Explaining differences in rates of evolution

Scientists look to fossils and evolutionary trees to help determine the rate of evolution – albeit with conflicting results. A new model has helped to resolve these contradictions.


I almost posted this one too.


But that isn’t about rates of evolution at all, but about rates of speciation and extinction. I also find much problematic about using fossils to determine speciation or to distinguish anagenesis from cladogenesis, and the confusion of species concepts represented by the notion of anagenetic speciation.

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This was strange for me too. Isn’t that just a chronospecies?

Yes, except that chronospecies is generally supposed to be an artificial notion, while anagenetic species are not. Wrongly.

Chronospecies are an artificial notion? I think you mean that in a chronospecies that any line we place down would be artificial, so that “anagenetic species” is an artificial notion.

Is that what you mean?

Partly. Chronospecies are intended to be artificial, just a matter of convenience. The problem is that I don’t think the authors intend anagenetic species to be like that, even though they are. The problem behind that is that when we talk about speciation, we usually mean that based on the biological species concept: the evolution of reproductive isolation between two populations. This concept can’t be extended very far either in space or, especially, time. Who can tell if two populations separated by thousands of years (or miles) would be reproductively isolated? And since anagenesis doesn’t involve splitting of populations, it isn’t speciation under the BSC.

That brings us to the second problem. It’s very difficult (most would say impossible) to trace actual ancestry and descent in fossil populations. Thus one can’t really determine anagenesis or distinguish it from cladogenesis (ordinary speciation). Nor, I would claim, can we reliably recognize speciation in the fossil record. Many species are cryptic, especially given the limited preservation in fossils. Species can vary through their range, and geographic sampling is likewise limited in fossils. If we see a new morphotype appear, it could be a new species, or migration from a previously unsampled location, or anagenesis in an existing but previously cryptic species. I can’t see a good way to distinguish them. Thus paleontologists use the morphological species concept, which is practical, but then it’s frequently equated with the biological species concept.