Dawkins, Richard (2009). The Greatest Show on Earth. New York: Free. 2011. ISBN 9781439164730. Pagine 470. 18.00 $
Durante il mese di dicembre del 2009 ho scritto due post (Pensare e studiare e Analogo e omologo) in cui anticipavo che stavo leggendo The Greatest Show on Earth, che stavo accumulando ritardi nelle mie recensioni su questo blog ma che avrei presto recuperato il ritardo. Eccomi qui: più di 2 anni.
Naturalmente, non sono più in grado di darvi le mie impressioni a caldo, e siete anche liberi di dubitare che mi ricordi tutti i dettagli del libro.
È ormai parecchio tempo che la produzione di Dawkins, anche quella rivolta al pubblico non specialistico, si è via via allontanata dalla frontiera della scoperta scientifica (The Selfish Gene, tanto per capirci, presentava idee piuttosto nuove al di fuori della cerchia degli addetti ai lavori, e infatti fece piuttosto discutere alla sua uscita: ne abbiamo parlato, anche se un po’ obliquamente, a proposito del recente libro di Trivers) per diventare un elegante apologeta delle teorie evoluzionistiche, anche e soprattutto contro l’attacco della destra religiosa americana e dei “creazionisti.”
Nei suoi libri precedenti sull’evoluzione – spiega l’autore nella prefazione – si era concentrato su temi quali la proposta di un punto di vista “non familiare” sulla familiare teoria dell’evoluzione (The Selfish Gene e The Extended Phenotype), la rimozione di ostacoli specifici alla comprensione dell’evoluzione stessa, cercando di rispondere a domande del tipo “a che serve mezzo occhio o mezza ala? come può funzionare l’evoluzione, se la maggior parte delle mutazioni sortisce effetti negativi?” (The Blind Watchmaker, River Out of Eden e Climbing Mount Improbable) e un pellegrinaggio chauceriano alla ricerca dei nostri antenati (The Ancestor’s Tale), in tutti i casi e sempre dando per scontati il fatto e la verità dell’evoluzione. In questo libro, Dawkins spiega che quella evoluzionistica non è “solo una teoria”, ma un fatto incontrovertibile:
This book is my personal summary of the evidence that the ‘theory’ of evolution is actually a fact – as incontrovertible a fact as any in science. [p. vii]
Dawkins stesso illustra molto bene il senso del suo libro in questa (lunga) intervista rilasciata in occasione dell’uscita del libro:
Come sempre con Dawkins, la lettura è appassionante anche se – a tratti – l’argomentazione è meno densa che in precedenti occasioni. Ma non saprei dire, sinceramente, se la stanchezza è la mia o la sua.
Non aggiungo altro di mio se non una serie di passi che (ormai oltre 2 anni fa) mi ero segnato. Vi invito a leggerli, però, questa volta, perché così vi renderete conto di come continuino a brillare la sua argomentazione scintillante e il suo talento per la spiegazione.
Plants have an energy economy and, as with any economy, trade-offs may favour different options under different circumstances. That’s an important lesson in evolution, by the way. Different species do things in different ways, and we often won’t understand the differences until we have examined the whole economy of the species. [p. 49]
Lenski’s research shows, in microcosm and in the lab, massively speeded up so that it happened before our very eyes, many of the essential components of evolution by natural selection: random mutation followed by non-random natural selection; adaptation to the same environment by separate routes independently; the way successive mutations build on their predecessors to produce evolutionary change; the way some genes rely, for their effects, on the presence of other genes. [p. 130]
It is hard to measure degrees of resemblance. And there is in any case no necessary reason why the common ancestor of two modern animals should be more like one than the other. If you take two animals, say a herring and a squid, it is possible that one of them resembles the common ancestor more than the other, but it doesn’t follow that this has to be the case. There has been an exactly equal amount of time for both to have diverged from the ancestor, so the prior expectation of an evolutionist might be, if anything, that no modern animal should be more primitive than any other. we might expect both of them to have changed to the same extent, but in different directions, since the time of the shared ancestor. […] Notice especially that ‘primitive’ in the sense of ‘resembling ancestors’ does not have to go with ‘simple’ (meaning less complex). A horse’s foot is simpler than a human foot (it has only a single digit instead of five, for example), but the human foot is more primitive (the ancestor that we share with horses had five digits, as we do, so the horse has changed more). [p. 157]
In animals, unlike bacteria, gene transfer seems almost entirely confined to sexual congress within species. Indeed, a species can pretty well be defined as a set of animals that engage in gene transfer among themselves. […] My colleague Jonathan Hodgkin, Oxford’s Professor of Genetics, knows of only three tentative exceptions to the rule that gene transfer is confined within species: in nematode worms, in fruit flies, and (in a bigger way) in bdelloid rotifers.
This last group is especially interesting because, uniquely among major groupings of eucaryotes, they have no sex. Could it be that they have been able to dispense with sex because they have reverted to the ancient bacterial way of exchanging genes? [p. 303]
[…] two organs – for example, bat hand and human hand – are homologous if it is possible to draw one on a sheet of rubber and then distort the rubber to make the other one. Mathematicians have a word for this: ‘homeomorphic’.
