March 27, 2006

Survival of the Fittest

The debate about evolution has raged in the comments below for several days. And it keeps returning to "survival of the fittest" and whether the concept is a tautology or not. And if not, whether it can produce falsifiable predictions. And finally, whether the answer to this has any bearing on whether evolution is a "scientific" theory.

I'll try to address at least the first two below.... The first part is a bit tricky. "Survival of the fittest" is a tautology in one sense because it is the logical consequence of a set of assumptions. But I'll argue that that's not relevant to how it's used in evolution. So.... here's my personal interpretation of "survival of the fittest" or selection (natural or artificial, ecological or sexual). Bear with me as I set up the assumptions:

  1. Imagine a replicator r that exists in an environment A
  2. The replicator will cease to operate (i.e. die) after some average period of time
  3. This replicator will replicate with some probability that is dependent on A such that the expected number of copies (i.e. offspring) is Er(A). (You might model this as Poisson reproduction and death processes, with λ's as functions of A, but it's not required.)
  4. The replicator has components (call them traits) that affect Er(A) and that are independently copied with a zero error rate to the "offspring"
  5. There is an arbitrarily large population of replicators with some variation in traits that deliver different Er(A)
Given this situation, it follows that for a constant A, replicators with traits that create a higher expected number of offspring will tend to increase as a proportion of the population. That's just what the math says (although, given the size of the margins, I'll leave the proof to others). And that, I believe, is all that natural selection, or "survival of the fittest", says.

So does it produce falsifiable predictions? Not really, not any more than the fact that "the circumference of a circle is equal to π times the diameter of the circle". If you find a "circle" that doesn't meet that condition, it means that your shape is not a circle or you mismeasured something — it doesn't mean you falsified the formula. (Note that the fact that the formula for the circumference is tautological is far from saying that knowing this relationship is useless.) Likewise natural selection tells us what will happen if our assumptions are true, and in fact it gives us a way to test if our assumptions are true.

And it turns out that, while the assumptions are remarkably close to the truth (especially given that DNA and molecular genetics were unknown in Darwin's time) and we can see selection take place over and over in experiments we run, the assumptions are not exactly true. Examining the assumptions that aren't true and when they are not true has helped us learn about other mechanism that affect genetic variation in populations. So what turned out not to be true?

  • The copying turned out not to be "error-free". Mutations happen. The rate is low enough in most cases to allow selection to occur (and it turns out to be important, which I'll return to below).
  • The "traits" are not independently copied. There are various kinds of linkage that cause alleles to be copied together.
  • The populations are not arbitrarily large. This is crucially important and was underappreciated in early evolutionary theory. The fact that populations are sometimes quite small means that genetic drift can play a major role, including the founder's effect. In small populations, even adaptive traits that increase Er(A) can, through the random walk of the stochastic process, vanish (or likewise for maladaptive traits, dominate) such that the population fixates.
  • The environment, A, is not fixed. In fact, it's changing constantly (though generally slowly compared to the rates of reproduction and death of individual replicators). Moreover, it changes because of the very replicators (and their traits) that we're talking about. And so we have things like evolutionary arms races.
There are others.... (And note that I've ignored sexual reproduction for the time being because it is, perforce, secondary to asexual and because in the view of some theorists (like Richard Dawkins) it is the "selfish gene" that should be considered the replicator, not the sexually-reproducing individual – but enough on that for now).

Regardless, natural selection is useful as a model of how this stuff works and helps us fine-tune our model of replication.

So finally, what's the big deal about natural selection and evolution. Not, I contend, that selection exists – because we see it all around us and can replicate it in controlled experiments whenever we wish. And not the way it specifies the assumptions of our model of replication – while novel in its time, this has been proven much more thoroughly by our modern understanding of molecular biology and genetics. So, what?

To me, the two big ideas of evolution are that:

  1. If you have mechanisms that increase and decrease trait variation in populations, you can account for the diversity of life we see on earth, and
  2. If natural selection is a non-trivial component of the mechanisms that decrease variation, you can account for how it all can look so well-designed.
The first point is critical because any theory has to be able to explain how we ended up with so many wondrous species. With mutation, recombination, gene transfer, and gene flow, you have mechanisms to increase variation in populations. With genetic drift and natural selection, you have mechanisms to decrease it. That, combined with various forms of isolation (e.g. geographic, reproductive), will give rise to diverse species.

But the second is equally important because any theory also needs to explain why everything looks so good at what it does. If there existed completely random mechanisms to increase and decrease variation in populations, you could still achieve some diversity, but it's unlikely that resulting species (assuming they didn't go extinct) would appear adapted to their surroundings. Evolution needs a mechanism to reduce variation – natural selection is one that makes things work better rather than worse.

Anyway, I don't know how clearly I've laid out my understanding. Or how convincing it is. Or how many holes in logic it currently has – it's my first time to write this stuff down, and the impedance mismatch between brain and paper is often higher than it appears. So please, comment away...

