Evolution 101

Sunday, July 23, 2006

What is Punctuated Equilibrium?

I received a message from a listener, DH, that I wanted to pass along to all of you. In the popular culture, oftentimes the idea is propagated that evolutionary theory is a controversial theory within science itself. This could not be further from the truth, and an international organization of science academies, the IAP, has released a statement that clarifies their official scientific position in regards to evolutionary theory. I’d like to read that statement to you now:

We, the undersigned Academies of Sciences, have learned that in various parts of the world, within science courses taught in certain public systems of education, scientific evidence, data, and testable theories about the origins and evolution of life on Earth are being concealed, denied, or confused with theories not testable by science. We urge decision makers, teachers, and parents to educate all children about the methods and discoveries of science and to foster an understanding of the science of nature. Knowledge of the natural world in which they live empowers people to meet human needs and protect the planet. We agree that the following evidence-based facts about the origins and evolution of the Earth and of life on this planet have been established by numerous observations and independently derived experimental results from a multitude of scientific disciplines. Even if there are still many open questions about the precise details of evolutionary change, scientific evidence has never contradicted these results:

1. In a universe that has evolved towards its present configuration for some 11 to 15 billion years, our Earth formed approximately 4.5 billion years ago.

2. Since its formation, the Earth – its geology and its environments – has changed under the effect of numerous physical and chemical forces and continues to do so.

3. Life appeared on Earth at least 2.5 billion years ago. The evolution, soon after, of photosynthetic organisms enabled, from at least 2 billion years ago, the slow transformation of the atmosphere to one containing substantial quantities of oxygen. In addition to the release of the oxygen that we breathe, the process of photosynthesis is the ultimate source of fixed energy and food upon which human life on the planet depends.

4. Since its first appearance on Earth, life has taken many forms, all of which continue to evolve, in ways which palaeontology and the modern biological and biochemical sciences are describing and independently confirming with increasing precision. Commonalities in the structure of the genetic code of all organisms living today, including humans, clearly indicate their common primordial origin.

We also subscribe to the following statement regarding the nature of science in relation to the teaching of evolution and, more generally, of any field of scientific knowledge: Scientific knowledge derives from a mode of inquiry into the nature of the universe that has been successful and of great consequence. Science focuses on (i) observing the natural world and (ii) formulating testable and refutable hypotheses to derive deeper explanations for observable phenomena. When evidence is sufficiently compelling, scientific theories are developed that account for and explain that evidence, and predict the likely structure or process of still unobserved phenomena. Human understanding of value and purpose are outside of natural science’s scope. However, a number of components – scientific, social, philosophical, religious, cultural and political contribute to it. These different fields owe each other mutual consideration, while being fully aware of their own areas of action and their limitations. While acknowledging current limitations, science is open-ended, and subject to correction and expansion as new theoretical and empirical understanding emerges.

Signed to this statement are the national academies of sciences from 66 countries, including the United States, the United Kingdom, Poland, Ireland, Italy, Greece, Hungary, Japan, Lithuania, Germany, Austria, Australia, and interestingly enough, Iran. It’s great to see such clear international support for science, and I think that everyone will eventually take notice of it.

On a similar note, I received a question from JM asking me about the beliefs of the current Pope about evolutionary theory. Pope John Paul II, as some of you are probably aware, was fairly well-educated about science, and accepted evolution explicitly in a 1996 speech, although he warned against the acceptance of any material explanation for the existence of the human soul. The current Pope Benedict XVI, before he was elected, made statements accepting of evolutionary theory as well. Recently, the Vatican’s chief astronomer explicitly endorsed evolutionary theory and rejected intelligent design as unscientific. However, other statements made by Catholic officials support the idea that the evolutionary process is guided by the divine, although the specific details of this are never made clear. This seems to be essentially the position that’s promoted by Dr. Kenneth Miller, the author of “Finding Darwin’s God.” In that book, as I’ve mentioned before, Dr. Miller explains evolutionary theory in very basic language, dismisses the claims of intelligent design proponents, and provides room for his faith. I would recommend that book to anyone interested in evolution just in a general sense, but especially to those who consider their faith as an obstacle to the acceptance of evolutionary theory.

Today I’d like to talk about punctuated equilibrium. This concept was coined by the famous paleontologist Stephen Jay Gould, and it has become somewhat of a controversy within evolutionary theory. This fact is, predictably, seized upon by various evolution deniers as a way to challenge evolutionary theory as a scientific theory, as if to say, “Aha! The scientists disagree about evolution! Therefore, it is wrong.” But this is unfounded. Scientists disagree all the time- by that logic, all scientific theories are null and void, because there’s always some controversy going on at some point in time about some aspect of just about every theory.

What is meant by the term, “punctuated equilibrium” is that, throughout evolutionary history, the evolution of one species to another has not been a constant process, but has instead been one of population stability followed by rapid speciation, followed by long periods of stability. You might be wondering, “Why is this controversial?” but in the definitive paper on punctuated equilibrium, published by Gould and Niles Eldredge in 1972, they made the case for their concept as an alternative to what they called, “phyletic gradualism.” This would be the opposite to what they proposed of the stability-speciation-stability model of evolution, and would describe the situation of whole populations speciating slowly, and constantly over time.

