Monday, March 26, 2007

Organisms, Groups, and Memes: Vehicles or Replicators? Part 2

I have never understood Punctuated Equilibrium in the manner that Dawkins, in his chapter, spends considerable time refuting. Evolution is gradualist, as Dawkins says, in the sense that there is no such thing as half an animal. A "transitional fossil" can be a confusing concept, giving one the idea that certain animals are "transitional" as opposed to being "normal" (when in fact every living thing is "transitional"). This misunderstanding is exploited by creationists in their tiresome demand for "transitional" fossils.

P.E. is, essentially, a reinterpretation of the fossil record which turns out not to be as radical or revolutionary as Gould portrayed it to be. Dawkins likens the difference between previous interpretations and the Gould-Eldredge proposal as the difference between walking up a hillside and ascending a staircase. There are no "gaps" in the fossil record, only steps.

Dawkins explores Gould and Eldredge's equation: P.E. + Wright's Rule = Species Selection

Wright's Rule: "The proposition that a set of morphologies produced by speciation events is essentially random with respect to the direction of evolutionary trends within a clade." In other words, in an apparent analogy with mutational randomness, if a species of horse shows marked progression toward larger body size, a new species of horse would not necessarily follow this trend, if Wright's Rule is preserved. As many new species would be smaller than the clade as larger. Therefore, Wright's Rule must be tested.

However, Dawkins points out (quoting Gould): "'If Wright's Rule be valid, and new species of horses arise equally often at sizes smaller and larger than their ancestors, then the trend is powered by species selection. But if new species arise preferentially at sizes larger than their ancestors, then we don't require species selection at all, since random extinction would still yield the trend.' Gould here simultaneously sticks his neck out and hands Occam's Razor to his opponents!" Dawkins adds that he can imagine some major speciational trends of the type described by Cope's Rule,* but points out that this is quite different that the original question, that of individual sacrifice behavior or adaptations "for the good of the species." The difference is between the concept of the group as replicator versus the group as a vehicle for replicators.

Palaeontological major trends, such as the length of horses' legs, are simple, requiring only a few replicator replacements, whether we mean genes or species at this point. However, many more replicator replacements are required for the evolution of a complex adaptation such as those that allowed a land mammal to evolve into whales, for example. Even if such a complex phenomenon could be broken down into smaller evolutionary events, the adaptation involves an interrelated web of changes that group-selectionism just could not produce them without yielding, over and over again, a highly unlikely barrage of parallel beneficial trends. (Indeed, this mistaken idea of group-selectionism could be what some creationists imagine evolution to be, leading them to dismiss evolution as improbable.)

Dawkins writes:

The theory of species selection, growing out of that of punctuated equilibria, is a stimulating idea which may well explain some single dimensions of quantitative change in macroevolution. I would be very surprised if it could be used to explain the sort of complex multidimensional adaptation that I find interesting, the 'Paley watch," or 'Organs of extreme Perfection and Complication' kind of adaptation that seems to demand a shaping agent at least as powerful as a deity. Replicator selection, where the replicators are alternative allels, may well be powerful enough. If the replicators are alternative species, however, I doubt if it is powerful enough, because it is too slow. Eldredge and Cracraft (1980, p. 269) appear to agree.

Part 3: Memes

Monday, March 19, 2007

Open Thread on Global Warming

Hello everyone:

While I finally work on my upcoming installment of The Extended Phenotype, you may have at it about global warming. Just a reminder: I am going to be a lot more strict about enforcing the respectful comments rule than I have been at Amused Muse. The subject is global warming - not each other. Puh-lease! Thank you so much. :-)

Friday, February 23, 2007

First Woman to Win Turing Award

(AP) -- One of the most prestigious prizes in computing, the $100,000 Turing Award, went to a woman Wednesday for the first time in the award's 40-year history.

Frances E. Allen, 74, was honored for her work at IBM Corp. on techniques for optimizing the performance of compilers, the programs that translate one computer language into another.
This process is required to turn programming code into the binary zeros and ones actually read by a computer's colossal array of minuscule switches.

