DNA analysis is being used to draw conclusions about when different lineages split. How reliable is this?

Question

The assumption is made that DNA mutates at a steady rate. But we see long periods of species stasis broken by brief periods of rapid evolution (punctuated equilibrium). The fossil record strongly supports the punctuated equilibrium interpretation. How does punctuated equilibrium square with the belief that DNA mutates at a steady rate?

Answer 1

Punctuated equilibrium does not contradict a constant rate of mutation.

Punctuated equilibrium is about a differential rate of *speciation* (rapid generation of new species or genera ... more technically called cladogenesis).

The mutation rate is just the rate at which transcription errors accumulate in the genotype. These mutations may or may cause change in the organism (evolution), and that change (evolution) may or may not cause speciation.

When I say that mutations may or may not cause change, I mean that many (if not most) mutations can be neutral ... small changes in the amino acid sequences of proteins that do not affect their function, or even transcription errors in junk DNA that has no function at all. Although these changes do not cause change (evolution) in the organism, they accumulate nonetheless at a constant and measurable rate.

And when I say that these changes may or may not cause speciation (cladogenesis), I mean that speciation also requires branching events caused by the environment.

So mutations can accumulate at a constant rate, while the rate of speciation itself can be quite non-constant.

In other words, a species can remain relatively constant for a long period, while still accumulating mutations in the genotype that have no effect, positive or negative, on the organism ... and then hit some period where some conditions or events can cause rapid and successive branching of that species line, without any change in the rate of mutation.