New Analyses Bolster Central Tenets of Evolution Theory

The first is that organisms will tend to adapt to their environments. If the planet's atmosphere contains lots of oxygen but very little methane gas, living things are going to end up tolerating oxygen -- and possibly even depending on it. But do not expect to see many methane-breathers.

This appearance of "perfect fit" makes it seem as if organisms must have been the product of an intelligent force. But this appearance of perfection is deceiving. It gives no hint of the numberless evolutionary dead ends -- lineages that, according to the fossil record, survived for a while but then died out, probably because changes in the environment made their once-perfect designs not so perfect anymore.

The second result of Darwin and Wallace's mechanism is that over time it will create species diversity. As additional "happy accidents" alter an organism's descendants over millions of years, those descendants will come to look less and less like other organisms with which they share a common ancestor. Eventually, the descendants will be able to mate only with each other. They will be lions and tigers -- each a distinct species, but both descended from the same ancient cat.

What is hard to understand about this process is that it is essentially passive. The mechanism is called "natural selection" because the conditions at hand -- nature -- determine which accidents are beneficial and which are not. Organisms do not seek ends.

Giraffes do not decide to grow long necks to browse the high branches above the competition. But a four-legged mammal on the savannah once upon a time was endowed with a longer neck than its brothers and sisters. It ate better. We call its descendants giraffes.

That a mechanism driven by random events should result in perfectly adapted organisms -- and so many different types -- seems illogical.

"Even today a good many distinguished minds seem unable to accept or even to understand that from a source of noise, natural selection alone and unaided could have drawn all the music of the biosphere," Jacques Monod, a French biologist and Nobel Prize winner, wrote in 1970 in the book "Chance and Necessity."

Natural selection was really hard to accept in Darwin's day. But it has become easier with the discovery of genes, DNA and techniques that have made it possible to watch natural selection happen.

DNA is a stringlike molecule made up of paired beads called nucleotides. It carries the instructions for making proteins and RNA, the chief building materials of life. Individually, these instructions are called genes.

The random changes Darwin knew must be happening are accidents that happen to DNA and genes. Today, they can be documented and catalogued in real time, inside cells.

Cells sometimes make errors when they copy their DNA before dividing. These mutations can disable a gene -- or change its action. Occasionally cells also duplicate an entire gene by mistake, providing offspring with two copies instead of one. Both these events provide raw material for new genes with new and potentially useful functions -- and ultimately raw material for new organisms and species.

Richard E. Lenski, a biologist at Michigan State University, has been following 12 cultures of the bacterium Escherichia coli since 1988, comprising more than 25,000 generations. All 12 cultures were genetically identical at the start. For years he gave each the same daily stress: six hours of food (glucose) and 18 hours of starvation. All 12 strains adapted to this by becoming faster consumers of glucose and developing bigger cell size than their 1988 "parents."