Y: The Descent of Men by Steve Jones

Compare the volume of their sperm, and you'll find the mighty zebra outguns human males. Gail Vines explores the biological oddity of men - and the genetic essence of Welshness

When it comes to stories of sexual prowess, most men can't help but inflate their exploits. Even Steve Jones, award-winning science writer and professor of genetics at University College London, is not immune to the temptation. As Jones wryly confesses in the preface to his latest book, in the first draft he likened mankind's daily output of semen to the flow of the Thames at London Bridge. Luckily, Jones checked the text with a scientist at the Environment Agency. In fact, the flow from a day's global copulations – around a million litres of semen – is equivalent merely to the trickle of water that is the Thames a few miles from its source.

Hardly such an impressive result from 50 billion couplings a year, you might say. Yet if people were Grevy's zebras, even the biggest river would fail to contain masculine emissions: on each ejaculation, a zebra delivers half a gallon of semen. It's a strategy that has evolved, apparently, as an attempt to flush out the seminal offerings of previous suitors.

Y: The Descent of Men is packed with juicy facts such as these, breezily presented in Jones's readable style. If it skates perilously close to glibness on occasion, it is usually rescued just in time by the author's abiding ambition to inform as well as entertain.

As in his earlier book, Almost Like a Whale, Jones takes as his starting-point a masterpiece of that wise old man of evolution, Charles Darwin. Here, he takes his inspiration from Darwin's treatise on human evolution, published as The Descent of Man in 1871. But while Darwin sought to explain the origins of humanity, Jones focuses exclusively on men. He claims that "at the turn of the 21st century a new science of maleness has emerged, almost unnoticed. This volume sets out to explore it."

All the same, Jones's pathfinding covers much well-trodden ground, rehearsing familiar stories, for instance, about the links between hormones and horniness. Occasionally, he presents controversy as fact, as when he claims that a menopausal woman's libido is enhanced by a lacing of testosterone in her HRT. His treatment of cutting-edge research on an oral contraceptive pill for men is disappointing too.

At times, his relentlessly journalistic language seems at odds with the import of the issues. When he mentions sperm donation, he assumes that cash payments are "essential if the precious fluid is to be found". Yet in Sweden, men who have already fathered children (and hence proven their fertility) donate sperm without payment, with the understanding that any child born of the donation can learn the donor's identity. Jones is sometimes in too much of a hurry.

The brief he has set himself – everything you need to know about the biology and evolution of males in 280 pages – is certainly daunting, and Jones has succeeded in producing a highly digestible yet sustaining read. And while understandably fascinated by the machinery of maleness, he is careful to stress that genes alone don't make the man.

The sheer variety of topics covered in Y: The Descent of Man is impressive. Baldness, intersex genitalia, the male menopause, erections and impotency ("Hydraulics for Boys"), castration and circumcision: all fall under Jones's penetrating eye. Everything from ICSI (injecting sperm into eggs to circumvent male infertility) to mice genetically engineered to produce valuable growth hormone in their semen gets a mention.

One chapter tackles sex selection, and the perverse propensity of cultures round the world to prefer boys to girls, even though, biologically speaking, females are the limiting sex.

Sex ratios skewed towards the male – as in modern China, where abortion and infanticide have led to a 20 per cent excess of males, or in certain Indian villages, where young boys outnumber girls by three to one – are an evolutionarily unstable strategy. Here, biologist Jones blithely predicts that the balance of the sexes will some day be restored, although in his final chapter he oddly contrasts the "ascent of women" with his perceived "descent of men".

You can't help feeling that Jones is happiest when he reaches the eighth chapter, which explores the genetics of Welshness. Jones is intrigued by the work of Bryan Sykes, professor of human genetics at Oxford University, who makes a living tracking down marker genes inherited exclusively through maternal or paternal lines.

Because only men have Y chromosomes, small random variations in gene sequences on that chromosome can be used as lineage markers. Sykes has collected data on a number of such Y markers, and linked them to geographical regions. If you're a British man, you can send a DNA sample to Sykes' firm Oxford Ancestors and learn whether you're descended from a Viking.

Of course, that Viking will be only one of thousands of ancestors that anyone now living can lay claim to. What's more, at every generation, all the genes (apart from the pathetically few marooned on the Y) are thoroughly mixed during the process of recombination. In his enthusiasm for Y gene tests, Jones tends to downplay such niggles. "The Y is an arrow of manhood that flies from Adam to every male alive today," he enthuses.

The enthusiasm is understandable, all the same. Despite his common surname (a genealogist's nightmare because shared by many unrelated individuals) his Y chromosome holds out the promise of an incontrovertible lineage. Research suggests that 70 per cent of Welsh male chromosomes share a single genetic subtype, Jones reports. "I have checked my own Y, and it too reveals an origin in ... the Little Dark People."

The record of the Y shows, he claims, that "the small dark men of the west kept the Anglo-Saxon male chromosomes (and those who bore them) at bay, but welcomed their wives and daughters." He goes on: "Welshmen, says the DNA, were able to retain their sexual identity in the face of a wave of alien females who rolled over them."

What he doesn't spell out here is that this much-vaunted genetic Welshness is confined to the Y chromosome, and in fact to a tiny genetic polymorphism that probably makes no discernible difference to its holder. The genes that matter, which make people tick, are now thoroughly mixed in that well-stirred genetic melting pot that is the British Isles. But that would spoil the story.