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spring books of parrots and . On the credit side, Pepperberg gives an excellent justification for her training procedures, the descriptions of the results are detailed and lucid, and, best of DAVID NEWTON DAVID all, the interpretations are calm and consid- ered. Pepperberg is well aware of Occam’s razor and is careful not to indulge in ridicu- lous overinterpretations. One might none- theless wonder what exactly was the point of this labour of love. To misquote Ludwig Wittgenstein, what would a parrot tell us if it could talk? Not a lot, seems to be the answer. Is a research programme to teach speech to parrots likely to lead to deeper insights than one devoted to teaching us the calls and songs of the parrot? Pepperberg’s justification is that her data might help to improve the of captive parrots, “prevent habitat destruction and capture of birds in the wild, or enable researchers to develop better models for various human dysfunctions”. I hope she’s right. I John C. Marshall is in the Neuropsychology Unit, University Department of Clinical Neurology, Radcliffe Infirmary, Woodstock Road, OX2 6HE, UK. since 1882, and Dar- genetic processes, primarily unequal crossing win, for his part, supplies appreciative yet over and conversion. is, inquisitive responses. in Dover’s words, an “umbrella term” cover- Things start rather formally — it’s “My ing these and other “non-Mendelian mecha- Dear Dover … Ever your most truly, Charles nisms of inheritance”. Communications Darwin” to begin with — but become Does molecular drive really rank beside increasingly chummy as the correspondence selection and drift as one of the primary from the dead develops — it’s “Dear Gabby…. Your most determinants of evolutionary change? Dear Mr Darwin: Letters on the sincere friend, Chas. Darwin” by the end. The Hardly. Darwin distinguished between two of and Human gimmick is almost painfully cute, but Dover fundamental aspects of the evolutionary handles it deftly: he is not unduly deferential, process: the genesis of variation, and the sub- by and his Darwin not overly impressed by what sequent fate of that variation. In creating new Weidenfeld & Nicolson: 2000. 268 pp. £20 Dover has to say. The result is a quirky but configurations of existing genetic variation, A. J. Berry readable account of the Dover perspective on molecular drive definitely contributes to modern evolutionary biology. step one. But does it contribute to step two? In 1876 contributed £10 — a Darwin’s education, however, is in idio- In principle, a variant can indeed spread substantial amount at that time — to the syncratic hands. At the outset, Darwin must through a gene family by molecular drive, costs of the criminal prosecution of Henry predictably swallow doses of Mendel and especially when there are asymmetries in the Slade, a renowned spiritualist medium. Hardy-Weinberg, but the textbooks are then drive process. For example, gene conversion Slade, his accusers charged, was a fraud, and quickly forsaken when, on the second page of is sometimes ‘biased’ such that an a allele is his séances were merely elaborate exercises in Dover’s second letter, we run into his pet the- more likely to be converted to an A than an A legerdemain. Remarkably, the case pitted the ory, ‘molecular drive’. This, Darwin learns, is, to an a; such a situation may result in a molec- two discoverers of against along with natural selection and , ularly driven increase of the Aallele. each other: Alfred Russel Wallace, author of one of “the three forces of evolution”. Much of But crucially, the ultimate fate of any vari- an approving book on spiritualism, was the the book is dedicated to explicating molecu- ant, whether subject to molecular drive or defence’s star witness. Despite Wallace’s char- lar drive and to justifying its exalted place in not, is determined by its impact on : acterization of the defendant as an “earnest Dover’s pantheon of evolutionary forces. natural selection will intervene if it either inquirer after truth in the department of Nat- Dover introduced the term in the early enhances or diminishes its bearer’s chance of ural ”, Slade was convicted. Darwin 1980s after DNA-sequencing studies of . If the variant has no such was delighted; he had no time for the “clever multi-gene families — groups of related impact — it is selectively neutral — then rogues” who preyed upon grieving relatives that often sit side by side along chromo- genetic drift is usually the major player, anxious to contact a loved one. somes — had revealed a striking and unex- although molecular drive may sometimes Darwin, who died in 1882, may now have pected evolutionary pattern now known as also play a role. Molecular drive’s contribu- cause to reconsider his attitude towards ‘concerted evolution’. Within a species, all tion to the second phase of the evolutionary posthumous communication as he himself members of a gene family may be identical, or process is thus subordinate to the ‘traditional’ has recently taken to holding forth from at least very similar, whereas between even forces determining the fate of genetic varia- beneath the flagstones of Westminster Abbey. closely related species we see plenty of tion in natural populations. Molecular drive The medium in this case is geneticist Gabriel sequence divergence between homologous is an interesting evolutionary phenomenon, Dover, whose book, Dear Mr Darwin, com- gene families. The homogenization of gene- but it is false advertising to bill it as a third prises a series of letters between Dover and family members within species is caused by major force of evolution. Darwin. Dover brings Darwin up to date on a number of simple and well-understood Dover’s Darwin, whose critical facilities

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© 2000 Macmillan Magazines Ltd spring books may have been dulled by a century or so we share with chimpanzees. So, suggests product is a slim volume that nevertheless underground, is more readily convinced of , we are faced with a packs in a richly articulated and challenging molecular drive’s significance than I am. puzzle: how could human vault this model of , backed by a wealth of pithily Having scripted Darwin’s endorsement of high so quickly? The question becomes more summarized comparative and develop- his theory, Dover then settles down to enjoy acute if one acknowledges little sign of any mental studies. his new role as Darwin’s speech-writer. accomplishment beyond basic ape mentality At least two-thirds of the book is devoted Responding to a lengthy Dover diatribe until two million years ago or even less. to tracing the origins and development of against , whose “selfish Tomasello’s solution — given how far he components of cultural in children, genery is genetically misconceived, opera- wants to push the — is a radical one. with a particular emphasis on language. This tionally incoherent and seductively danger- Depending on the , I suspect it will is a masterly survey, covering pre-linguistic ous”, Darwin reports that he will conscien- elicit excitement, irritation or incredulity. scaffolding for language, the acquisition of tiously hunt up Dawkins’s books in a library: These different reactions may be more or less symbol and syntax use, discourse and the “I hope they are not filed under ‘Science’!”