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The Pattern of Evolution

The Pattern of Evolution, Niles Eldredge, Freeman, 1999, ISBN 0-7167-3064-4, hardcover, 219pp


This book appears to be fairly light; it is written in a casual style and does not go into any great depth. Much of the material will be familiar to readers who have read other popular works on evolution and paleontology. Appearances are deceptive; there is an important thesis presented in the book.

Niles Eldredge is the more modest of the punc eek twins, Gould and Eldredge, at least if one judges by media presence and number of books written. He and Gould are opposite sides of a fence from Dawkins. The nature of this dividing line is fairly obscure except to the participants who seem to be clear in their minds that the other side is hopelessly wrong headed. The label for the fence, at least on the Gould/Eldredge side, is hyper-darwinism. The dividing line seems to be a matter of focus, one side (Dawkins) focussing on evolution in the small with an emphasis on theory, and the other side (Gould and Eldredge) focussing on evolution in the large.

Eldredge opens his thesis with Mayr's observation that the sciences can be divided into the functional and historical sciences. The functional sciences focus on physical law, e.g., physics and chemistry. The historical sciences focus on the reconstruction of the past, e.g., paleontology and geology. One can quarrel with this dichotomy - there is a difference between observational sciences which focus on the present and historical sciences which focus on the reconstruction of the past. Moreover the observational and historical sciences are paired with sciences which focus on process rather than on observed happenstance. For example, geology is paired with geophysics and paleontology is paired with taphonomy.

The sciences may not quite fit into Mayr's neat formula but there is a considerable difference between sciences which focus on general law and those which focus on particulars of the universe. The perception of this difference is the basis for the well known "physics envy" which, in turn, leads to efforts to cast various sciences into the style of physics. Evolution is, par excellence, a historical science. Eldredge suggests that there are two roads to rescuing the study of evolution from being a second class citizen among the sciences.

One road is the low road, that taken by Dawkins, John Maynard Smith, and the microbiologists. The gene, microbiochemistry, and population genetics is all. "Change in allele frequencies in populations" is the watchword and real plants, animals, and history are replaced with mathematical plants, animals, and game theory.

The other road, that which Eldredge proposes to take, is the high road. On that road one looks at life in all its bewildering complexity and surprising turns of history, attempts to ascertain what patterns may be found in the large, and from those patterns deduce general principles.

Not surprisingly, Eldredge focuses on punctuated equilibrium. Gould and Eldredge established twenty odd years ago that there is a normal pattern in the history of species called punctuated equilibrium. A species will arise fairly rapidly (in terms of geological time) and then remain almost unchanged for the rest of its history, a matter of some millions of years for successful species.

It turns out that the thing that needs to be explained is the species stasis. Life can evolve quite rapidly in favorable circumstances - in one of the African rift lakes some 200 distinct species of cichlid fish arose within 12,000 years. The travellers on the low road have established that evolution of the genome goes at a good pace.

It must be kept in mind that the geneticists and the paleontologists are talking about two different things when they talk about evolution. The geneticist is talking about change in the collective genomes of populations over time, i.e., evolution of the genotype. The paleontologist (and biologists generally) are talking about changes in the physical morphology in species over time, i.e., evolution of the phenotype. The shape of the body of an organism, its phenotype, is a product of its genotype and its environment, sometimes called nature versus nurture. Environmental influences cannot be inherited; it is only the genome that can truly evolve.

We have here the appearance of a paradox: The collective genotype of a species changes over time; the species phenotype does not. However the species phenotype is the product of the genotype. If the genotype changes over time why doesn't the phenotype also change over time?

The answer to this paradox lies in the nature of possible changes to the genome and to the way natural selection works. Natural selection, as worked out by Darwin, operates on the phenotype. Those organisms which are reproductively fit, i.e., those organisms which survive and reproduce, are the ones which pass on their genes to the next generation. Most changes to the genome are selectively neutral, i.e., they have no perceptible affect on the morphology and hence are invisible to natural selection.

The question to be answered is: Why does natural selection act to preserve the species phenotype once a species is established? It is this question that The Pattern of Evolution seeks to answer.

In earlier works Gould and Eldredge have proposed various mechanisms for explaining stasis. The most important of these was habitat tracking. The idea is that species tend to respond to environmental change by moving rather by adaptation. Thus, as the climate warms, semi-tropical plants and animals move into the temperate zone. When habitat tracking is not possible species usually become extinct. The principle is sound enough: Moving is easier and faster than evolving. Eldredge concedes that habitat tracking cannot be a complete explanation for stasis. Indeed it is not; is explains changing ranges of habitation but not stasis.

In the present work Eldredge takes another tack. He introduces the claim that ecologies exhibit stasis as well as species. Now this points to a common failing in discussions of evolution, namely the failure to recognize that species do not exist in an abiotic vacuum but rather within the context of local ecologies. Life lives within life.

He goes on to make the obvious point that the species of life are organized within two hierarchies, one genealogical and one economic. Much of the focus in evolutionary theory is on genealogy and reproduction; this may reflect the obsession of our species with our exotic and erotic sex life. In truth, though, reproduction is a sometime thing; in many species it is a once in a life time affair. The principal business of life is business, the securing of the next meal. Fitness in reproductive capacity, in mating and producing the next generation, is necessary and natural selection selects only those who are reproductively fit. Economic fitness, the ability to secure food, to eat, and to live until the next day, is far more pervasive; the dead do not breed. Economic fitness, in turn, depends principally on how well one fits into the great chain of eaters and the eaten.

This is, I think, a key insight - one cannot understand evolution in the large until one understands how ecologies organize and evolve. The true weakness of the book, in my opinion, is that Eldredge does not do anything with the insight. Having arrived at the insight he simply stops. The light bulb flashes but the scene is not illuminated.


This page was last updated February 10, 1999.

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