What's Punctuated Equilibrium?

Like a baby rocking gently in the womb suddenly faced with forceful ejection into the world, punctuated equilibrium describes a model of evolution in which species serenely coast for tremendous stretches of time unchanged, only to have a spurt of evolution suddenly give rise to nascent new species in the blink of an eye.

The idea that evolution only occurs at a slow, drawn-out pace with a stream of transitions between older and newer species was almost the downfall of evolution as we know it -- detractors often cited the fairly chronic lack of intermediary fossil evidence as proof that evolution was false. That's because this model, commonly known as phyletic gradualism, should have yielded a plethora of fossils morphing slowly from one form to another. Instead, the geological strata tends to serve up completely new specimens in a relatively abrupt fashion.

"Many breaks in the fossil record are real," wrote Niles Eldredge and Stephen Jay Gould in 1972's book Models in Paleobiology -- in an essay called "Punctuated Equilibria: An Alternative to Phyletic Gradulism."

"They express the way in which evolution occurs, not the fragments of an imperfect record," Eldredge and Gould wrote, regarding these breaks.

In other words, punctuated equilibrium looks at this lack of data as critical evidence as opposed to tragic flaw -- it offers an alternative. If instead of a long, stately evolution with variations continually arising along the way, what if there was a sudden flurry of activity and then everything became calm again? That would help explain the lack of transitory fossil samples.

The question now is -- what can have such a drastic effect on a population that it goes through an evolutionary whirlwind?

One theory for this is the concept of allopatric speciation -- a model which describes evolution triggered by geographic location. If members of a species become geographically separated or isolated from their parent species, punctuated equilibrium suggests they'll rapidly alter to adapt to their new environment.

To be clear: "rapid" can mean several hundred to thousands of years, but if fossils are dated to the closest million -- that's just a blink of an eye. And it's another reason it makes sense that little fossil evidence of change would survive -- these evolving peripheral populations would tend toward smaller numbers. The periods of stability in between swings can be accounted for with reproductive gene flow.

In short, if populations are not isolated, they usually continue to intermingle genetically, geographically or both, maintaining a static nature.

But what about humans? Are we going through an evolutionary growth spurt of our own?

That's what a research team led by University of Wisconsin-Madison anthropologist John Hawks found in a recently published a study in the Dec. 10, 2007 issue of the Proceedings of the National Academy of Sciences (PNAS) that details how humans have changed in the last several thousand years. Using the data gathered for the International HapMap Project -- an undertaking related to the Human Genome Project -- the researchers estimate that seven percent of the genes found in modern human have recently been tweaked.

The study identified several examples of this evolution, from adults' ability to enjoy a cold glass of milk to an increased resistance for a number of nasty diseases.

"In evolutionary terms, cultures that grow slowly are at a disadvantage, but the massive growth of human populations has led to far more genetic mutations," Hawks recounted in a 2007 interview with the University of Wisconsin-Madison News. "And every mutation that is advantageous to people has a chance of being selected and driven toward fixation. What we are catching is an exceptional time."

Article posted March 3, 2009.

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