Acorn shortage? Maybe …

ResearchBlogging.orgThe Washington Post reports that this fall’s acorn crop is apparently very poor, at least in parts of the Eastern Seaboard. There are lots of interviews with naturalists concerned about starving squirrels:

For 2 1/2 miles, Simmons and other naturalists hiked through Northern Virginia oak and hickory forests. They sifted through leaves on the ground, dug in the dirt and peered into the tree canopies. Nothing.

“I’m used to seeing so many acorns around and out in the field, it’s something I just didn’t believe,” he said. “But this is not just not a good year for oaks. It’s a zero year. There’s zero production. I’ve never seen anything like this before.”


Photo by Martin LaBar.

Accompanying the article is a photo of a northern flying squirrel, a species so cute that Disney is probably trying to copyright their genome. There is, however, no reference to a systematic survey of acorn production in any of the areas affected. As the saying goes, the plural of anecdote is not data.

That might sound like a picky thing to ask for, but without hard numbers we have no way of knowing whether this really is an unusually bad year for acorns. Oaks are masting species, meaning that their seed production varies a lot from year to year. It’s been suggested that this is actually a defense against seed predators [$-a], such as squirrels. In “mast” years, trees produce a huge seed crop, and seed predators cache more seeds than they will eventually eat, so that some seeds survive to sprout. Masting works for long-lived trees because an oak that lives for decades can afford to take a year off from reproduction every so often, if it means that when it masts a larger fraction of its seeds survive to adulthood.

So it’s hard to say whether this acorn shortage is unusual. If it continues two years in a row in the same regions, that would be surprising. Of course, by that time, populations of cute seed predators may have already declined precipitously. Common squirrel species are probably in no real danger, but critters like flying squirrels and anything else that can’t make a living on suburban bird feeders could be in trouble.

Reference

D.H. Janzen (1971). Seed predation by animals. Annual Review of Ecology and Systematics, 2 (1), 465-92 DOI: 10.1146/annurev.es.02.110171.002341

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Earliest-known turtle had only half a shell

ResearchBlogging.orgFresh in this week’s Nature: a newly-discovered fossil turtle, the oldest ever found, has a lower shell, but no upper one [$-a]. Odontochelys semitestacea, as it’s called, is a really neat potential transitional fossil – the ribs are flattened like butter knives, but not fused into an upper shell. Apparently, this is suggestive of the way in which the upper shell forms in embryonic modern turtles, and the authors are careful to point out that, in other respects, the fossil is clearly an adult.


See that thing on its back? Its
ancestors may not have had one.

Photo by raceytay.

In an accompanying News and Views piece, Reisz and Head suggest that the lower half of a shell would be quite useful [$-a] if Odontochelys lived mostly in the water, where predators are more likely to attack from below than from above. They argue, though, that Odontochelys may not represent a transitional step between shell-less ancestors and full-shelled modern turtles, but a case of “secondary loss,” in which a full-shelled turtle took to the water and subsequently lost its unnecessary and cumbersome upper shell. I’m no turtle anatomist, but this sounds like a plausible alternative hypothesis. The only way to test it is is to dig up an even older turtle, and see what its shell looks like.

(See also coverage by All Things Considered, which is pretty good if unnecessarily snarky about the degree to which paleontologists specialize. It’s not like it’s that odd to think someone might build a career comparing birds’ beaks to turtles’ beaks.)

References

C. Li, X.-C. Wu, O. Rieppel, L.-T. Wang, L.-J. Zhao (2008). An ancestral turtle from the Late Triassic of southwestern China. Nature, 456 (7221), 497-501 DOI: 10.1038/nature07533

R.R. Reisz, J.J. Head (2008). Palaeontology: Turtle origins out to sea. Nature, 456 (7221), 450-1 DOI: 10.1038/456450a

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Funny how these things look, in retrospect

On BoingBoing Gadgets, an almost-entirely-random but excellent essay from John Brownlee about a particular Christian edutainment series, which in retrospect is pretty creepy and philosophically squishy and maybe a bit scary when you think about it. I don’t think I ever saw the video franchise in question, but I definitely got a lot of exposure to that sort of thing growing up. (“McGee and Me” anyone?) My parents are anything but fundamentalists, but no congregation is ideologically uniform – and so I was subjected to more-conservative-than-at-home Sunday School lessons maybe 75% of the time. (I’d say my family falls into the leftmost quartile of most of the churches we’ve attended.) This phenomenon probably reached its apogee in early middle school, when I was recruited, along with all my peers, into a “Children’s Sunday” service culminating in an elaborate choral number – performed in front of the whole congregation, with one of us* dancing around in a gorilla costume – about how silly and nonsensical and fundamentally un-cool it is to believe in evolution.

I guess I can’t really claim to have been brainwashed.

—————–
* Not me.

