Wednesday, February 01, 2006


The following paragraphs are from Richard Dawkin's book THE ANCESTOR'S TALE (what else?) and commence on page 359. Again Dawkins argues for the idea that evolution is more about genetic change than the morphological change that fossils and taxonomy reveal. It takes the whole argument out of the primitive hands of the Creationists who still like to sound the "missing link" sound bite meme. Anybody who says "missing link" so reveals their ignorance of the field of evolution that one can only laugh. Just last week a home-schooling mother in a letter to the editor crowed that she teaches her kids science and that the missing link still refutes natural selection. She actually thinks she's educating her kids. Poor dumb kids born to that mother. They might be loved, but they'll be ignorant as door posts. Yes, I know, to be loved is a wonderful thing even if your brain remains as inactive as a stone.


THE REASON IT FALLS to the lamprey to tell this tale will be revealed at the end. It is a reprise on a theme we have met before: there is a separate gene's-eye view of ancestry and pedigree that is surprisingly independent of the view we get when we think about family trees in more traditional ways.

Haemoglobin is well known as the vitally important molecule that carries oxygen to our tissues and gives our blood its spectacular colour. Human adult haemoglobin is actually a composite of four protein chains called globins, knotted around each other. Their DNA sequences show that the four globin chains are closely related to each other, but they are not identical. Two of them are called alpha globins (each a chain of 141 amino acids), and two are beta globins (each a chain of 146 amino acids). The genes coding for the alpha globins are on our chromosome 11; those coding for the beta globins are on chromosome 16. On each of these chromosomes there is a duster of globin genes in a row, interspersed with some junk DNA that is never transcribed. The alpha cluster, on chromosome 11, contains seven globin genes. Four of these are pseudogenes—disabled versions of alpha with faults in their sequence, never translated into protein. Two are true alpha globins, used in the adult. The final one is called zeta, and it is used only in embryos. The beta cluster, on chromosome 16, has six genes, some of which are disabled, and one of which is used only in the embryo. Adult haemoglobin, as we've seen, contains two alpha and two beta chains, wrapped around each other to form a beautifully functioning parcel.

Never mind all this complexity. Here's the fascinating point. Careful letter-by-letter analysis shows that the different kinds of globin genes are literally cousins of each other—members of a family. But these distant cousins still coexist inside you and me. They still sit side by side with their cousins inside every cell of every warthog and every wombat, every owl and every lizard.

On the scale of whole organisms, of course, all vertebrates are cousins Of each other too. The tree of vertebrate evolution is the family tree we are all familiar with, its branch-points representing speciation events— the splitting of species into daughter species. In reverse, they are the rendezvous points that punctuate this pilgrimage. But there is another family tree occupying the same time scale, whose branches represent not speciation events but gene duplication events within genomes. And the branching pattern of the globin tree looks very different from the branching pattern of the family tree, if we trace it in the usual, orthodox way, with species branching to form daughter species. There is not just one evolutionary tree in which species divide and give rise to daughter species. Every gene has its own tree, its own chronicle of splits, its own catalogue of close and distant cousins.

The dozen or so different globins inside you and me have come down to us through the entire lineage of our vertebrate ancestors. About half a billion years ago, in a jawless fish perhaps like a lamprey, an ancestral globin gene accidentally split in two, both copies remaining in different parts of that fish's genome. There were then two copies of it, in different parts of the genome of all descendant animals. One copy was destined to give rise to the alpha cluster, on what would eventually become chromosome 11 in our genome, the other to the beta duster, now on our chromosome 16. There is no point in trying to guess which chromosome either of them sat on in the intermediate ancestors. The locations of recognizable DNA sequences, indeed the number of chromosomes into which the genome is divided, are shuffled and changed with surprisingly gay abandon. Chromosome numbering systems, therefore, do not generalize across animal groups.

As the ages passed, there were further duplications, and doubtless some deletions as well. Around 400 million years ago the ancestral alpha gene duplicated again, but this time the two copies remained near neighbors of each other, in a cluster on the same chromosome. One of them was destined to become the zeta of our embryos, the other became the alpha globin genes of adult humans (further branching gave rise to the non-functional pseudo genes I mentioned). It was a similar story along the beta branch of the family, but with duplications at other moments in geological history.

Now here's a fascinating point. Given that the split between the alpha cluster and the beta cluster took place half a billion years ago, it will of course not be just our human genomes that show the split, and possess both alpha genes and beta genes in different parts of our genomes. We should see the same within-individual split if we look at the genomes of any other mammals, at birds, reptiles, amphibians or bony fish—for our common ancestor with all of them lived less than 500 million years agO. Wherever it has been investigated, this expectation has proved correct Our greatest hope of finding a vertebrate that does not share with us the ancient alpha/beta split would be a jawless fish like a lamprey or a hagfish, for they are our most remote cousins among surviving vertebrates. They are the only surviving vertebrates whose common ancestor with the rest is sufficiently ancient that it could have predated the alpha/ beta split. Sure enough, these jawless fish are the only known vertebrates that lack the alpha/beta divide. Rendezvous 22 is so ancient, in other words, that it predated the split between alpha and beta globin.

Something like the Lamprey's Tale could be told for each one of our genes, for they all, if you go back far enough, owe their origin to the splitting of some ancient gene. And something like this entire book could be written for each gene. [Close quote.]

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