Zoologists recognized homology in pre-Darwinian times, and pre-evolutionists would describe, say, bat wings and human hands as homologous. If they had known enough mathematics, they would have been happy to use the word ‘homeomorphic’. In post-Darwinian times, when it became generally accepted that bats and humans share a common ancestor, zoologists started to define homology in evolutionary terms. Homologous resemblances are those inherited from a shared ancestor. The word ‘analogous’ came to be used for resemblances due to shared function, not ancestry. [pp. 312-313]
The often-quoted figure of about 98 per cent for the shared genetic material of humans and chimps actually refers neither to numbers of chromosomes nor to numbers of whole genes, but to number of DNA ‘letters’ (technically, base pairs) that match each other within the respective human and chimp genes. [p. 318]
A new mutation, if it is genuinely new, will have a low frequency in the gene pool. If you revisit the gene pool a million years later, it is possible that the mutation will have increased in frequency to 100 per cent or something close to it. If that happens, the mutation is said to have ‘gone to fixation’. […] The obvious way for a mutation to go to fixation is for natural selection to favour it. But there is another way. It can go to fixation by chance […] , given a large enough number of generations. And geological time is vast enough for neutral mutations to go to fixation at a predictable rate. The rate at which they do so varies, but is characteristic of particular genes, and, given that most mutations are neutral, this is precisely what makes the molecular clock possible. [p. 335]
When we are talking about natural selection, we think in terms of rare beneficial mutations turning up and being positively favoured by selection. But most mutations are disadvantageous, if only because they are random and there are many more ways of getting worse than there are ways of getting better.
Note: This is especially true of mutations of large effect. Think of a delicate machine, like a radio or a computer. Al large mutation is equivalent to kick it with a hobnailed boot, or cutting a wire at random and reconnecting it in a different place. It just might improve its performance, but it is not very likely. A small mutation, on the other hand, is equivalent to making a tiny adjustment to, say, one resistor, or to the tuning knob of a radio. The smaller the mutation, the more closely the probability of improvement approaches 50 per cent. [p. 352]
In a typical mature forest, the canopy can be thought of as an aerial meadow, just like a rolling grassland prairie, but raised on stilts. The canopy is gathering solar energy at much the same rate as a grassland prairie would. But a substantial proportion of the energy is ‘wasted’ by being fed straight into the stilts, which do nothing more useful than loft the ‘meadow’ high in the air, where it picks up exactly the same harvest of photons as it would – at far lower cost – if it were laid flat on the ground.
And this brings us face to face with the difference between a designed economy and an evolutionary economy. in a designed economy there would be no trees, or certainly no very tall trees: no forest, no canopy. Trees are a waste. Trees are extravagant. Tree trunks are standing monuments to futile competition – futile if we think in terms of a planned economy. But the natural economy is not planned. Individual plants compete with other plants, of the same and other species, and the result is that they grow taller and taller, far taller than any planner would recommend. Not indefinitely taller, however. There comes a point when growing another foot taller, although it confers a competitive advantage, costs so much that the individual tree doing it actually ends up worse off than its rivals that forgo the extra foot. It is the balance of costs and benefits to the individal trees that finally determines the height to which trees are press to grow, not the benefits that a rational planner could calculate for the trees as a group. And of course the balance ends up at a different maximum in different forests. The Pacific Coast redwoods (see them before you die) have probably never been exceeded.
Imagine the fate of a hypothetical forest – let’s call it the Forset of Friendship – in which, by some mysterious concordat, all the trees have somehow managed to achieve the desiderable aim of lowering the entire canopy to 10 feet. The canopy looks just like any other forest canopy except that it is only 10 feet high instead of 100 feet. From the point of view of a planned economy, the Forest of Friendship is more efficient as a forest than the tall forests with which we are familiar, beacause resources are not put into producing massive trunks that have no purpose apart from competing with other trees.
But now, suppose one mutant tree were to spring up in the middle of the Forest of Friendship. This rogue tree grows marginally taller than the ‘agreed’ norm of 10 feet. Immediately, this mutant secures a competitive advantage. Admittedly, it has to pay the cost of the extra length of the trunk. But it is more than compensated, as long as all other trees obey the self-denying ordinance, because the extra photons gathered more than pay the extra cost of lengthening the trunk. Natural selection therefore favours the genetic tendency to break out of the self-denying ordinance and grow a bit taller, say to 11 feet. As the generations go by, more and more trees break the embargo on height. When, finally, all the trees in the forest are 11 feet tall, they are all worse off than they were before: all are paying the cost of growing the extra foot. But they are not getting any extra photons for their trouble. And now natural selection favours any mutant tendency togrow to, say, 12 feet. And so the trees go on getting taller and taller. Will this futile climb towards the sun ever come to an end? Why not trees a mile high, why not Jack’s beanstalk? The limit is set at the height where the marginal cost of growing another foot outweighs the gain in photons from growing that extra foot.
We are talking individual costs and benefits throughout this argument. The forest would look very different if its economy had been designed for the benefit of the forest as a whole. In fact, what we actually see is a forest in which each tree species evolved through natural selection favouring individual trees that out-competed rival individual trees, whether of their own or another species. [pp. 378-380]