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Citation: Benson RBJ, Campione NE, Carrano MT, Mannion PD, Sullivan C, et al. (2014) Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage. PLoS Biol 12(5): e1001853. doi:10.1371/journal.pbio.1001853

Citation: Benson RBJ, Campione NE, Carrano MT, Mannion PD, Sullivan C, et al. (2014) Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage. PLoS Biol 12(5): e1001853. doi:10.1371/journal.pbio.1001853

Most of the time I’m focusing on population genetic time scales when I think of evolutionary change. That is, allele frequency shifts within a species level lineage, or narrower. Since this is amenable to experimental analysis obviously there are advantages. But sometimes I really wonder if I’m doing a disservice to myself not paying more attention to examinations of evolutionary change on the scale of tens of millions of years and across whole clades which might have thousands of species. A new paper in PLoS BIOLOGY, Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage
is very interesting. Here’s the author summary:

Animals display huge morphological and ecological diversity. One possible explanation of how this diversity evolved is the “niche filling” model of adaptive radiation—under which evolutionary rates are highest early in the evolution of a group, as lineages diversify to fill disparate ecological niches. We studied patterns of body size evolution in dinosaurs and birds to test this model, and to explore the links between modern day diversity and major extinct radiations. We found rapid evolutionary rates in early dinosaur evolution, beginning more than 200 million years ago, as dinosaur body sizes diversified rapidly to fill new ecological niches, including herbivory. High rates were maintained only on the evolutionary line leading to birds, which continued to produce new ecological diversity not seen in other dinosaurs. Small body size might have been key to maintaining evolutionary potential (evolvability) in birds, which broke the lower body size limit of about 1 kg seen in other dinosaurs. Our results suggest that the maintenance of evolvability in only some lineages explains the unbalanced distribution of morphological and ecological diversity seen among groups of animals, both extinct and extant. Important living groups such as birds might therefore result from sustained, rapid evolutionary rates over timescales of hundreds of millions of years.

As this paper is predicated on nifty statistical analysis one has to be careful at taking the results at face value. Subsequent reanalysis might yield a different conclusion. But it is certainly an intriguing possibility that clade-level selection of some sort might be operating. I’m still very skeptical of what to even think about this, or how to conceptualize the dynamic. But that’s often a good thing.

 
• Category: Science • Tags: Dinosaurs, Evolution, Macroevolution 
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  1. How often do you think about clade-level selection in intraspecific studies? Might it be that “evolvability” of certain populations is advantageous just as the authors suggest that evolvability of larger clades of species is?

  2. selection above the level of the individual is not super popular in evolutionary genetics. behavioral ecologists have totally different ideas though 🙂 obviously above-individual-level selection can occur in theory, but the assumption is that the conditions are too stringent.

  3. jb says:

    Dinosaurs were the dominant land animals for much longer than mammals have been. I’ve sometimes wondered what mammals would look like after an additional 100 million years of evolution (without the influence of humans). It’s only been 66 million years since the extinction event that cleared the way for mammalian dominance. 66 million years after the Permian-Triassic extinction put you in the early Jurassic, when the dinosaurs were just getting started. Maybe the only reason there have never been any Supersaurus sized mammals is that they just haven’t had time to evolve yet!

  4. I remember a discussion in The Selfish Gene (I think) about how a population of monkeys (or anything subject to parasites) might develop a practice of mutual grooming, or alternatively develop a practice of never grooming anyone else (other strategies being unstable). The author was careful to point out that since evolution is blind, if you’re looking at a given population you can’t predict whether mutual grooming or mutual apathy will evolve, even though grooming is more advantageous.

    But to my way of thinking, if instead of taking the perspective that you’re looking at a population as it evolves over time, you take the perspective that you’re looking at the world and seeing how the animals in it tend to behave, it should be easy to predict that there will be a lot more grooming than apathy: populations that develop the apathy strategy should tend to die out under parasite load, and populations that develop a grooming strategy should flourish better and therefore be present for much longer stretches of history. So if your sampling method is “choose a point in time, and see whether things that might groom actually do groom”, your results should be weighted towards grooming.

    I’m not sure how relevant this is, but it seems related to me.

  5. Similarly, I was under the impression that the standard explanation of sexual reproduction is precisely that it enables faster evolution at the group level, even though it is obviously highly detrimental to the individual organisms practicing it (compared to asexual reproduction).

  6. Bruce says:

    jb >Maybe the only reason there have never been any Supersaurus sized mammals is that they just haven’t had time to evolve yet!

    Atmospheric oxygen fluctuated from 15% to 35% back when insects and amphibians were huge. If I was breeding animals for size in space, I’d try a higher oxygen level. At 35% you’d get a lot of forest fires. Imagine a T Rex with a hummingbird metabolism, scooting into fire zones for fresh BBQ, needing scales for fireproofing. If I was a T Rex with the brains of a crow, I might use those little arms to carry a burning branch upwind of tasty critters.

    So when I get my time machine, I need an asbestos suit and self-contained breathing apparatus to go with my Holland and Holland.

  7. jb says:

    Atmospheric oxygen fluctuated from 15% to 35% back when insects and amphibians were huge. If I was breeding animals for size in space, I’d try a higher oxygen level.

    My understanding though has been that oxygen levels were lower than today when the dinosaurs were around, which strikes me as strange.

    Checking Wikipedia makes it more confusing, because there are two charts there that seem to completely contradict each other. (The second chart is linked beneath the first, and supposedly it is the first chart “in more detail,” but in fact it is quite different).

