Plants are locked in a long twilight struggle with herbivores, particularly insects – sometimes they evolve a new defensive mechanism, “escaping” to diversify into new groups [$-a], but mostly natural selection works with the traits they already have. That means arms races – plants evolving greater concentrations of defense chemicals, and herbivores evolving greater tolerance of those chemicals. In this month’s Evolution, a new study of defensive chemistry evolution in milkweed [$-a] documents exactly this process.
Asclepias viridis, a milkweed
Photo by gravitywave.
The study by Agrawal et al. follows up on earlier work in the same group, which established the evolutionary relationships between the members of the milkweed genus, Asclepias. Milkweeds are named for their defense against insect herbivores, a milky sap full of nasty chemicals – coumaric acids, caffeic acids, cardenolides, and flavonoids. The authors raised a large sample of milkweed species in a controlled environment, then measured the levels of these chemicals in each species. By mapping the chemical profiles onto the previously-developed phylogeny of Asclepias, they could estimate how milkweeds’ chemistry has evolved since the genus first arose.
Aphids on Asclepias
Photo by aroid.
This analysis revealed that milkweeds have gotten nastier over their evolutionary history. But it’s not that clear-cut: the diversity of defensive chemicals present in Asclepias decreased, even as the total production increased – so the plants seemed to be paring down an initial diversity of defenses into a few chemicals that worked especially well. Coumaric and caffeic acids, which are produced from the same biochemical precursors, forced a trade-off so that as one increased, the other decreased. On the other hand, cardenolides and flavonoids, which are both produced in another biochemical pathway, were positively associated.
If this sounds complicated, that’s because it is. As Agrawal and his coauthors point out, we actually don’t have a good sense at what timescale an arms race should manifest – that is, are we talking about plants evolving greater defenses over a few generations, or over millions of years, as this study? Natural selection can appear to be moving a population strongly in one direction for a year or two – and then turn out to be fluctuating all over the place [$-a] if you watch for decades. How year-to-year selection acting on multiple traits translates into the grand trends of evolution – whether the explosive diversification of flowering plants or the emergence of human intelligence – remains one of the big puzzles for those of us who study the living world.
A.A. Agrawal, J.-P. Salminen, M. Fishbein (2009). Phylogenetic trends in phenolic metabolism of milkweeds (Asclepias): Evidence for escalation. Evolution, 63 (3), 663-73 DOI: 10.1111/j.1558-5646.2008.00573.x
P.R. Ehrlich, P.H. Raven (1964). Butterflies and plants: a study in coevolution Evolution, 18, 586-608 DOI: http://www.jstor.org/pss/2406212
P.R. Grant, B.R. Grant (2002). Unpredictable evolution in a 30-Year study of Darwin’s finches Science, 296 (5568), 707-11 DOI: 10.1126/science.1070315
This week’s PNAS has another (open access!) paper taking a crack at the problem of how cooperation can evolve. The authors create a world where cooperation arises spontaneously in a population of selfish individuals by modeling a fundamental human drive: the desire for a good neighborhood.
Helbing and Yu set up a model world ruled by the Prisoner’s Dilemma, a common game theory scenario in which pairs of interacting individuals can choose to cooperate or not cooperate with each other. If both refuse to cooperate, neither gets anything; if one cooperates and the other doesn’t, the cheater gets a reward, but the cooperator pays a cost; if both cooperate, then they both get a smaller reward. If neither interactor can predict the other’s choice, the most sensible strategy is to just never cooperate – you make out pretty well when the other guy is silly enough to cooperate with you, and you’re no worse off than you started out if you both refuse to cooperate.
Previous models have made cooperation work in Prisoner’s Dilemma situations a few different ways. One way is to allow individuals to remember how they have treated each other over multiple iterations of the PD interaction, so that cheaters can be punished [$-a]; another is to let the game play out across space in such a way that cooperators can cluster together, so that they are more likely to interact with other cooperators [$-a].
Helbing and Yu’s model is a variation on the “spatial” flavor – individuals occupy cells in a grid, and interact with those in adjacent cells. Strictly speaking, it isn’t an evolutionary model (even though the authors describe it as such), because there doesn’t seem to be any inheritance of behavior from one generation to another; instead, individuals “learn” from their neighbors, imitating the ones who are most successful in terms of interaction rewards. There’s a random element to individual behavior, to approximate trial and error strategies. Perhaps most importantly, individuals can migrate across the grid, moving to adjacent unoccupied cells where they expect to find a greater reward.
