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Evolutionary pressures, within and without

Gene Expression
By Razib Khan
Apr 20, 2011 10:16 AMNov 20, 2019 3:49 AM


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Foraminifera, Wikimedia Commons

The Pith: The tree if life is nourished by agon, but pruned by the gods. More literally, both interactions between living organisms and the changes in the environment impact the pulsing of speciation and extinction.

No one can be a true "Renaissance Man" today. One has to pick & choose the set of focuses to which one must turn one's labor to. Life is finite and subject to trade offs. My interest in evolutionary science as a child was triggered by a fascination with paleontology. In particular the megafauna of the Mesozoic and the Cenozoic, dinosaurs and other assorted reptilian lineages as well as the hosts of extinct and exotic mammals which are no more. Obviously I don't put much time into those older interests at this point, and I'm as much of a civilian when I read Laelaps

as you are. More generally when it comes to evolution I focus on the scale of microevolution rather than macroevolution. Evolutionary genetics and the like, rather than paleontology. This is in part because I lean toward a scale independence in evolutionary process

, so that the critical issue for me has been to understand the fundamental lowest level dynamics at work. I'm a reductionist.

I am not quite as confident about the ability to extrapolate so easily from evolutionary genetic phenomena upwards in scale as I was in the years past. But let's set that aside for a moment, and take a stroll through macroevolution. When I speak of natural selection I often emphasize that much of this occurs through competition within a species. I do so because I believe that the ubiquity of this process is often not properly weighted by the public, where there is a focus on competition between species or the influence of exogenous environmental selective pressures. The intra- and inter- species competition dynamic can be bracketed into the unit of selection

debate, as opposed to the exogenous shocks of climate and geology. The former are biotic and the latter are abiotic variables which shape the diversity and topology of the tree of life. A new paper in Science attempts to quantify the effect of these two classes of variables on the evolutionary arc of a particular marine organism over the Cenozoic, roughly the last 65 million years since the extinction of the dinosaurs. Interplay Between Changing Climate and Species’ Ecology Drives Macroevolutionary Dynamics


Ecological change provokes speciation and extinction, but our knowledge of the interplay among the biotic and abiotic drivers of macroevolution remains limited. Using the unparalleled fossil record of Cenozoic macroperforate planktonic foraminifera, we demonstrate that macroevolutionary dynamics depend on the interaction between species’ ecology and the changing climate. This interplay drives diversification but differs between speciation probability and extinction risk: Speciation was more strongly shaped by diversity dependence than by climate change, whereas the reverse was true for extinction. Crucially, no single ecology was optimal in all environments, and species with distinct ecologies had significantly different probabilities of speciation and extinction. The ensuing macroevolutionary dynamics depend fundamentally on the ecological structure of species’ assemblages.

The foraminifera went from 2 species early in the Cenozoic to over 30. Additionally, as noted in the paper they're well sampled across the whole time period. It is a cliché that paleontology suffers from a deficit of thick data sets, but this seems far less the case with marine organisms which are numerous and mineralize copiously, such as the foraminifera. Ecology here seems to be defined both by position in the water column as well as morphology of the species. Presumably this intersection defines specific niches inhabited by the species of this lineage. Figure 2 and 3 illustrate the primary results of this paper:

The scatter plots in figure 2 are pretty striking. Using one parameter there's almost no prediction of clade growth. Remember that R-squared simply tells how how much of the variance of axis y can be explained by axis x. But, when you include the interaction between two variables, the R-squared starts to become significant. And when you have three variables, it isn't too shabby at ~0.66. That means the interaction between clade diversity, climate, and ecology, can explain 2/3 of the variance in clade growth. Diversity just measures inter-specific competition and interaction. A diversity focused model would predict that clades rapidly expand to fill available niches when it is low, and that one attains a steady state equilibrium when species richness has increased. Climate is rather self-evident. Finally, as I note above, ecology seems to be a compound of characteristics and indicates the positioning of a population in relation to others and their environment. In this paper the authors refer to the Red Queen's Hypothesis

, as well as the "Court Jester Model." Honestly I don't really know specifically what the latter is aside from what is mentioned in the paper. That certainly highlights my ignorance. But from what I can tell the Red Queen Hypothesis of evolutionary arms races correspond to biotic pressures, while the Court Jester Model denotes the climatic shocks and shifts which are outside of the closed system of species' interactions. So figure 2 shows that both forces are critical in determining the specific state of species' richness. But the third figure illustrates that they have somewhat different roles. "E" is ecology and "C" climate, while "D" is diversity. You see that diversity (or lack of more accurately) correlates with speciation, while ecology & climate are more relevant for prediction of of extinction. The former is due to the "early burst" of adaptive radiation which occur in a low diversity state. Why is the diversity low? Probably because of a massive extinction event due to an exogenous shock. So the two classes of variables do influence each other, insofar as biotic dynamism surges in the wake of an abiotic perturbation. Much of the above is common sense, and we understand it non-quantitatively. Of course both exogenous and endogenous dynamics are at work in shaping the specific nature of the tree of life. By exogenous, I'm referring to climatic shifts, comets, geologic activity, etc. By endogenous I'm referring to the cycles of interactions which might be triggered by a sequence of co-evolutionary arms races. Many readers of this weblog with some biological background will be familiar with chaotic phenomena

bubbling out of purely endogenous parameters. In theory a cycle of extinctions and clade radiations could be due to endogenous processes. But the above data suggest that at least for life on earth, that is not so. Perhaps in a low energy universe trillions of years in the future, in a universe with few surprises, we'll see purely closed ecosystems at work. But not right now. A surprise is always in the cards! Citation:

Ezard TH, Aze T, Pearson PN, & Purvis A (2011). Interplay between changing climate and species' ecology drives macroevolutionary dynamics. Science (New York, N.Y.), 332 (6027), 349-51 PMID: 21493859

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