Evolutionary pressures, within and without

By Razib Khan | April 20, 2011 1:16 am

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.

ResearchBlogging.orgI 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

CATEGORIZED UNDER: Ecology, Evolution

Comments (3)

  1. The Red Queen hypothesis is a term coined by Leigh Van Valen in 1973, but as recently as 1985, the main alternative was called the “stability hypothesis” (coined by Stenseth and Maynard Smith in 1984).

    The first references I can find in the literature to the “Court Jester” hypothesis, a term specifically coined in reference to the Red Queen hypothesis is in Anthony D. Barnosky, “Does evolution dance to the Red Queen or the Court Jester?”, 3 Annual Meeting of the Society of Vertebrate Paleontology USA (1999), and in a 2001 paper on the subject (“DISTINGUISHING THE EFFECTS OF THE RED QUEEN AND COURT JESTER ON MIOCENE MAMMAL EVOLUTION IN THE NORTHERN ROCKY MOUNTAINS”), he uses the term without citation, suggesting that he is the one who coined it. Westfall and Millar attribute the term to him(citing his 2001 paper) in a paper of their own from 2004. The 2001 paper, alas, does not spell out the metaphor underlying the “Court Jester” terminology and the 1999 presentation that apparently introduced the metaphor isn’t easily available on line in full text. In the 2001 paper he states that the debate is over:

    “[W]hether this march of morphology and species compositions through time, so well documented not only for mammals but throughout the fossil record, is more strongly influenced by interactions among species (Red Queen hypotheses), or by random perturbations to the physical environment such as climate change, tectonic events, or even bolide impacts that change the ground rules for the biota (Court Jester hypotheses). . . . A class of alternative ideas, here termed Court Jester hypotheses, share the basic tenet that changes in the physical environment rather than biotic interactions themselves are the initiators of major changes in organisms and ecosystems. Some of the more prominent ideas are listed in Table 1. Court Jester hypotheses imply that events random in respect to the biota occasionally change the rules on the biotic playing field. Accelerated biotic response (relative to background rates) is the result.”

    The Court Jester hypothesis builds on the punctuated equalibrium theory of Gould (1972) by providing a primary mechanism for it. Table 1 in the 2001 paper appropriates for the Court Jester side of the debate the Stability hypothesis, Vrba’s Habitat Theory (1992), Vrba’s Turn-over pulse hypothesis (1985), Vrba’s Traffic light hypothesis and Relay Model (1995), Gould’s Tiers of Time (1985), Brett and Baird’s Coordinated Statis (1995), and Graham and Lundelius’ Coevolutionary Disequalibrium (184) theories.

    The Jester reference, metaphorically, is not to the Lewis Carroll books about Alice that give us the Red Queen. There is no Court Jester in either of them. The best metaphorical sense I can find to it is that it plays on the notion of a royal court and uses the term in the sense of its meaning in the context of the Tarot, where the Court Jester is the symbol of death triumphing over all:

    “In Tarot, “The Fool” is the first card of the Major Arcana. The tarot depiction of the Fool includes a man (or less often, a woman) juggling unconcernedly or otherwise distracted, with a dog (sometimes cat) at his heels. The fool is in the act of unknowingly walking off the edge of a cliff, precipice or other high place. Another Tarot character is Death. In the Middle Ages, Death is often shown in Jester’s garb because “The last laugh is reserved for death.” Also, Death humbles everyone just as jesters make fun of everyone regardless of standing.”

    Alternately, the Court Jester terminology may be a metaphorical reference to the Joker card in a deck of cards that can upset the settled order, or alternatively a reference to the notion that a Court Jester is a disinterested player that has no stake or interests in the impact of his actions on any of the competing parties (since this theory posits a natural, abiotic cause). As Wikipedia explains: “The position of the Joker playing card, as a wild card which has no fixed place in the hierarchy of King, Queen, Knave, etc. might be a remnant of the position of the court jester. This lack of any place in the hierarchy meant Kings could trust the counsel of the jesters, as they had no vested interest in any region, estate or church.”

    Barnowsky aknowledges in the 2001 paper that the Court Jester hypothesis is not necessary inconsistent with the Red Queen hypothesis: “Indeed, as Ned Johnson
    remarked (after listening to a lecture expressing these ideas), ‘‘Maybe it is time for the Court Jester to marry the Red Queen.’’ That is, perhaps the dichotomy between the two hypotheses is really a dichotomy of scale, and that as we look for ways to travel across biological levels, we will find ways to resolve the dichotomies.”

    Not wanting to miss out on the fun, Bradshaw and Brook in 2009 dubbed a similar notion the “Chronus hypothesis” in opposition to the Gaia hypothesis which more or less corresponds to the stability hypothesis, while acknowledging the similar prior “Media hypothesis” of Peter Ward which argues that life itself carries a self-destructive seed within it that sooner or later manifests catastrophically. The Chronus hypothesis lays out the metaphors:

    Cronus (Κρόνος) was the patricidal (or patri-emasculating) youngest son of Gaia, the Earth mother. Cronus was also the leader of the first generation of Titans, the giant descendants of Gaia and Uranus, the sky father. Cronus was incited by his mother to kill Uranus for perceived crimes against Gaia’s other descendants, and Cronus himself was overthrown by his own son, Zeus, and banished to Hades (Atsma 2009). Given the tumultuous and competitive life-and-death history of Cronus, we believe this metaphor better captures the processes of inter-species competition and mutualisms that our population analogy of speciation and extinction embodies. Under the Gaia model, self-regulation works to avoid extinction because it is akin to the loss of a body part (function is reduced), whereas under Cronus, extinction is part of the process of natural selection (providing restoration of function through subsequent diversification). . . .

    [T]he sorceress Medea (Μήδεια) was the granddaughter of Helios the sun god and wife to Jason of the Argonauts who later killed her own sons as revenge for Jason’s unfaithfulness (Atsma 2009). Instead of the self-regulating super-organism Gaia, Ward describes the Earth’s mass extinctions as Medean events – large biodiversity loss driven by life itself (Ward 2009a). Arguing that the Gaia hypothesis cannot account for large shifts in the Earth’s temperature over geological time, Medea describes how the massive flux of atmospheric carbon dioxide and methane by the processes of plant, microbial and animal respiration was the very cause of such volatile conditions which lead to (at least some) mass extinctions (Ward 2009a,b). In essence, the Medean perspective describes a self-destructive, or anti-order component where life “seeks” to destroy itself, and it can do so on a massive scale due to amplifying feedbacks under certain circumstances (Ward 2009a,b). Modern human society might eventually merit the Medean soubriquet.


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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com


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