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I am reading Punctuated Equilibria: The Tempo and Mode of Evolution Reconsidered, which mentions the "tempo" and "mode" of evolution. I'm not familiar with the related literature and when I google the concept, what I got is something like:

One general topic, suggested by the word "tempo," has to do with "evolutionary rates..., their acceleration and deceleration, the conditions of exceptionally slow or rapid evolutions, and phenomena suggestive of inertia and momentum." A group of related problems, implied by the word "mode," involves "the study of the way, manner, or pattern of evolution, a study in which tempo is a basic factor, but which embraces considerably more than tempo''

This is as elusive as a definition can be. A clear definition should be something like "tempo is ...."

So what are the definitions of mode and tempo in evolution?

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Actually, we can go right to the source on this: George Gaylord Simpson's Tempo and Mode in Evolution (1944).

Tempo is the rate of evolution. Simpson was a paleontologist, so he was trying to answer questions like "what is the percent change in the size of this tooth per million years?" He has a chapter estimating these rates for various characters in horses, which have a very good North American fossil record.

Mode encompasses the mechanisms responsible for evolution and their patterns. Simpson was working at the time of the Modern Synthesis (the wikipedia page is decent and brief enough on the topic, about which books are written), coming from the perspective of a paleontologist trying to reconcile genetic evolution with the macroevolutionary patterns he was seeing in the fossil record.

Simpson was really the first paleontologist to look at fossils in a quantitative way, trying to understand of connections between microevolutionary processes and macroevolutionary patterns. He even co-authored one of the first quantitative and statistical methods books for "organismal" biologists: Quantitative Zoology.

Specifically, of tempo, he says (p. xvii):

"...evolutionary rates under natural conditions, their acceleration and deceration, the conditions of exceptionally slow or rapid evolutions, and phenomena suggestive of inertia and momentum."

Of mode, he says (p. xviii)

"...the study of the way, manner, or pattern of evolution, a study in which tempo is a basic factor, but which embraces considerably more than tempo. The purpose is to determine how populations became genetically and morphologically differentiated, to see how they passed from one way of living to another or failed to do so, to examine the figurative outline of the stream of life and the circumstances surrounding each characteristic element in that pattern."

Mode basically encompasses all the processes of evolution. Tempo is the subset of mode that concerns how fast evolution is (or is not) happening.

The great thing is that 70 years after Simpson's book, biologists are still trying to understand tempo and mode of evolution.

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It's great that you clarify tempo IS the rate of evolution. However, I'm still not clear about the relation between mode and tempo. Since tempo is a "rate" of evolution, in the context of dynamics, it is a rate of the underlying process, or generally speaking, a property of the process. How should I understand "mode ... encompasses all ... processes .... Tempo is the subset of mode..."? The sentence seems to suggest tempo is also a process. –  wdg Apr 25 at 3:01
    
Does it help to think of mode as all the processes involved in evolution? The rate of evolution is one of those processes, but there are additional processes (e.g., classical modes of speciation: allopatric, parapratric, etc.). So an "evolution" textbook probably has a chapter on rates. The rest of the book would be classified as "mode". –  kmm Apr 25 at 13:18
    
the textbook metaphor is a nice one. –  wdg Apr 27 at 2:43

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