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Poison dart frogs can’t get too creative

ResearchBlogging.orgBeing a poisonous animal isn’t much help if your predators don’t know about it. That’s why lots of poison-defended critters – monarch butterflies or poison dart frogs, for instance – advertise with bright “warning” colors. This is called aposematism. The idea is that predators will learn (or even evolve) to avoid bad-tasting, poisonous prey if they’re well-marked for future reference.

The trouble with aposematism, though, is that it requires giving up another, more common defensive color scheme: camouflage. If you’re a poisonous critter, and you evolve bright coloration for the first time, predators don’t yet know that you’re poisonous – but you’re really brightly colored and easy to see. How, then, does aposematism evolve from non-aposematic ancestors?


Photo by dbarronoss.

A new study on early release from Biology Letters suggests that it isn’t easy. The authors, Noonan and Comeault, set out to determine whether brightly-colored poison dart frogs are more likely to be attacked when they evolve new color patterns [$-a]. It’s possible that the frogs’ predators avoid all brightly-colored prey regardless of pattern, in which case new frog patterns would be just as good for predator deterrence as the old ones. But it’s also possible that predators only avoid patterns they’ve run across (and spat out) before – so that new, rare patterns would have all the disadvantages of giving up camouflage with none of the benefits of aposematism.

Noonan and Comeault performed an elegant behavioral experiment, setting out clay model frogs in an area where frogs of one color pattern predominate. One set of models matched the local color pattern, another was brightly colored but different from the local pattern, and a third was drab and camouflaged. Birds were much more likely to attack the “new” color pattern than either the “local” version or the drab one. This result is hard to understand at the first pass – if new color patterns are vulnerable to attack, how can aposematism evolve in the first place? The answer is, not by natural selection, but by genetic drift.

Genetic drift is a natural, mathematical consequence of finite populations: imagine a bag full of marbles, half of them black and half white. If you pull a sample of marbles from the bag, you expect them to be half black and half white on average (i.e., over many samples) – but any individual sample might have a very different frequency of white and black marbles, especially if it’s small. If the probability of picking a white marble from the bag is 0.5 (because half the marbles are white), then the probability of picking a sample of four white marbles is 0.5 × 0.5 × 0.5 × 0.5 = 0.0625. That’s a small probability, but not zero. Drift is a very real effect in the natural world, especially during the establishment of new local populations, when the population size is initially quite small.

The key to understanding Noonan and Comeault’s result is that aposematism is frequency dependent – it favors not the old pattern as such, but whatever bright color pattern is most common in the frog population. Birds attacked the “local” color pattern at a low rate, which suggests that they’re always re-learning which pattern to avoid. A new color pattern might be hard to establish within a population of frogs that look very different from it, but if a new pattern pops up in the course of establishing a new population, then – thanks to genetic drift – it may be common enough for predators to learn to avoid it.

Reference

B.P. Noonan, A.A. Comeault (2008). The role of predator selection on polymorphic aposematic poison frogs. Biology Letters DOI: 10.1098/rsbl.2008.0586

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Invasive plants turn out to be native

ResearchBlogging.orgBotanists digging through bog sediments on the Galapagos island Santa Cruz have discovered that six plant species thought to be invasive were actually already there when humans first arrived [$-a]. The key data are fossilized pollen grains buried in the sediments – pollen from all six plants are found in sediments formed up to 7,000 years before humans settled the Galapagos.

Reference

J.F.N. van Leeuwen, C.A. Froyd, W.O. van der Knaap, E.E. Coffey, A. Tye, K.J. Willis (2008). Fossil Pollen as a Guide to Conservation in the Galapagos Science, 322 (5905) DOI: 10.1126/science.1163454

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Conservative talk show host worried about “honesty” at Mennonite colleges

Someone attending a sporting event at Mennonite-affiliated Goshen College got his or her panties in a knot because Goshen, doesn’t play the National Anthem before games. (Just as with my alma mater, Eastern Mennonite University, Goshen takes the Mennonite loyalty to Christ over the state very seriously.) So this disgruntled sports fan called conservative radio talk show host Mike Gallagher to berate a liberal arts school with a student body somewhere south of 2,000. MWR reports that, apart from the McCarthyite concern with pledging allegiance to state authority, Gallagher is worried that pacifist Mennonites may not represent war fairly:

On his New York-based The Mike Gallagher Show, eighth in the nation in audience size, Gallagher criticized Goshen in a Nov. 7 broadcast, then invited Bill Born, dean of students, to speak on the show Nov. 10.

In that broadcast, Gallagher said he appreciated “the Christian nature of the Mennonite church,” but was concerned about whether Goshen was teaching against war in U.S. history.

“How would any student get an honest assessment of war at the Goshen College environment?” Gallagher said.

What Gallagher means, of course, is that pacifist history professors can’t be trusted to represent war as useful or necessary. And frankly, he’s right. In eight years of Mennonite private-school education, I took a lot of history classes, and I can’t say I ever got the impression that war was worthwhile. But that wasn’t because my teachers were teaching propaganda – it was because they fully represented the costs and consequences of armed conflict.