. appropriate according to the evolutionary implications of internalization for other Dear Mr Darwin, however, is not confined timescale Tomasello truly aspires to address. aspects of cognition. to molecular drive and having Darwin say The key proposition is that there was just Certain features of Tomasello’s thesis are nasty things about Richard Dawkins. Dover one critical step in biological evolution which less compelling. He considers the possibility writes at length on recent advances in transformed our ancestors’ capacity to sus- that the vital change may have happened two developmental , and adds his voice to tain culture. A new ‘ratchet’ effect arose, in or even six million years ago. But his argu- those objecting to ’s which cultural advances were built upon pro- ment appears to neglect enormous changes insistence on attributing every quirk of gressively in a way not seen in the social tradi- in the brain, which has tripled in size since six human behaviour to the action of natural tions of other . Human cognition million years ago and roughly doubled in the selection. Given that evolutionary psycholo- would thenceforth become increasingly past two million. It seems more likely that gy is an implicitly genetic theory (a trait must complex and differentiated, eventually whatever elaboration of social and cultural have a genetic basis to be subject to natural achieving modern levels of sophistication practices occurred in this period, it was selection), it is interesting to note that many without need of further biological change. underwritten by equally massive and rapidly of its most persistent critics are geneticists. To see how radical a proposition this is, driven neural changes. Dear Mr Darwin is an engaging tour of consider the case of language. Tomasello is A predominant role for cultural change Dover’s passions, even if some are announced arguing that the structures of our highly elab- becomes more likely in the context of the past with more fanfare than they merit. Let us orated language capacities have noth- quarter of a million years of Homo sapiens’ hope, however, that Dover’s com- ing to do with the evolution of a dedicated existence. If the greater part of existing lan- munications with Darwin do not and, in some views, highly structured lan- guage structure arose over this period, the create a literary fad based on the guage (he gives short shrift to the idea idea that this happened through cultural harassment of dead scientists. of innate mental modules, of any kind). learning and ratcheting processes still consti- The thought of Linnaeus Instead, he proposes that syntax and all the tutes a major challenge to those who argue for being badgered by manic other complex aspects of human language innate language systems. Tomasello notes modern cladists is alarming. have simply been built up over the genera- that the main diversification of the Romance On receiving one of Wallace’s tions, by cumulative, ratcheted, cultural evo- languages occurred in a few hundred years; so spiritualist publications, T. H. lution. The biological substrate did not need why could not cultural processes of syntacti- Huxley replied, “I never cared to change. What makes such a scenario plau- cization turn an embryonic language into a for gossip in my life, and dis- sible, according to Tomasello, is evidence vastly more complex one over hundreds of embodied gossip, such as these that, quite early on, the human child demon- thousands of years? worthy ghosts supply their strates a capacity to translate between the A further doubt is whether Tomasello has friends with, is not more inter- perspectives of self and other that goes correctly identified the critical cognitive step esting to me than any other.” I beyond anything seen in apes. Perceiving that elevated our ancestors’ social sophisti- A. J. Berry is at the Museum of Comparative others as intentional agents, in particular, cation over existing anthropoid psychology. Labs, 26 Oxford Street, Cambridge, permits the child to become the kind of ‘imi- His conclusion is largely founded on experi- Massachusetts 02138, USA. tation machine’ needed to participate in the mental findings in captive apes far removed powerful ratchet effect. from the rich inputs of their natural environ- Tomasello’s ambitious thesis requires ments. Field researchers tend to perceive accounts of changes on three very different more advanced cultural processes at work, timescales; evolutionary, historical and onto- although these perceptions are difficult to On being genetic. To this task he brings almost un- substantiate without experimental controls. rivalled authority, based on an influential Accordingly, we are at something of an human suite of both comparative and developmental impasse on this question. Tomasello’s thesis The Cultural Origins of Human studies; he cites more than 40 observational probably depends less than he implies on the Cognition and experimental studies conducted by his exact difference between chimpanzee and by Michael Tomasello group on monkeys, apes and children. These human cultural propensity, especially if the Harvard University Press: 1999. 248 pp. studies, mostly conducted in the 1990s, cover thesis gets its main application in the recent $29.95, £18.50 an impressive array of socio-cognitive capaci- rise of Homo sapiens. ties, including imitation, joint attention, Nevertheless, students of primate behav- theory of mind and language acquisition. iour are one of several groups who should Human language and thought elevate us This substantial empirical base is coupled read this important book. It spells out force- mentally to a grade far removed from any- with a sophisticated grasp of the theoretical fully what appears to make human develop- thing known in other animals. Yet it has hap- issues at stake, particularly when it comes to ment so distinctive, and does so from the per- pened in just a twinkling of evolutionary Tomasello’s prime area of expertise, the spective of an expert in language acquisition time. Less than six million years separate us development of language. Written with who has also devoted much time to compara- from the non-human, non-verbal ancestor refreshing simplicity and directness, the tive work with apes. It is strong medicine for

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