Darwin's Blind Spot: Evolution Beyond Natural Selection, by Frank Ryan. Houghton Mifflin, 310 pages, $25.

There is a species of hermit crab that scuttles around with a pink anemone atop its shell. If an octopus or other predator approaches the crab, the anemone jabs it with a poisoned tentacle, scaring off the would-be assailant. In return for playing bodyguard, the anemone is permitted to piggyback and feast on the crab's leftovers and excrement.

One can debate the equity of this deal. But as physician and science writer Frank Ryan argues in his latest book, it's an example of one of the most powerful -- and he thinks underestimated -- partnerships in nature, what scientists call symbiosis.

Symbiosis is a relationship in which two dissimilar organisms join forces, sometimes to the point where they swap genetic material. In his book, Ryan traces the scientific and intellectual history of this idea, and attempts to show that symbiosis "has played a formative, if largely unacknowledged, role in the origins and subsequent diversification of life on Earth."

Ever since British naturalist Charles Darwin first published On the Origin of Species by Means of Natural Selection in 1859, evolutionary theory has been dominated by the idea that the diversification of life on the planet arose from the gradual accumulation of random mutations within an organism's DNA. The mutations that gave an organism the best shot of mating -- the "survival of the fittest" clause -- are passed on to the next generation.

But Ryan believes Darwin, and generations of evolutionary biologists since, didn't consider the whole story.

Drawing on examples from fields ranging from molecular biology to epidemiology, Ryan shows that the natural world is filled with unusual unions -- from lowly rock-clinging lichens, which are a marriage of algae and fungi, to humans, who over millions of years have formed biological partnerships with a slew of bacteria that now inhabits everywhere from the gut to the innermost reaches of the cell. Through such examples, Ryan attempts to show that symbiosis is a force to be reckoned with and how it nudged evolution along.

The most accessible and interesting moments in the book occur when Ryan leads readers into the laboratories of scientists studying symbiosis -- people such as James Lovelock, who proposed the Gaia theory, which views all life on Earth as a single living entity interacting with and shaping the environment.

And biologist Lynn Margulis at the University of Massachusetts, an early champion of symbiosis, who first proposed that mitochondria, tiny structures that serve as cellular power plants, were once free-living bacteria. She speculates that even our nerve cells billions of years ago may once have been bacteria and over the eons became naturalized citizens.

But as a popular chronicler of science, Ryan, who has written two other books, Virus X and The Forgotten Plague, has some blind spots.

He often lapses into evolutionary jargon and makes intellectual and logical leaps, assuming that his readers are following merrily along. As a result, those who lack a grounding in evolutionary biology may find the book tough going at times.

While Ryan does make some fascinating stops along the way -- to discuss, for example, who coined the phrase "survival of the fittest" and why -- his book ultimately perhaps tries to cover too much ground too quickly, leaving readers somewhat fuzzy about how Darwinism and symbiosis really do influence one another.

Michael Stroh has covered technology issues for four years, the last two and a half for the Sun, for which he covered the Microsoft trial. He previously worked at the Sacramento Bee and the Los Angeles Times.