2001 : WHAT QUESTIONS HAVE DISAPPEARED?

Different people on the Edge list seem to have chosen to understand 'questions that have disappeared' in three very different senses:

1. Questions that were once popular but have now been answered 
2. Questions that should never have been asked in the first place 
3. Questions that have disappeared although they never received a satisfactory answer.

This third meaning is, I suspect, the one intended by the organizer of the forum.  It is the most interesting of the three since it suggests real science that we should now be doing, rather than just raking over the historical coals.

The three meanings are too disparate to bring together easily, but I'll try. The popular question 'Has there been enough time for evolution to take place?'  can now confidently be answered in the affirmative.  It should never have been put in the first place since, self-evidently, we are here. But what is more interesting is that the real question that faces us is almost the exact opposite.  Why is evolution so slow, given that natural selection is so powerful?   Far from there being too little time for evolution to play with, there seems to be too much.

Ledyard Stebbins did a theoretical calculation about an extremely weak selection pressure, acting on a population of mouse-sized animals to favor the largest individuals.  His hypothetical selection pressure was so weak as to be below the threshold of detectability in field sampling studies.  Yet the calculated time to evolve elephant-sized descendants from mouse-sized ancestors was only a few tens of thousands of generations: too short to be detected under most circumstances in the fossil record.  To exaggerate somewhat, evolution could be yo-yo-ing from mouse to elephant, and back again, so fast that the changes could seem instantaneous in the fossil record.

Worse, Stebbins's calculation assumed an exceedingly weak selection pressure.  The real selection pressures measured in the field by Ford and his colleagues on lepidoptera and snails, by Endler and his colleagues on guppies, and by the Grants and their colleagues on the Galapagos finches, are orders of magnitude stronger.  If we fed into the Stebbins calculation a selection pressure as strong as the Grants have measured in the field, it is positively worrying to contemplate how fast evolution could go.  The same conclusion is indirectly suggested by domestic breeding.  We have gone from wolf to Yorkshire terrier in a few centuries, and could presumably go back to something like a wolf in as short a time.

It is indeed the case that evolution on the Galapagos archipelago has been pretty fast, though still nothing like as fast as the measured selection pressures might project.  The islands have been in existence for five million years at the outside, and the whole of their famous endemic fauna has evolved during that time.  But even the Galapagos islands are old compared to Lake Victoria.  In the less than one million years of the lake's brief lifetime, more than 170 species of the genus Haplochromis alone have evolved.

Yet the Coelacanth Latimeria, and the three genera of lungfish, have scarcely changed in hundreds of millions of years.  Surviving Lingula ('lamp shells') are classified in the same genus as their ancestors of 400 million years ago, and could conceivably interbreed with them if introduced through a time machine.  The question that still faces us is this.  How can evolution be both so fast and so leadenly slow?  How can there be so much variance in rates of evolution?  Is stasis just due to stabilizing selection and lack of directional selection?  Or is there something remarkably special going on in the (non) evolution of living fossils?  As William Blake might have written to a coelacanth: Did he who made the haplochromids make thee?

RICHARD DAWKINS is an evolutionary biologist and the Charles Simonyi Professor For The Understanding Of Science at Oxford University; Fellow of New College; author of The Selfish Gene, The Extended Phenotype , The Blind Watchmaker, River out of Eden (ScienceMasters Series), Climbing Mount Improbable, and Unweaving the Rainbow.