Paleontological term, coined by Stephen Jay Gould, to describe the uneven progression of evolution. Instead of being a continuous, gradual process, evolution is characterized by very short periods of extreme diversification and growth interspersed between long periods of slow change. This idea is based on evidence in the fossil record where strata for many ages contain the same organisms. Abruptly, within a short geological period of time, a whole new set of organisms are found. The theory is based on the idea that periods of intense growth are initiated by a novel adaptation which gives a particular species an amazing advantage over its environment. All interacting species are quickly selected for those that can adapt to this new change. Other causes of puctuated equilibrium can be quick environmental changes such as a meteor or ice-age.

An example would be the Cambrian explosion, if I remember correctly, where the evolution of tissues that can form hard structures (i.e. teeth and body armor) resulted in an astounding amount of growth in a short period of time as predator and prey counter-evolved to outdo each other with these new weapons and defense systems. The trilobites are a prime example of this with their hard chitinous bodies. The large soft bodied organisms of the Precambrian Era all but disappeared during this prolific diversification.

The reason I thought of this topic, I must sheepishly admit, is from watching the movie version of X-men where they say in the introduction that evolution is a slow and gradual process, but every now and then, it proceeds very rapidly - hence the sudden emergence of mutants within the human population. Although the selective pressure for mutants isn't obvious, its a fun idea. There are certainly worse perversions of science in the movies (e.g. the new version of Godzilla)

"Punk Eek" is often used erroneously by creationists to claim that "scientists don't agree that evolution is a valid theory". This is of course, bullshit. It's not saying that a) evolution is disproven, or b) Darwin was wrong. The hypothesis suggests an additional, alternate mode of evolution, which occurs at a faster rate than the standard "neo-Darwinian" model of evolution by successive small degrees.

The novel adaptation mentioned in the first writeup is often referred to as a "hopeful monster". Imagine a fish born with a skeletal deformity which gave it bony plates around certain vital organs. It could survive predation to produce more offspring than softer fish. Eventually, most of the fish would be hard-bodied. You would not find a complete range of intermediate forms between the original soft fish body and the "armoured" body, would you? Just a sudden appearance of "armoured fish"! It's a just-so story, but it illustrates the point.

The most common misinterpretation of this theory is by people who seem to think of evolution as some kind of magical force, a tangible 'thing'. "Evolution goes slowly for a while, then speeds up, then goes slowly again... But if evolution is so scientific and all, how can it vary like this?"

Evolution is simpy a process. If one creature develops an improvement, this will allow the creature's species to wipe out other less developed species, to expand its habitat, etc. This dominant species will then evolve into several species over a sufficient period of time. (We're still talking big numbers here).*

You can then say that the average 'advancedness' of species on Earth has increased; evolution has 'sped up'. But this is a rather strange way of looking at it. Instead of punctuated equilibrium being a replacement to evolution, or an 'add-on', it is simply an explanation of what path evolution will take.

* Of course, the dominance of one species will not totally destroy all others. In fact, the pressures on the non-dominant species will cause natural selection on them to increase, so they will either evolve improvements of their own or die out.

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