By Robert Hazen, George Mason University
Charles Darwin’s theory of evolution by natural selection has led to two hot areas of current research. Biologists accept evolution as a fact, but there remain several theories about the details of the rate of evolution and the mechanisms of evolution.
The Gradualist Model
Let’s look at the contention of Darwin’s original thesis: that change in species occurs slowly, in small increments. According to this gradualist model, changes are all but invisible over the short time span of modern observations, and they are usually obscured by innumerable gaps in the imperfect fossil record.
But the fossil record doesn’t always conform to this idea, and every scientific theory has to be subject to modification as new data comes in.
The Punctuated Equilibrium Model
In 1972, American paleontologists Stephen Jay Gould and Niles Eldridge challenged conventional wisdom with an opposing viewpoint, called punctuated equilibrium, which claims that species change one to another in relatively sudden bursts, without a leisurely transition period. These episodes of rapid change are separated by relatively long, static spans, in which a species may change hardly at all, perhaps for millions of years.
The punctuated equilibrium hypothesis attempts to explain a very curious, and a very well-established, feature of the fossil record. This is one that’s been familiar to paleontologists for more than a century, but usually has been ignored, because it didn’t fit with the Darwinian idea.
Many distinctive fossil species appear to remain unchanged for millions of years; that’s a condition of stasis that’s very much at odds with Darwin’s model. Numerous distinctive markers or index fossils, for example, are abundant throughout the world’s rocks. They prove useful in dating sediments, because they remain unchanged for periods of time around the world.
All the intermediate forms that are predicted by Darwin’s gradualism seem, for the most part, to be lacking. In most localities, you can see a given species of clam, or a given species of coral; it persists essentially unchanged in layer after layer of rock, only to disappear, and new species of clam or coral will come into play at higher levels of strata.
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The Case Study of Horses
One great illustration of this is North American horses. These horses were once used as a classic textbook example of the gradual evolution of a horse, but they’re now providing equally compelling evidence for punctuated equilibrium. It’s interesting how data can change, as you look at it in new ways.
There’s a convincing fifty-million-year sequence of modern horse ancestors, each of them slightly larger, each with more complex teeth, with a longer face, with a more prominent central toe, and shorter side toes. These seem to prove Darwin’s contention that there was gradual evolution.
But if you look very closely at these fossil deposits, it reveals a somewhat different story. It turns out that horses evolved in discrete steps, each of which persisted almost unchanged for millions of years, and was eventually replaced by a distinctive newer model. You had the four-toed Eohippus, which preceded the three-toed Miohippus, for example. But North American fossil evidence suggests a jerky transition of distinct species between the two types.
Gradualism Versus Punctuated Equilibrium
If horse evolution has been continuous, if it’s been a gradual process, one might expect that almost every fossil specimen of horse would be slightly different from every other fossil, because the fossil-horse specimens are scattered across the country, and there are very few of them. There may be only several hundred fossil-horse specimens known.
The fact that you see certain distinct species, and not those intermediates, would lend credence to this idea of punctuated equilibrium. If it seems difficult to understand how such a major change could occur rapidly, consider this: an alteration of a single gene in fruit flies is enough to turn a normal fly, with a single pair of wings, into a fly that has two pairs of wings. That’s a single change in a single gene, so it certainly is possible that other changes could cause dramatic differences between two types of horses.
So, which is correct? Is it gradualism? Is it punctuated equilibrium? Most likely, both ideas are correct at times and at places; we really have to go to the evidence of the rocks to understand. Evolution is what evolution is; it isn’t what we impose on it because of our theories, which may be much more narrow.
Common Questions about the theory of Punctuated Equilibrium
According to the gradualist model, changes in species are all but invisible over the short time span of modern observations, and they are usually obscured by innumerable gaps in the imperfect fossil record.
In 1972, American paleontologists Stephen Jay Gould and Niles Eldridge challenged conventional wisdom with an opposing viewpoint, called punctuated equilibrium, which claims that species change one to another in relatively sudden bursts, without a leisurely transition period. These episodes of rapid change are separated by relatively long, static spans, in which a species may change hardly at all, perhaps for millions of years.
The study of these horses were once used as a classic textbook example of the gradual evolution of a horse, but they’re now providing equally compelling evidence for punctuated equilibrium. It turns out that horses evolved in discrete steps, each of which persisted almost unchanged for millions of years, and was eventually replaced by a distinctive newer model. You had the four-toed Eohippus, which preceded the three-toed Miohippus, for example. But North American fossil evidence suggests a jerky transition of distinct species between the two types.
