|
|
|
Fossil Dunkleosteus terrelli, a bony-headed |
The popular view of evolution is that it’s something like an arms race. Predators develop longer claws and sharper teeth; so prey develop scales, armor plates, and detachable tails.1 That’s the old “red in tooth and claw” view of evolution. We saw some of that, to be sure, during and after the Cambrian explosion with development of exotic forms like the bony-headed fishes.2 But life as an arms race is a simplistic view, focusing on the drama we can see on the African veldt or the ocean depths, which is really just a small segment of life. Most of the mechanics of evolution goes on at the level of basic chemistry: proteins evolving to get better at their jobs or to undertake new jobs.
The idea of a quid pro quo arms race was also undercut by Darwin’s early understanding of evolutionary processes. He believed that bodily changes developed slowly and gradually. Sure, gradualism was a popular notion in the 19th century: that mountains wore away grain by grain; rivers cut their valleys inch by inch; and elephants grew their trunks or giraffes their necks by tiny increments over many generations.3 The one problem with gradualism in evolution was that the fossil record didn’t seem to support these gradual changes. There just weren’t the equivalent of proto-elephants with vestigial trunks and proto-giraffes with medium-sized necks. And of course, most protein evolution doesn’t get recorded in the fossil record at all in the way teeth and bony plates do.
Of course, since the groundbreaking study of Galapagos Island finches by Peter and Rosemary Grant,4 today we know that evolution can proceed almost as we watch. Environmental factors like seasonal rainfall, which can affect the size and toughness of seeds year by year, can also change the shape and strength of a finch’s beak within a generation.
But lacking this novel insight into the rapid pace of evolution, Stephen Jay Gould and Niles Eldredge in 1972 came up with the idea of “punctuated evolution.” They argued that populations in equilibrium do not change much over time, as wide-scale interbreeding tends to absorb and diffuse minor genetic changes throughout the gene pool rather than let them flower into new forms. But out at the edges of a population, groups of individuals might occasionally become isolated. Then new forms can appear and, if the two populations are later reunited, appear to be a sudden change or even the appearance of a new species.
But what about populations that formerly were in equilibrium with their environment and then, because the environment changes, begin to fall out of equilibrium?
Genetic mutations are happening all the time. Because of the vast redundancy in the DNA protein-coding system, most of them don’t mean anything. The code uses four different bases—adenosine (A), cytosine (C), thymine (T), and guanine (G)—which are read in groups of three to identify the next amino acid to be added to the growing chain of any protein. Four bases in three positions yield 64 different ways (4 X 4 X 4) to identify that amino acid. But the system uses a total of only 20 different amino acids to build up all the millions of proteins used in life processes. So each amino acid can be called by two or three different coding combinations. Knock down one of those bases, and chances are the other two will still call for the correct amino acid.
At the same time, proteins are big, messy molecules. They use their folded configuration and the positive and negative charge domains that the folding exposes to work their special magic: forming a membrane or a structure, speeding up or slowing down a chemical reaction, or signaling changed conditions from one cell to the next. Swapping out one amino acid in the chain for another doesn’t always have much effect. The same goes for leaving one out or adding an extra amino acid. Of course, adding, subtracting, or changing a key player in the sequence will occasionally have major effect, sometimes lethal effect. But one change usually doesn’t mean much. However, when you get change on change on change, interesting things can start to happen.
If you’re part of a species that’s flourishing under a stable environment, any change that emerges at the level of protein function is likely to be bad for you. Your species was optimized for your environment; so making random changes is likely to make you suboptimal, less able to survive, less likely to attract mates who like the healthy look of prime specimens. But if the environment has drifted in subtle ways—more moisture or less, more sunlight or less, more acidic or basic, or a combination of these changes—then the competition from all those perfect specimens falls off. All members of the species are facing stressors, lack of fitness, tough times.
If you have accumulated enough genetic modifications to put your protein functions into play, that’s still no guarantee that you will succeed where others are failing all around you. In fact, you have an equal chance that your new protein regimen will be meaningless or even harmful in the new environment as that it will confer any benefit. Poor you. But still, you have a chance. And members of your species who get no mutations at all, preserving their pristine status as top players under the old conditions, will suffer even worse.
A changing environment is like a storewide clearance. Customers no longer care for last year’s fashion, the older model, the same old same old. It’s a time when evolution blindly tries new forms, like a safe cracker randomly spinning the dial hoping to stumble on the right combination, trying to find one that will work. A lot of random numbers fall out in this process. A lot of random mutations don’t happen to be the winning combination of function and form.
Evolution is not directed. The environment in all its guises—sometimes sharper teeth in a predator, but more often a shortfall in the amount of rain—changes the conditions of the test and the criteria for survival. Some members of the species come up with the right genetic number and win the lottery. Most don’t and die off.
Evolution is cruel, especially on the level of the individual. Your baby gets a bad gene and dies, bang dead. You get a bad gene and your life never rises to the level of your hopes. Evolution requires a lot of wrong choices and sudden lethality to work—and that’s why most people don’t like it. It seems too cold, too random, too capricious.
But consider the alternative. Your beautiful, pristine species meets a change in the environment—perhaps one you can’t even detect, like a change in acidity or loss of a trace element—and everyone withers and dies. Nobody survives. Nobody carries on. You and everyone you know goes bang dead.
We’d like to have an all-seeing, all-knowing presence guiding the process. He/She/It would allow a change to occur in the environment and then artfully, painlessly, and overnight change all our genes at once so that everybody survives and can still recognize each other, still interbreed successfully, in our new state. That would be a kinder, gentler universe. A world without death and futility. It would also be a fairytale.
Humans, with our growing knowledge of the molecular basis of life and our developing ability to link gene to protein to function, may one day reach that stage. We may one day be able to redesign ourselves to meet strange new environments, along the lines of James Blish’s The Seedling Stars. Until that day, however, we have to work the old-fashioned way: blind luck, random chance, and the genetic lottery.
1. Why do you think a squirrel has a thick, fluffy tail almost as long as its body that moves with the same galumphing motion? If a predator is chasing the squirrel from behind and makes a desperate leap, it may take a bite out of sacrificial hair rather than actual squirrel meat.
2. This was a time, half a billion years ago, when most of the major animal phyla burst upon the scene in a geologically short time, 20 million years, and the rate of evolution as shown in the fossil record accelerated by an order of magnitude over the next 70 or 80 million years.
3. This view was opposed by catastrophism, which said gullies like the Grand Canyon could be cut by massive floods—here the gradualists were shying away from the Biblical story of Noah—and mountains thrown up by violent eruptions. Of course, catastrophes do happen, as demonstrated by volcanoes and the floods unleashed at the end of the last Ice Age. But mountains and valleys sometimes also develop slowly. In my opinion, too much of science has become embroiled in ding-dong battles about “either/or,” where everything has to work either like this or like that, rather than “both/and,” where you have to examine the facts of the situation and choose the model that fits the data.
4. Jonathan Weiner, The Beak of the Finch (Vintage Books, 1994).