Conrad Waddington demonstrated a phenomenon called `genetic assimilation'. He started with a genetically diverse population of wild fruit flies, and found that about one in 15,000 of their pupae, when warmed, produced a fly with no cross-vein in its wing. These 'crossveinless' flies looked just like some very rare mutant flies that turned up occasionally in the wild, just as occasional genetically phocomelic children turned up before thalidomide. By breeding from the flies that responded to the treatment, Waddington selected for a lower and lower threshold of response. In a few tens of generations, he had selected flies that bred true for the cross-veinless trait, exhibiting it regularly without anyone warming the pupae. This may look like Lamarckian inheritance, but it's not. It's genetic assimilation. The experiments were selecting flies that had no cross-vein at lower and lower temperature thresholds. Eventually, they selected flies that had no cross-vein at `normal' temperatures.
Similarly, genetic assimilation provides a much better explanation than Lamarckism for the phocomelic children of thalidomidemodified parents. We have selected, from some 4 million foetuses, those that respond to thalidomide with phocomelia. It is not surprising that when they marry each other, they produce a few progeny whose threshold is very low - below zero in fact. They are so liable to produce phocomelia that they do it without thalidomide, just as Waddington's flies came to produce cross-veinlessness without warming the pupae.
One of the things that really worried Darwin was the existence of parasitic wasps - a fact that has influenced our Discworld tale, but has gone unremarked until now in the scientific commentaries. Parasitic wasps lay their eggs in other insects' larvae, so that as the wasp eggs grow into wasp larvae, they eat their hosts. Darwin could see how this might have happened on evolutionary grounds, but it seemed to him to be rather immoral. He was aware that wasps don't have a sense of morality, but he saw it as some kind of flaw on the part of the wasps' creator. If God designed each species on Earth, for a special purpose - which is what most people believed at the time - then God had deliberately designed parasitic wasps, whose purpose was to eat other species of insect, also designed by God. To be so eaten, presumably.
Darwin was fascinated by such wasps, ever since he first encountered them in Botafogo Bay, Brazil. He eventually satisfied himself - though not his successors - that God had found it necessary to permit the existence and evolution of parasitic wasps in order to get to humans. This is what the quote at the end of Chapter 10 alludes to. That particular explanation has fallen out of favour among biologists, along with all theist interpretations. Parasitic wasps exist because there is something for them to parasitise - so why not? Indeed, parasitic wasps play a major role in controlling many other insect populations: nearly one-third of all of the insect populations that humans like to label `pests' are kept at bay in this manner. Maybe they were created in order for humans to be possible ... At any rate, the wasps that so puzzled Darwin still have much to tell us, and the latest discovery about them threatens to overturn several cherished beliefs.
Strictly, the discovery is not so much about the wasps, as about some viruses that infect them ... or are symbiotic with them. They are called polydnaviruses.
When mother wasp injects her eggs into some unsuspecting larva, such as a caterpillar, she also injects a solid dose of viruses, among them said polydnaviruses. The caterpillar not only gets a parasite, it gets an infection. The virus's genes produce proteins that interfere with the caterpillar's own immune system, stopping it reacting to the parasite and, perhaps, rejecting it. So the wasp larvae munch merrily away on the caterpillar, and in the fullness of time they develop into adult wasps.
Now, any self-respecting adult parasitic wasp obviously needs its own complement of polydnaviruses. Where does it get them? From the caterpillar that it fed on. And it gets them (just as mother did) not as a separate infective `organism', but as what is called a provirus: a DNA sequence that has been integrated into the wasp's own genome.
Many genomes, probably most if not all, include various bits of viruses in this way. Our own certainly does. Transport of DNA by viruses seems to have been an important feature of evolution.
In 2004 a team headed by Eric Espagne worked out the DNA sequence of a polydnavirus - as one does - and what they found was dramatically different from what anyone had expected. Typical virus genomes are very different from those of 'eukaryotes'- organisms whose cells have a nucleus, which includes most multi-cellular creatures and many single-celled ones, but not bacteria. The DNA sequences of most eukaryote genes consist of `exons', short sequences that collectively code for proteins, separated by other sequences called introns, which get snipped out when the code is turned into the appropriate protein. Viral genes are relatively simple, and typically they do not contain introns. They consist of connected code sequences that specify proteins.-This particular polydnavirus genome, in contrast, does contain introns, quite a lot of them. The genome is complex, and looks much more like a eukaryote genome than a virus genome. The authors conclude that polydnavirus genomes constitute `biological weapons directed by the wasps against their hosts'. So they look more like the enemy's genome than that of an ordinary virus.
Numerous examples, old and new, disprove every aspect of the folk version of evolution and DNA. We end with one that looks especially important, discovered very recently, and whose significance is just becoming seriously apparent to the biological community. It is probably the most severe shock that cell biology has received since the discovery of DNA and the wonderful `central dogma': DNA specifies messenger-RNA which specifies proteins. The discovery was not made through some big, highly publicised research programme like the human genome project. It was made by someone who wondered why his petunias had gone stripy. When all the world is chasing `the' human genome, it's not easy to get research grants to work on stripy petunias. But what the petunias revealed is probably going to be far more important for medicine than the entire human genome project.
Because proteins are the structure of living creatures, and because as enzymes they control the processes of life, it has seemed obvious that DNA controls life, that we can `map' DNA code on to all the important living functions. We could assign a function to each protein, so we could assume that the DNA that coded for that protein was ultimately or fundamentally responsible for the corresponding function. Dawkins's early books reinforced the idea of one gene, one protein, one function (although he carefully warned his readers that he didn't want to give that impression), and this encouraged such media exaggerations as calling the human genome the Book of Life. And the `selfish gene' image made it entirely credible that huge stretches of the genome were present for solely selfish reasons - that is, for no reason related to the organism concerned.
Biologists employed - as so many now are - in the biotechnology industries serving agriculture, pharmacy, medicine, even some engineering projects (we don't mean just `genetic engineering' but making better motor oils), all subscribe to the central dogma, with a few minor modifications and exceptions. All of them have been informed that nearly all of the DNA in the human genome is `junk', not coding for proteins, and that although some of it may be important for developmental processes or for controlling some of the `real' genes, they really don't need to worry about it.