Allen joined IBM in 1957 after completing a master's degree in mathematics at the University of Michigan. At the time, IBM recruited women by circulating a brochure on campuses that was titled "My Fair Ladies."

Read the rest here.

Tuesday, February 13, 2007

Organisms, Groups, and Memes: Vehicles or Replicators? Part 1

An active replicator is that which exerts at least some kind of influence over the likelihood of its being copied. A DNA molecule is a replicator; a piece of sheet music considered valuable enough to be xeroxed can be considered a replicator (Dawkins, p. 83); but an individual organism, sexual or asexual, cannot be considered a replicator.

Here's why.

Because we reject Lamarckism, we know that characteristics acquired during an organism's life cannot be passed down to its offspring. Even in the case of asexual reproduction in which the entire genome is passed down, any acquired features--a lost limb, developmental factors, etc.--is not replicated in the lineage, and in order for the organism as a whole to be called a replicator it must pass these on. "There is a causal arrow," writes Dawkins, "going from gene to bird, but none in the reverse direction." However, the genome of an organism that reproduces asexually could be a candidate for the term "replicator."

Can a species be considered a replicator? Again, Dawkins points out that a species, being mutable, is not comparable to a gene and its alleles. However, the gene pool of a reproductively isolated species could be a candidate.

In examining this possibility, Dawkins explores "differential lineage extinction"--for example, the statistical rates of extinction of ammonites and bivalves that have a high rate of evolving a larger size, say, than those who do not (those that more rapidly increase their size over successive generations are also more likely to die off). However, while these different rates of extinction are a form of selection, they do not drive "progressive evolutionary change" and thus these lineages are not replicators, either. They are merely, as Dawkins terms them, "survivors."

(And at this point, people, I confess I must read the chapter again.)

Part II: Punctuated Equilibrium and Memes

Sunday, January 28, 2007

And I Was Going For LeGuin!

I am:
Samuel R. "Chip" Delany
Few have had such broad commercial success with aggressively experimental prose techniques.

Which science fiction writer are you?

Wednesday, January 17, 2007

The Active Germ-Line Replicator

What is a "gene"? Dawkins admits to using the term loosely. In this chapter he tries to nail down the term more precisely, in order to give a clearer, "gene's eye-view" of the replicator as the unit of selection.

In 1957 S. Benzer split the gene into three units: the muton (minimum unit of mutational change); the recon (minimum unit of recombination); and the cistron (roughly equivalent to the unit need for one polypeptide synthesization). Dawkins proposes the optimon (and Ernst Mayr the selectron) as the unit of natural selection.

Dawkins briefly describes Gould's objection to his proposal, and critiques Gould't characterization of the selfish gene replicator as located "below" the level of the individual (which for Gould was the unit of selection), as the species group-unit is located "above" the level of the individual. Dawkins points out that this "single-dimensional ladder" analogy is not apt in that genes are replicators, whereas vehicles (individuals and species) are not. Individual vehicles are selected, but they are not replicators (more on this later). Likewise, replicators are selected by proxy as individuals (vehicles) are selected.

The issue of species versus individual selection is a question of what is to be properly considered a vehicle; the issue of individual/species versus gene selection is a question of what is to be considered a replicator.

"I define a replicator as anything in the universe of which copies are made," says Dawkins. These may be active (displaying some ability to influence their being copied) or passive, "germ-line" (such as gametes), or "dead-end" (somatic). It is the active, germ-line replicator that Dawkins names as his optimon.

Because chromosomes, and the genes within them, are subject to splitting due to crossing-over in sexual reproduction, replicators may consist of various codon lengths, but, depending upon their length and the strength of the particular selective pressure on them, various replicators will have different "half-lives," and the most successful replicators will, by exerting their phenotypic effects, have the longest half-lives with respect to their alleles.

It now becomes obvious why no individual, nor even an individual's genome, can be considered a replicator.

In the next chapter, Dawkins explains what individuals and species are, and why species also cannot be defined as replicators.