To give you a better idea of the distinction between the two, just imagine a child growing. When it is born to just prior to puberty, the average child grows pretty gradually and steadily. This is roughly the kind of change that is thought of with phyletic gradualism. Now imagine that same child going through puberty, and experiencing growth spurts. It could go one or two years without growing much at all, and then over a three month period shoot up several inches. Then maintain that height for another year, and then experience another growth spurt. This is roughly the kind of change that is thought of with punctuated equilibrium. There are long periods of little to no change, or equilibrium, which are punctuated by short periods of rapid change.

This does not imply in any way that new species spring into existence instantly and magically. Just like a growing child doesn’t snap its fingers and shoot up three inches, new species don’t just “appear” out of nowhere. This is called “saltation,” and it’s associated with ideas like creationism, not evolutionary theory.

So, why did Gould and Eldredge even come up with this concept anyway? Were they just sitting around, trying to come up with ways to pick fights with other scientists?

The reason for the theory comes from the natural evidence, actually the fossil evidence. Throughout the course of paleontological investigation, fossils of any given species were found, more or less with the same basic anatomy, throughout the range of their presence in the fossil record. As one moved along the geological column closer to present day, one would find descendent species that were obviously related by their anatomy, but were different enough to be classified as different species. And so on and so forth, with daughter species continuing to be found in more recent strata, but again, with very little gradation among the species population. If you recall from the podcast on transitional species, this same phenomenon is latched onto by evolution deniers as being “gaps” in the fossil record, and is seen as a refutation of evolutionary theory.

The common objection is that, if evolution is true as Darwin described it, then we should be able to dig into the ground, find the fossil of a species, dig a little farther, find the fossil of its parent that looks slightly different, dig again and find its grandparents that look even more different, and dig once again to find its great-grandparents that look different enough to be classified as a different species altogether. That’s somewhat of a simplified account of that objection, but it encompasses the intended spirit. Essentially, deniers of evolution claim that we should have an unbroken chain of fossils that demonstrate the minute evolutionary changes over time. That is, we should not only have fossils that demonstrate separate species, but we should also have the fossils of the organisms that are in between those species.

If you remember from the podcast about the concept of a species, you’ll of course realize how silly a request that is. Fossil species are defined retroactively- there isn’t some kind of label that switches from one species name to another as populations change.

In addition, consider that the geological record is not perfect. It’s not a filing cabinet where fossils are neatly ordered, ready for discovery by paleotologists. It is incredibly unlikely that the remains of an organism will become fossilized instead of decomposed, and even for those that become fossilized, their continued preservation is not certain. Fossils are destroyed all the time, by seismic activity, volcanic activity, and erosion.

However, even given that consideration, what punctuated equilibrium does is explain the pattern of the fossil record in a way that is consistent with evolutionary theory. Namely, that speciation involves a small group of organisms with the parental population, this group is typically isolated geographically from the parental population, and this genetic isolation promotes rapid morphological change in the daughter species population. Because of this, fossil records of the parent and daughter populations will appear to be geographically and chronologically distinct in the geological column. This concept really isn’t anything that revolutionary when you think about it- the famous evolutionary biologist Ernst Mayr had conceived of both of those mechanisms long before Gould and Eldredge came around. In trying to figure out mechanistically how species develop, he came up with peripatric speciation, in which a small subset of a population forms into a new species population, and allopatric speciation, in which geographically isolated members of a population form a new species population. If that sounds exactly like what happens in punctuated equilibrium, you’re basically right- and this is what the real controversy is about.

Within the scientific community, it’s never been a question of whether evolution was right or wrong, or whether punctuated equilibrium was right or wrong. For the most part, punctuated equilibrium was accepted by the scientific community almost immediately. What is in dispute, rather, is to what extent punctuated equilibrium was in agreement with Darwin, and to what extent it differed from evolutionary theory prior to the Gould and Eldredge’s introduction of the concept. It’s true that Darwin wrote extensively about the concept of gradualism in his work, but he seems to have anticipated an idea like punctuated equilibrium, as he suggested that the period of time in which a species doesn’t change is likely much longer than the period of time in which it does. You’ll remember that previously I mentioned that Gould and Eldredge had introduced the concept of phyletic gradualism as a counter to their own concept. Many scientists see this as a strawman, in that they made the case that Darwinism implied phyletic gradualism, to which they were proposing their own scientific alternative. This is simply not true of Darwin himself, and I’ve already mentioned that he anticipated the concept of punctuated equilibrium himself. Why Gould and Eldredge would have painted Darwin with such an inaccurate brush is a question I can’t answer, although it should be noted that Gould was known for speaking about his discoveries in a manner which some would consider overemphasizing their revolutionary nature, to the extent to which many people thought he was criticizing the existing tenets of evolutionary theory.

Essentially, punctuated equilibrium is an accepted part of evolutionary theory, and not only that, it’s a clear concept that follows clearly from the original theory as first conceived of by Charles Darwin all the way to present-day. It’s a very useful concept for understanding the patterns within the fossil record, and the only controversy that still exists is in regards to how revolutionary of an idea it actually was within evolution.


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