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I recently read The Dinosaur Heresies by Robert T. Bakker, a 1986 popular science book presenting arguments for an active lifestyle and high metabolic rate in dinosaurs.

One of the arguments that Bakker presents relates to the pattern of species and genera in the fossil record over time. He states that "warm-blooded" animals (i.e. animals with high metabolic rates) are expected to have higher turnover of taxa and faster rates of diversification. Dinosaurs have high species and genus turnover and rapid diversification, which Bakker interprets as evidence for a high metabolism.

This argument rests on the claim that high metabolism is correlated with high evolutionary turnover. Setting dinosaur physiology aside, I wonder how reliable the underlying assumption really is.

Is there good evidence that metabolism and evolutionary rates are causally linked, perhaps due to the ecological strategies that warm-blooded animals tend to adopt?

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  • $\begingroup$ The question is very interesting. I don't think you express any argument though but only a hypothesis. From your post, the hypothesis that "animals with high metabolism have higher rate of speciation" seem to come from nowhere. Can you try to express Bakker's logic behind this hypothesis? $\endgroup$ – Remi.b Dec 28 '15 at 2:18
  • $\begingroup$ @Remi.b Bakker argues that dinosaurs had a high metabolism, based on the rates of divergence and extinction in the fossil record. However, this argument relies on the claim that high metabolism is in fact correlated with high evolutionary turnover. My question is whether this underlying claim is indeed well-supported by evidence. $\endgroup$ – augurar Dec 28 '15 at 2:26
  • $\begingroup$ Oh ok. It is not an argument then. It is a claim that such evidence exist. An argument should follow a logical thinking, an argument is not a non-referenced claim. So, you are asking if the claim is supported by evidence or not. $\endgroup$ – Remi.b Dec 28 '15 at 2:35
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Metabolism and evolutionary rates are linked, but not in the way you describe.

First consider that metabolism is intrinsically linked to body size. The larger an animal is, the smaller amount of energy per unit mass it takes to maintain that body. This can then be extended to many things about an organism, such as life span, gestation time, population growth, etc. (see Brown et al. 2004 for more detail).

Next consider that metabolism changes with temperature. Take an ectotherm (an organism who's internal temperature is the same as the ambient environment, traditionally called cold-blooded creatures) like a lizard, which will go thru temperature fluctuations throughout the day. It will have higher metabolism when the temperature is higher. A lizard living in a warm environment like the tropics will have an overall higher metabolism than a lizard of the same size living in a temperate climate. With that higher metabolism comes a different life span, gestation time, population growth, etc. even though their body sizes are the same.

When temperature is considered and corrected for, all organisms have attributes that scale directly with body size. A plant literally has the same metabolic rate as a lizard if they are a similar mass and live at the same temperatures. (Seriously, see the Brown paper).

Now onto evolution. Rates of evolution depend on many things, but primarily two. Generation time (the time for 1 generation of a population to reproduce) and mutation rates of DNA. A longer generation time decreases the rate of evolution. For example a single celled organism reproducing every month will evolve faster than an elephant reproducing every decade. Larger rates of mutation in DNA increase the rate of evolution. If DNA does not change then evolution cannot happen. So the faster DNA changes (thru mutation rates) the faster the potential evolution can be.

Generation time increases with increased body size/metabolism (Brown et al. 2004). Mutation rates decrease with increased size/metabolism (Gillooly et al. 2005).

With those two general patterns, one can say that the rate of evolution decreases with increased metabolism.

Now onto dinosaur evolution. You said that R. Bakker observed that dinosaurs had a higher rate of evolution than expected. With the hypothesis that a higher metabolism caused this. I'll assume they had the same physiological constraints as animals of today, which is reasonable considering they are ancestors of birds. Which everything I just laid out I would propose R. Bakker hypothesis is false, since a higher metabolism means a higher generation time and lower mutation rates, which would lead to decreased rate of evolution.

The latest science says that dinosaurs were endothermic (regulate their own temperature) up to a point (Witze 2014), so their internal temperatures were not as warm as mammals, but probably warmer than most fish and reptiles. So when accounting for size they likely had lower evolutionary rates than mammals, and higher rates than fish, reptiles, insects, or other ectotherms.

Why then does R. Bakker observe higher evolutionary rates in the fossil record then? I can think of just a general reason why. The timescale of dinosaurs is on the order of 10's of millions of years. Much longer than the tiny amounts of evolution that we have observed in the past 100 years of science. There could easily be processes happening at those long time scales that we aren't considering.

References

Brown, J. H., Gillooly, J. F., Allen, A. P., Savage, V. M., & West, G. B. (2004). Toward a metabolic theory of ecology. Ecology, 85(7), 1771-1789.Link

Gillooly, J. F., Allen, A. P., West, G. B., & Brown, J. H. (2005). The rate of DNA evolution: effects of body size and temperature on the molecular clock. Proceedings of the National Academy of Sciences of the United States of America, 102(1), 140-145. Link

Witze, Alexandra (2014). Dinosaurs neither warm-blooded nor cold-blooded. Nature. 2014 Link

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  • $\begingroup$ Interesting, and thanks for the "metabolic theory" links. However, what you say in the third-to-last paragraph is actually the opposite of what you said in the preceding paragraphs. Higher metabolism means lower generation time and higher mutation rates, and hence a faster rate of evolution. This is in agreement with Bakker's arguments. $\endgroup$ – augurar Aug 7 '16 at 2:56
  • $\begingroup$ Regarding dinosaur mesothermy, the debate is far from settled. While some researchers favor a "mesothermic" model, one can find other papers arguing for complete endothermy like that of modern birds and mammals (e.g. Seymour 2013). $\endgroup$ – augurar Aug 7 '16 at 3:05
  • $\begingroup$ Regarding the last paragraph — much of what we know about evolution comes from the fossil record, so I'm not sure what you mean by "processes happening at those long time scales that we aren't considering". When Bakker says that evolutionary rates for dinosaurs in the fossil record are high, he's comparing them to other groups in the fossil record, not just to recent observations over the last hundred years. $\endgroup$ – augurar Aug 7 '16 at 3:08
  • $\begingroup$ Pretty sure my comments are consistent throughout. As metabolism increases generation time increases and mutation rates decrease. That's what observations from today tell us. Assuming dinosaurs followed the same patterns then you can't say that their higher than expected evolution rates are due to high metabolism. $\endgroup$ – Shawn Aug 8 '16 at 14:37
  • $\begingroup$ Process at long time scales could be many things. For example, how should we interpret the evolution of a small bodied creature to a large bodied one over several million years? I have no idea, but something like that could affect how we compare evolution in the fossil record with evolution we see in laboratories. $\endgroup$ – Shawn Aug 8 '16 at 14:42

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