Wednesday, December 14, 2005


Sorry I missed Monday's entry but a cold came along and took me away to far realms of fever, chills and pain. At my age, some of these colds that come along really hammer me. I told my wife I can understand, if a mere cold can do this to me, how disease can kill, because the body is powerless in the grip of such things. Even cold medicines didn't help much to alleviate the symptoms of this monster cold.


The sophisticated discrimination of observation demonstrated by Richard Dawkins in the following passage from my current reading is precisely why I enjoy reading science. Granted the subtlety of observation demonstrated by a truly masterful creative writer is also awesome, but scientific clarity gets us much closer to a grasp on the realities of the natural world. Now there I go, dismissing my own fields of study in English and Creative Writing when I truly know that emotional realities are quite adequately revealed by good creative writing in a way that no psychological text can do it, that is until the advent of evolutionary psychology.

I also note that I didn't write the page or pages down upon which these paragraphs appear, but they're from THE ANCESTOR'S TALE.

[Open quote.] And now, an important afterthought on evolutionary trees, drawing in lessons from Eve's Tale and the Neanderthal's Tale. We might call it the gibbon's decline and fall of the species tree. We normally assume that we can draw a single evolutionary tree for a set of species. But Eve's Tale told us that different parts of DNA (and thus different parts of an organism) can have different trees. I think this poses an inherent problem with the very idea of species trees. Species are composites of DNA from many different sources. As we saw in Eve's Tale and reiterated in the Neanderthal's Tale, each gene, in fact each DNA letter, takes its own path through history. Each piece of DNA, and each aspect of an organism, can have a different evolutionary tree.

An example of this comes up every day, but familiarity leads us to overlook its message. A Martian taxonomist shown only the genitals of a male human, a female human, and a male gibbon would have no hesitation in classifying the two males as more closely related to each other than either is to the female. Indeed, the gene determining maleness (called SRY has never been in a female body, at least since long before we and the gibbons diverged. Traditionally, morphologists plead a special case for sexual characteristics, to avoid 'nonsensical' classifications. But identical problems arise elsewhere. We saw it previously with ABO blood groups, in Eve's Tale. My B-group gene relates me more closely to a B-group chimpanzee than an A-group human. And it is not just sex genes or blood groups, but all genes and characteristics which are susceptible to this effect, under certain circumstances. The majority of both molecular and morphological characteristics show chimps as our closest relatives. But a sizeable minority show that gorillas are instead, or that chimps are most closely related to gorillas and both are equally close to humans.

This should not surprise us. Different genes are inherited through different routes. The population ancestral to all three species will have been diverse—each gene having many different lineages. It is quite possible for a gene in humans and gorillas to be descended from one lineage, while in chimps it is descended from a more distantly related one. A11 that is needed is for anciently diverged genetic lineages to continue through to the chimp-human split so humans can descend from one and chimps from another.

So we have to admit that a single tree is not the whole story. Species trees can be drawn, but they must be considered a simplified summary of a multitude of gene trees. I can imagine interpreting a species tree in two different ways. The first is the conventional genealogical interpretation. One species is the closest relative of another if, out of all the species considered, it shares the most recent common genealogical ancestor. The second is, I suspect, the way of the future. A species tree can be seen as depicting the relationships among a democratic majority of the genome. It represents the result of a 'majority vote' among gene trees.

The democratic idea—the genetic vote—is the one that I prefer. In this book, all relationships between species should be interpreted in this way. All the phylogenetic trees I present should be viewed in this spirit of genetic democracy, from the relationships between apes to the relationships between the animals, plants, fungi and bacteria. [Close quote.]

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