Monday, August 1, 2011

Why is Evolution so Slow?

The following is an excerpt from my new book, Life of Earth: Portrait of a Beautiful, Middle-Aged, Stressed-Out World, released earlier this year by Prometheus Books.

As we have seen in previous entries, evolution can occur rapidly enough to account for the diversity of life on this planet. Why, then, has evolution occurred so slowly? There are lots of mutations out there in populations, and selection can operate rapidly. If all of the conditions had been just right, life could have evolved into its many complex forms within just a few million years. Why didn’t it?

The answer to the question just posed is that the environmental conditions in which each population lives keep changing back and forth. Sometimes warm, sometimes cool. Sometimes wet, sometimes dry. And the interactions of species with one another keep changing. Evolution could make rapid progress, if there were some force directing it toward a goal, but instead it wanders around aimlessly—because evolution has no goal.

Consider one of the best-studied examples of evolution: the finches of the Galápagos Islands. In 1835, a British naval ship, H. M. S. Beagle, stopped at the Galápagos Islands, off the coast of Ecuador. The young naturalist on board, Charles Darwin, had just enough time to explore some of the islands. Based on what he saw, and what he heard from others, it appeared that each island may have had its own species of birds and tortoises, even though the islands all had a similar climate—brutally hot and dry, except in the misty mountains. When Darwin got back to England, the ornithologist John Gould identified many of the birds in Darwin’s collection as finches. It became obvious to Darwin that all of the finches, which differed greatly in size and in the way they gathered food, were the recent descendants of a kind of finch that lived on the mainland of South America. Evolution had occurred; it had occurred rapidly; and before long Darwin had figured out how it occurred: by means of natural selection. The original immigrant finch populations evolved different ways of getting food. These finches, now called Darwin’s finches, have become famous as the group of organisms in which the process of evolution was discovered.

American evolutionary biologists Peter and Rosemary Grant began an intensive study of these birds in the 1970s. They established their base on the island that the British called Daphne Major to study the process of natural selection, mostly in the medium ground finch Geospiza fortis. In order to study natural selection, it is necessary to demonstrate that the offspring of some of the birds survived better, and laid more eggs, than the offspring of other birds. Sounds easy? Not on your life. In order to do this, the Grants had to keep track of each bird. They had to know which birds mated with which other birds, how many eggs they laid, and then keep track of their offspring. And then do it again and again, every year. Even on that small island there are hundreds, sometimes thousands, of medium ground finches. The Grants, with the help of colored bands that they placed on the birds’ feet, became personally acquainted with hundreds of birds each generation, over the course of more than thirty generations.

In order to study natural selection, you have to choose a trait or feature that evolves and that can be measured. The Grants chose to measure beak size. Why beak size? Because a small difference in beak size, as little as a half of a millimeter, can make all the difference in what kinds of seeds the birds can crack open and eat. With a slightly larger beak, a finch can crack open and eat the seeds of puncture vines; with a slightly smaller beak, a finch can eat smaller seeds from other plant species.

The birds with the larger beaks preferred to crack and eat the larger seeds: they could get more food with each bite that way. Their beaks, however, were a little too unwieldy to efficiently handle the smaller seeds. The birds with smaller beaks simply could not eat the larger seeds, but were fairly adroit with handling the smaller ones. The population of finches always contained a mixture of small-beaked and large-beaked birds—in fact, a whole range of beak sizes.

Enter El Niño and La Niña. El Niño is a set of conditions in which strong ocean currents from the warm waters of the west bring lots of rain to the Galápagos Islands, as well as to much of North and South America. As a result, plants that produce small seeds grow profusely over the ground. El Niño is not necessarily a time of plenty for all organisms. During La Niña periods (when El Niño conditions are not occurring), strong ocean currents bring up cold water from the depths along the coast of South America, feeding marine life such as shrimp and fish and offering a bonanza to birds that eat them. El Niño (and the storms that come with it) is a blessing to some organisms, and a catastrophe to others.

The Grants measured finch beaks during a La Niña drought that began in 1977. Small seeded plants became rare, and large seeded puncture vines became common. Because more large seeds were available, it would make sense that natural selection would favor large beaks at that time. The Grants did not simply find that the average beak size in the finch population increased during the drought. They found that large-beaked parents laid more eggs, and their large-beaked offspring also laid more eggs, each generation. They had observed natural selection in the act of occurring.

Then in 1982 El Niño brought lots of rain. Very quickly, plants with small seeds sprang up all over the island, and puncture vines became rare. This time, natural selection favored the smaller-beaked birds, and it was the larger-beaked birds that went hungry and laid fewer eggs.

The fluctuation in beak sizes—first larger, then smaller—that the Grants observed was very small, on the order of about a millimeter. But this small change is very important to the survival of the birds. A change of a millimeter could occur within just a few years in a population. The Grants had observed rapid evolution: a rapid increase, then a rapid decrease, in beak size. The Grants had seen, and showed the scientific world, how rapidly evolution could occur.

Even more importantly, their work revealed why evolution usually does not occur rapidly even though natural selection does. The direction of natural selection changes often. Every ten years or so, the climate flops back and forth between El Niño and La Niña. The changes in beak size, on the order of a few years, are rapid; but on the order of centuries, the changes largely cancel one another out. The course of evolution wobbles rapidly along a very slow and often unvarying path. If Nature would just make up its mind, the way animal breeders make up their minds, Nature could breed its species a lot faster than it does. If only Nature had a mind and a purpose, which it does not.

1 comment:

  1. Interesting excerpt from your book - peaks my interest.