Dienstag, 6. März 2007

How does evolution "work"?

A new book about the ways of evolution ("The Plausibility of Life") has some thoughts worth to think through (Natalie Angier in NYT):

... I am, nevertheless, a multicellular organism of reasonably complex structure, and we complex bioforms can’t help but appreciate novelty. We are the fruits of it. If not for evolutionary novelty — that is, the periodic and often radical overhauling of an existing cell type, body plan, limb shape or brain design into something new and useful, or at least entertaining — we might still be so many daubs of blue-green algae decorating an Australian rock. ...

... biological novelty. Under its tutelage, early groups of cells made the leap from the sleepy expulsion of oxygen as waste to the aerobic consumption of oxygen to grow at a hastier pace; and groups of single cells learned to pool their talents into multicellular collectives of specialized body compartments that could then go out and hunt other multicellular collectives; and fishy fins became amphibious feet and crept onto the beach, and some land-weary feet changed their mind and flippered back to the sea, while still other limb bones lengthened and found skin flaps for flying, and, hey, this airborne business is pretty handy, let’s rearticulate the forelimbs of three separate lineages and take wing as a pterodactyl, a bird, a bat.

As scientists see it, these and others of nature’s fancy feats forward are clearly the result of large-scale evolutionary forces, but the precise mechanisms behind any given innovation remain piquantly opaque. For some researchers, the conventional gradualist narrative, in which organisms evolve over time through the steady accretion of many mincing genetic mutations, feels unsatisfying when it comes to understanding true biological novelty.

“The standard Darwinian view always sounds like a better theory for making improvements than for making inventions,” said Dr. Marc W. Kirschner, a professor of systems biology at Harvard Medical School. If incremental, additive genetic changes were responsible for all the boggling biodiversity we see around us, he said, how can it be that humans have hardly more genes than a microscopic nematode, and that many of those genes are nearly identical in roundworms and humans besides?

In their recently published book, “The Plausibility of Life,” Dr. Kirschner and Dr. John C. Gerhart of the University of California,Berkeley, offer a fresh look at the origins of novelty. They argue that many of the basic components and systems of the body possess the quality of what they call “evolvability” — that is, the components can be altered without wreaking havoc on the parts and systems that connect to them, and can even produce a reasonably functional organ or body part in their modified configuration. For example, if a genetic mutation ends up lengthening a limb bone, said Dr. Kirschner, the other parts that attach to and interact with that bone needn’t also be genetically altered in order to yield a perfectly serviceable limb. The nerves, muscles, blood vessels, ligaments and skin are all inherently plastic and adaptable enough to stretch and accommodate the longer bone during embryogenesis and thus, as a team, develop into a notably, even globally, transformed limb with just a single mutation at its base. And if, with that lengthened leg, the lucky recipient gets a jump on its competitors, well, g’day to you, baby kangaroo.

Dr. Kirschner also observes that cells and bodies are extremely modular, and parts can be moved around with ease. A relatively simple molecular switch that in one setting allows a cell to respond to sugar can, in a different context, help guide the maturation of a nerve cell. In each case, the activation of the switch initiates a tumbling cascade of complex events with a very distinctive outcome, yet the switch itself is just your basic on-off protein device. By all appearances, evolution has flipped and shuffled and retrofitted and duct-taped together a comparatively small set of starter parts to build a dazzling variety of botanic and bestial bodies.

The combined modularity and bounciness of body parts suggest that life is spring-loaded for change, for outrageous commixtures, the wildest fusion cuisine. And who knows whether our organismic suppleness, our deep evolvability, isn’t related to our mental thirst for the new, and our hope that behind the door lies the best surprise yet?

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