailed changes in the behavior, so that natural selection has no opportunity to pick out changes which are beneficial from those which are not.
Fourth, if random mutations can only, say, increase or decrease a length, then even if one mutation goes in the wrong direction, it is easy for another mutation to recover by going in the opposite direction. But if there are in effect many possible directions, it becomes much more difficult to recover from missteps, and to exhibit any form of systematic convergence.
And finally for anything beyond the very simplest forms of behavior, iterative random searches rapidly tend to get stuck, and make at best excruciatingly slow progress towards any kind of global optimum .
It has often been claimed that natural selection is what makes systems in biology able to exhibit so much more complexity than systems that we explicitly construct in engineering. But my strong suspicion is that in fact the main effect of natural selection is almost exactly the opposite: it tends to make biological systems avoid complexity, and to be more like systems in engineering.
When one does engineering, one normally operates under the constraint that the systems one builds must behave in a way that is readily predictable and understandable. And in order to achieve this one typically limits oneself to constructing systems out of fairly small numbers of components whose behavior and interactions are somehow simple.
But systems in nature need not in general operate under the constraint that their behavior should be predictable and understandable. And what this means is that in a sense they can use any number of components of any kind ¯ with the result that the behavior they produce can often be highly complex.
However, if natural selection is to be successful at systematically molding the properties of a system then once again there are limitations on the kinds of components that the system can have. And indeed, it seems that what is needed are components that behave in simple and somewhat independent ways ¯ much as in traditional engineering.
At some level it is not surprising that there should be an analogy between engineering and natural selection. For both cases can be viewed as trying to create systems that will achieve or optimize some goal .
[I]n the end, therefore, what I conclude is that many of the most obvious features of complexity in biological organisms arise in a sense not because of natural selection, but rather in spite of it.
One wag described it as the costliest vanity book ever.
But what I believe instead is that the vast majority of the complexity we see in biological systems actually has its origin in the purely abstract fact that among randomly chosen programs many give rise to complex behavior .I suspect Wolfram has just independently discovered neutral mutations and is needlessly agog. But then I read things like,
Third, whenever the overall behavior of a system is more complex than its underlying program, almost any mutation in the program will lead to a whole collection of detailed changes in the behavior, so that natural selection has no opportunity to pick out changes which are beneficial from those which are not.and think that Wolfram does not appreciate the way in which natural selection sculpts a population. Although of course the genetic composition of a population changes under selection pressure, individual genes are not often directly selected in or out. Individual organisms are.
In sexual species, just about every organism is genetically unique. A species is a swarm of similar genomes. A species under pressure to change its adaptation is being sculpted by selection in that those best able to get along in a new way are the most likely to make it. The pruning is continuous so long as the selection pressures stay the same. It's done at the organism level. Nothing is operating at the level where Wolfram is imagining the difficulty.
He is a mathematician looking at how complexity arises from simple computer programs.
I know you can draw very nice mountains with fractals, but that doesn't exactly mean that mountians come from computer algorithms.
He treats with and disposes of chaos at about page 100.
He treats with and disposes of fractals by page 30.
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