Improbable as it may sound, Paris Hilton�s accessory pooch, the one that looks like a rodent and fits in her bag, is basically a wolf.

Improbable as it may sound, Paris Hilton's accessory pooch, the one that looks like a rodent and fits in her bag, is basically a wolf. All dogs, the actor Martin Clunes notes in his book A Dog's Life, share 99.9% of their DNA with wolves - huskies, the toy dogs at Crufts and all the collies, terriers and labradors in between. The active agent of all that diversity? Us. "By meddling with nature," as Clunes notes, "we've created the most diverse species on the planet."

In his travels round dog shows, sheep trials and homes, Clunes finds that many of these breeds thrive. He himself has two pedigree dogs - a cocker spaniel and a labrador - both of which are the picture of good health. But like many dog lovers, he believes that human interference in breeding can go too far. "If you want my opinion of the downsides of in-breeding and over-breeding, it's disgusting," he says. Careful breeding can most certainly have benefits, he says, but adds: "Man has been tampering with species for years, there's bound to be a downside. It's down to unscrupulous breeders, just as aggressive dogs have unscrupulous trainers and owners."

Clunes's book, a celebration of dogs and an exploration of the history of their relationship with man, comes out in the wake of a PR earthquake in the world of pedigree dogs. On Monday, the RSPCA issued a forthright statement confirming that it would not be attending Crufts in 2009, in a protest against the incidence of genetic illness among some breeds. It comes after a BBC documentary last month highlighted severe health problems among certain pedigree dogs, including a prize-winning Cavalier King Charles spaniel with syringomyelia, a skull that was too small for its brain. Calling for the prioritisation of welfare and health over looks among show dogs, the RSPCA's chief veterinary adviser Mark Evans said that dog shows using current breeding standards as the main judging criteria "actively encourage both the intentional breeding of deformed and disabled dogs and the inbreeding of closely related animals", adding that such intentional behaviour was "morally unjustifiable and has to stop". The Kennel Club said the comments were "extremely regrettable".

Clunes is unequivocal in condemning unscrupulous breeders, but stresses the need for perspective. "I'm not a fan of a lot of those old-fashioned Kennel Club breed regulations, but I believe from what I've been told by it that it is addressing that," he says. "There are appalling breeders out there; but I don't think necessarily it's appropriate to condemn all dog breeders, because I've met some very good ones."

In this, he echoes the sentiments of the Scottish SPCA, which shares the RSPCA's concerns over certain breeds, noting that 10%-15% have severe problems, but says that "a lot are fine".

"All of my dogs have been manufactured and I love them to bits. It's just part of the fascination. It's interesting without it having a value judgment; to me, anyway."

Clunes's fascination with all things dog-related is palpable. He has yet to meet a dog that he doesn't like, he says. Where some people love lolloping big dogs, but can't stand yappy little ones, Clunes has an all-embracing, inclusive love of all canines. Sure, he says, the size of his labrador Arthur is "part of the joy of him", but he's also lobbying for a Jack Russell to control the rats on his Dorset farm.

Mongrel or pedigree, he doesn't care, the fact that he has two pedigrees himself being down to luck in Arthur's case and family tradition when it came to Mary. He loved bonding with wolves for the book and accompanying TV series - you could see the sheer delight in his eyes as a wolf bitch nuzzled against his cheek in a north Devon wolf santuary - but the "honour and privilege" was just as great this Monday when he met a Staffordshire bull terrier at Battersea Dogs Home. He wouldn't even define himself narrowly as a "dog person", saying he loves cats and has one at present. No, Clunes just loves animals, cherishes the relationship he has with his and wanted to find out how dogs and people came to become such dear friends. "There has been such a special relationship between humans and dogs that we often blur the distinction, quite wrongly," he says. "We forget that we are a species apart."

There was the time when he and his wife Philippa's beloved Mary was a puppy, for instance. "Quite pathetically, we stopped going out when Mary was tiny in case she got lonely or upset," writes Clunes. The first time they left the flat, they asked a friend round to keep the dog company and were gone no longer than an hour. The second time, they set up a video camera to see what she got up to without them.

Clunes describes how gutted he and Philippa were to discover that their dog had severe hip dysplasia, erosion of the dog's hip joint leading to arthritis, a condition caused partly by genetic influences, though to what extent is debatable. She had to have two operations and the couple decided then that "we owed it to her to give her the best possible passage through her life".

Unfortunately, their treatment of Mary could have been part of subsequent problems. These began on the arrival of a second cocker spaniel, Tina. Fights broke out between the two dogs quickly and continued even after the intervention of trainers, with Tina becoming increasingly aggressive. Eventually, Rosemary Young, a dog trainer of 40 years' experience, told them that the dogs would have to be permanently separated, or one would have to be given away. After further attacks, they reluctantly gave Tina away, something that was particularly hard for the couple's daughter, Emily.

It has all been part of the journey for Clunes, who admits they made mistakes: "Absolutely, mistake after mistake. In our defence I will say that a lot of our problems stemmed from Mary's birth defects, surgery and fragility, but then that prompted our appalling behaviour. Tina is now "flourishing" and Mary is living out her old age in peace.

With such dramatic scenes playing out in his own back yard, the closeness of the relationship between domestic dogs and the wolves of Yellowstone, dingoes of Australia and wild dogs of Tanzania, all of whom Clunes saw on his travels, becomes much easier to grasp.

But the biggest lesson of all? "That I'm not a freak," he says with his big, bellowing infectious laugh. It was a revelation to Clunes to see how much other big blokes love their dogs. He cites Charlie, who looks after the hounds at the local hunt, who kissed one of the dogs. "He loves them. The guys with the ratters, the Jack Russells, they love them; Shaun loves his wolves, and even tough old Tony Fitzjohn down in Africa, waded in to rescue an ageing bitch who was about to be attacked. Now he's got to look after the old girl for the rest of her life. I'm OK, they're OK, we're OK."

A Dog's Life by Martin Clunes, Hodder and Stoughton, £20, is published tomorrow.