Posted by richard at 11:59 PM | Comments (2)

Apropos "absurdity"

In The New Yorker, H. Allen Orr reviews Daniell Dennett's new book, Breaking the Spell: Religion as a Natural Phenomenon. The book, an attempt to look at the phenomenon of religion from a scientific view, resonates with my absurd belief below. (Which is, of course, not surprising since I referenced Dennett myself in my post.) But it's funny how similar it sounds:
According to Dennett, the earliest stages of religion were likely characterized by speculations about supernatural or quasi-natural beings. These questions arose out of an aspect of human nature we take for granted: the recognition that the world contains not only other bodies but also other minds. We recognize, in other words, that the world includes "agents" independent minds that possess their own sets of beliefs and desires. This recognition allows us a wide range of cognitive moves and countermoves presumably unavailable to most other species: "I know he thinks that I have a stone in my hand." The ability to attribute agency is, Dennett says, almost surely an evolutionary adaptation. It is probably encoded genetically in our species (no one taught you that other minds populate the planet), and it plays a key role in everything from fighting ("He doesn't know that I dropped the stone") to seduction ("Would you like to see my cave paintings"). But its appearance during evolution led to an unexpected possibility: attributing agency where no agent exists. Human beings are skilled at positing agents — whispering winds, turnip ghosts, and monsters under the bed — for which the evidence is less than overwhelming, and this tendency might explain why nearly all peoples talk about creatures like elves and goblins. (Emphasis added)
Anyway, it just struck me, coming so closely on the heels of the other post.
Posted by richard at 06:02 PM | Comments (0)

July 20, 2004

Global Warming

The Telegraph (somewhat sensationally) reports on the latest studies about global warming. While I'd hesitate to say that we finally have "the truth about global warming," the study by the Max Planck Institute for Solar System Research in Gottingen, Germany is definitely interesting. Basically, it claims that the sun is burning brighter and emitting more energy now, and over the last 150 years, than it has in over 1000. While it's unclear that this accounts for all or most of the global average temperature increase over the last century, it certainly could account for a significant portion.

In my mind, this is just another example of why the Bush administration was right to scrap Kyoto over the complaints of the transnationalists.

First, as this study shows, we don't know enough about the problem (how big it is, what causes it, the degree to which we can change it) to be imposing drastic, world-economy-shaking limits on carbon emissions. Hell, we've only been able to accurately measure temparature for a couple hundred years. It would suck to waste billions limiting carbon emissions only to find that solar output was the real driver and oh, by the way, it's about to drop and get much colder.

Second, while imposing significant costs, the accords were far from "reversing" the warming of the last century – in fact, they only promised to return developed economies to carbon outputs that were a fraction below 1990 levels.

Third, Kyoto neglected to put caps on developing nations like China and India, which are sure to provide the bulk of the increase over the next 25 years.

We are, in my mind, much better off with a growing economy that can afford the studies necessary to understand the scope of the problem and the R&D to design technology (like new methods of carbon sequestration and massive carbon sinks) to fix it. Human output of carbon has been growing quickly for 150 years and, even if we put the severest curbs in place today, will continue at high levels for decades, if not centuries. The limits of our knowledge and our technology now means that a "reversal" is practically impossible without destroying all of our gains in standard of living since the industrial revolution (which might be the point for some in the anti-globalist crowd).

I am reminded somewhat of supercomputers. Some very smart people at a government lab were working on some very hard modelling problems that require years of computer time to solve. They were trying to decide how big a supercomputer to buy with the money they had in their budget. So, being geeks, they plotted price/performance over time, extrapolated Moore's law and historical trends, and what they found was that they were actually better off not buying a supercomputer now, but waiting a year and buying one then that was much faster. They could then start their calculations 12 months later, but still finish before any computer they could buy today would be able. This is the weird logic of what's possible when the time frames of technological progress outstrip the time frames of the problem to be solved. (In fact, you might argue that this is the very definition of technology, or of a tool.)

There is, of course, a limit to this line of reasoning because if everyone decides to wait a year then the supercomputers of tomorrow will never be built, because no one will invest in them today. But the point still holds: except for the coordination problem, i.e. freeloaders, if we want to solve five-year problems today, we are probably better off getting together and investing in building faster computers for the first 2 years.

My belief is that the same dynamic holds with carbon output – we are better off waiting for the better technology, but we should make sure that some of the returns from our carbon-producing economies today are invested in the right technologies. Kyoto, and the rest of the chicken-little reaction, is not the answer.

Posted by richard at 12:53 AM | Comments (5)

January 06, 2004

Crichton on "Consensus Science"

Michael Crichton has another interesting speech on the role of science in public policy: Aliens Cause Global Warming. While he ratchets up the rhetoric pretty high, he has a point about how science gets used in political debates, particularly the relatively frequent use of "consensus" when instead science should be silent on the issue. The way Bjorn Lomborg was treated was pretty outrageous — when political correctness muzzles scientific research, we have a problem.

To head off a critique from the Wife: yes, I think Crichton does put too much stock in the ideal of "disinterested" science. Science is a social institution, one of many, and while I, on pragmatic and instrumentalist grounds, accept it as one worth protecting and perfecting, it's worth remembering that there was no golden age of disinterested science. It has always been and will always be used to further agendas, some of them dispicable.

That being said, it is worth striving for the unobtainable ideal of pure science and condemning "consensus science" is one way to do that.

Update: here's another speech where he labels "environmenatlism" the new religion. Again, over the top, but some more interesting points.

Posted by richard at 12:58 PM | Comments (1)

December 05, 2003


Just hope they're not talking about someone's grandfather that way: Fossilised crustacean boasts oldest penis

Posted by richard at 05:15 PM | Comments (1)