  8. Wiki confused me too, and that’s where I got 15-35%. I’d bet there was a lot more seaweed/algae/(confusing fossils that may have floated) in the Oxygen Explosion, critters evolved to eat it, and things fluctuated. The Tethys looks like a great place to have a Sargasso Sea. I don’t think today’s oxygen levels have been stabilized by The Balance of Nature for ever, because I don’t think there’s a balance of nature. Boom and bust forever and ever, amen. (I’m not a scientist, or even very informed here).

    But if I got in a time machine I’d wear a space suit.

  9. ohwilleke says: • Website

    I can imagine a mechanism that could cause disparate rates of evolutionary change to be linked to body size.

    Suppose that any given ecological niche can support X pounds of animal biomass. If an individual animal weighs Y pounds, then the number of animals in existence at any given time is equal to X/Y and for a given X, the number of animals alive at any given time is inversely related to the average body mass of an individual animal.

    Then suppose that in a given generation this is a certain probability, one in a million to be cliche, for example, that a significantly fitness enhancing mutation will occur.

    “Macro-evolution” can be defined as evolutionary changes that are significantly fitness enhancing relative to the pre-evolved version. Macro-evolution, then, cannot happen in a given generation until one or more individuals in the population has a significantly fitness enhancing mutation.

    The larger absolute value of a population’s effective population size is at any given moment, the more likely it is that it will not be prevented from experiencing macro-evolution as a result of a mutational limit (i.e. the absence of anyone in the population who has a mutation that could significantly enhance fitness at that particular point in time).

    A 1 kg animal in a particular ecological niche is 100 times a likely to have a mutation in the population that makes further macro-evolution possible at any given time, than a population of 100 kg animals in that particular ecological niche (assuming that the ecologically permitted biomass is the same).

    Also, small animals, as a consequence of basic bio-mechanical constraints, have faster metabolisms and for some biochemical reason or other, “advanced” animals all tend to have the roughly the same lifespans after adjusting for metabolism. Thus, 1 kg animals will have far more generations per century than 100 kg animals do. Indeed, there is an established formula that quantifies that relationship has been known for roughly eighty years. Per Wikipedia: “Kleiber’s law, named after Max Kleiber’s biological work in the early 1930s, is the observation that, for the vast majority of animals, an animal’s metabolic rate scales to the ¾ power of the animal’s mass. . . . Thus a cat, having a mass 100 times that of a mouse, will have a metabolism roughly 31 times greater than that of a mouse. In plants, the exponent is close to 1.”

    The probability that a macro-evolution facilitating mutation will take place in a species with a given population over a fixed period of time like a century, is directly proportional to the number of generations in that time period.

    So, given the assumptions of fixed lifetime after controlling for metabolism, and fixed animal species biomass independent of animal body weight in any given ecological niche, neither of which are exactly or universally true, but both of which are approximately true, the probability of a macro-evolution facilitating mutation per year in a species as a function of body size s proportionate to: (1/M^3/4)*(1/M).

    Thus, as a heuristically motivated first order approximation, we can expect the rate at which macro-evolutionary change takes place in a species with body mass M is proportionate to M^-1.75.

    When you look at a data set that is the product of that kind of exponential distribution and there are other significant bounds on M from other constraints, a qualitative description of the data in which cases of M under some threshold value fit in one category (e.g. rapidly evolving), and cases of M over some threshold value fit into a second category (e.g. slowly evolving) is a very natural and common way to qualitatively describe this data distribution until you grasp the functional form of the equation that is really at work in driving these differences.

    Now, while it isn’t obvious that this is the case, in reality, one important confounding factor is that the number of SNPs per genome, which is a reasonable way to approximate the number of sites at which mutations could possibly happen, vary widely from species to species of animal in a manner that shows little or no connection to an animal’s body size. Yet, the number of mutations per generation in a genome has been shown empirically to be largely a function of the size of the genome.

    Genome size is quite unlikely to be independent of clade affiliation (since all genomes in the clade have a common ancestor), so naively, genome size should be just as important as body size, if not more so, in determine the rate at which animals in a clade evolve. This factor would not be wiped out by the law of averages within a clade relative to other clades, although this factor might be pretty modest in a macro-clade of a rapidly radiating new kind of animals like dinosaurs with recent common origins since genome size differences between subclade might differentiate from each other fairly slowly relative to rates of mutational change within their genomes.

    But, maybe total genome size isn’t the relevant thing to be looking at in any case. Maybe the number of points at which a mutation could produce macro-evolutionary change in any given genome are about the same in all animals, even though the size of the complete genome differs greatly between species. Even though one animal’s genome might have 1,000,000 base pairs and another animal’s genome might have 100,000,000 base pairs, both animals might have just 1,000 critical base pair points at which a mutation could lead to macro-evolutionary change in that species. In this case, the increased number of mutations in an animal with a larger genome is exactly balanced by the reduced number of sites at which macro-evolutionary change in that species can be triggered.

    It would be interesting to see how well the data in the cited study of dinosaur evolution fits the M^-1.75 macro-evolution rate that is observed as a function of animal body size.

  10. Looking at the supplemental materials, the evolution rate does appear to have a roughly exponential relationship with body size, although the parameters of that exponential relationship are bit hard to eyeball from the data presented in graphic form, and there does seem to be something of a kink in those parameters at a critical mass point, with one set of parameters before that point and another after it.

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