Neither imitation nor migration alone allow cooperation to survive in this model world, but some interaction between the two does. This result holds, apparently, for a wide range of possible combinations of payoff conditions. For some conditions, the model will even allow cooperators to “invade” a world full of non-cooperators. The speed with which individuals can move across the grid – cooperators seeking other cooperators, and avoiding cheaters – is critical, say the authors. They call this “success-driven migration” – and it does seem to allow cooperation – though not altruism – to arise out of selfishness.
See also Wired Science’s coverage.
M. Doebeli, C. Hauert (2005). Models of cooperation based on the Prisoner’s Dilemma and the Snowdrift game Ecology Letters, 8 (7), 748-66 DOI: 10.1111/j.1461-0248.2005.00773.x
D. Helbing, W. Yu (2009). The outbreak of cooperation among success-driven individuals under noisy conditions PNAS DOI: 10.1073/pnas.0811503106
M.A. Nowak, R.M. May (1992). Evolutionary games and spatial chaos Nature, 359 (6398), 826-829 DOI: 10.1038/359826a0
R.L. Trivers (1971). The evolution of reciprocal altruism Quarterly Rev. Biol., 46, 35-57
Liberals: The stupidest and weakest members of the political triumvirate, they allowed conservatives to turn their name into a slur against them, exposing them as the political equivalent of the kid who lets the school bully pummel him with his own fists (Stop hitting yourself. Stop hitting yourself. Stop hitting yourself).
Conservatives: Self-hating moral relativists, unless you can convince me that an intellectual class that publicly praises family values but privately engages in sodomy, coke and trophy wives is more aptly described in some other way.
Libertarians: Never got over the fact they weren’t the illegitimate children of Robert Heinlein and Ayn Rand; currently punishing the rest of us for it. Unusually smug for a political philosophy that’s never gotten anyone elected for anything above the local water board.
Prefiguring, it turns out, the best entry in the 50 most loathsome people in America in 2008, #43: You.
You’re hopping mad about an auto industry bailout that cost a squirt of piss compared to a Wall Street heist of galactic dimensions, due to a housing crash you somehow have blamed on minorities. It took you six years to figure out what a tool Bush is, but you think Obama will make it all better. You deem it hunky dory that we conduct national policy debates via 8-second clips from “The View.” You think God zapped humans into existence a few thousand years ago, although your appendix and wisdom teeth disagree.
I have just learned about the simplified English version of Wikipedia from xkcd. It is Wikipedia, written for beginning English readers. That means the writers use simple words and short sentences. This makes them sound like Ernest Hemmingway. Simple English Wikipedia does not have an entry for Ernest Hemmingway. A search for “Ernest Hemmingway” on Simple English Wikipedia finds only a reference to Fall Out Boy and an article about Aleister Crowley.
There is one word I always look up when I want to try a new reference source.
In the study of life and living things, evolution is the term used to describe the way a type of living thing changes over a long period of time. “Evolution” is a scientific theory (an explanation) that is used by scientists to explain why different creatures and plants are the way that they are, and act the way that they do.
That is pretty good.
A horse has a single hoof on each foot, a cow has two, a bird has its whole arm changed into a wing, and a human has a hand. But if we look at fossils – made when very old dead things got squashed between clay or sand, which hardened into rocks, we can see all these animals were once one type of animal: Fishes.
That is not as good. I am not sure why this is.
I lied. There are two words I always look up to try new reference sources. Can you guess the other one?
The Mennonites are a group of Christian Anabaptists named after Menno Simons (1496–1561). His teachings were a relatively minor influence on the group,though. They are of the historic peace churches. Mennonites are committed to nonviolence, nonviolent resistance/reconciliation, and pacifism.
That is double plus not as good. I am afraid that if I write like this much longer, I will forget how to write long sentences.
Just discovered: Stephen Fry joins Mark Carwardine in returning to the places and creatures visited by Carawardine and chronicled by Douglas Adams in the excellent little book Last Chance to See, a travelogue of desperately endangered animals. The second Last Chance, like the first, is principally a BBC documentary project – we shall have to see if a book grows out of Fry’s new journey. Regrettably, none of the video seems to be viewable this side of the Atlantic.
Via kottke.org: The Onion AV Club rehabilitates Eyes Wide Shut. I don’t remember much of the critical panning that accompanied the movie’s original release, and I didn’t see it till some grad-student friends and I committed to watch Stanley Kubrick’s major films in chronological order a couple years ago. But I agree with the review that it’s up to Kubrick’s usual high standards.
Given the choice, though, I tend to prefer 2001: A Space Odyssey and Dr. Strangelove, which between the two of them account for my interests in evolution, hard science fiction, conscientious objection to war, and Peter Sellers.