My question to Gallagher is, how can a history teacher honestly tell her students that war is useful or necessary?

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Stephenson on AV Club

The Onion’s A.V. Club interviews author Neal Stephenson in the wake of his new novel Anathem, which I have, coincidentally, just finished reading. Anathem isn’t quite as good as Stephenson’s Baroque Cycle (and I have some specific quibbles with some of the biology in it), but it’s a fine book about a well-build imaginary world. Stephenson has a good eye for detail, and a great talent for weaving big ideas into narrative. He also, apparently, uses a chalkboard in the writing process.

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The smallest possible eye

ResearchBlogging.orgThe eye is the original instance of “irreducible complexity,” a biological structure supposedly too complicated to have evolved by undirected mutation and natural selection. Darwin made a point to deal with the evolution of the eye in The Origin of Species. He argued that, in spite of appearances, a surprisingly complete gradation of eye complexity is seen in nature, and it’s not too hard to connect the dots. Brace for Victorian prose:

In the Articulata [Arthropods] we can commence a series with an optic nerve merely coated with pigment, and without any other mechanism; and from this low stage, numerous gradations of structure … can be shown to exist, until we reach a moderately high stage of perfection. In certain crustaceans, for instance, there is a double cornea, the inner one divided into facets, within each of which there is a lens-shaped swelling. In other crustaceans the transparent cones which are coated by pigment, and which properly act only by excluding lateral pencils of light, are convex at their upper ends and must act by convergence; and at their lower ends there seems to be an imperfect vitreous substance. With these facts … I can see no very great difficulty (not more than in the case of many other structures) in believing that natural selection has converted the simple apparatus of an optic nerve merely coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the great Articulate class.

Since Darwin’s day, biologists have developed much more detailed descriptions of how eyes might have evolved [$-a] from simple light-sensitive “eyespots” all the way up to the complex structure of mammalian eyes. But we haven’t had a good description of how those original, hyper-simple eyes actually work. Light hits them, and their owner responds to it – but what’s the connection between stimulus and response?


Photo by wakima.

As part of an early kickoff for Darwin’s 200th birthday celebration next year, this week’s issue of Nature has a paper that provides the answer: eyespots directly control how their owners move [$-a]. The authors, Jékely et al., use a variety of molecular biology methods to dissect the connection between eyespots and movement in the larvae of a marine flatworm, Platynereis dumerilii. Most of the experimentation wasn’t too kind to the larvae.

Platynereis larvae are tiny spheres with belts of cilia, their only means of propulsion, and an eyespot on either side of one hemisphere. The eyespots consist of only two cells each, a pigment cell and a photoreceptor, and they seem to be useful in helping the larva move toward light sources (i.e., further up in the water column). This tendency to move toward light is called “phototaxis.”

First, the authors burned off one eyespot or the other using a laser, and showed that larvae missing both eyespots were unable to move toward light, but those missing only one were mostly able to do so. Then they cut larvae in cross sections and, under an electron microscope, traced the body of an eyespot’s photoreceptor cell – which turned out to extend all the way to the cells in the equatorial cilia. It’s as if a human’s eyes were directly connected to her legs. The authors further show, in fact, that the Platynereis larvae swim in a manner perfectly adjusted for steering by eyespots; when one spot receives light, it makes the cilia on its side beat harder, and the larva banks toward the light source.

Like evolution itself, science proceeds slowly, step by tiny, hopefully useful step. This paper is one more piece in the enormous puzzle of life on Earth – the kind of work that has moved biology as far beyond Darwin’s first conjectures as the human eye is from a flatworm’s.

References

G. Jékely, J. Colombelli, H. Hausen, K. Guy, E. Stelzer, F. Nédélec, D. Arendt (2008). Mechanism of phototaxis in marine zooplankton. Nature, 456 (7220), 395-9 DOI: 10.1038/nature07590

T. Lincoln (2008). Cell biology: Why little swimmers take turns. Nature, 456 (7220) DOI: 10.1038/456334b

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What puts the “co” in coevolution?

ResearchBlogging.orgCoevolution is like the opposite of pornography – lots of scientists can define it with nicety, but most of us have trouble saying for sure whether any given pair of species are actually coevolving. “Coevolution” literally means “evolving together” – more formally, that an evolutionary change in one species causes a reciprocal change in another [$$]. But this process can be quite complicated to demonstrate in practice. In the latest example of this conundrum, a paper in this month’s Evolution suggests that one relationship we thought was coevolutionary maybe isn’t [$-a].


Leafcutter ants at work
Photo by rofanator.

Leafcutter ants have been thought to be involved in clear-cut coevolutionary relationships with a number of microbial species. Leafcutters, as is pretty well known, harvest leaf fragments to feed fungal gardens, which the ants use as a food source. That’s one relationship: ant-fungus. Less well-known are the ants’ relationships with bacteria – bacteria that fight off the fungus-garden-killing diseases, in the genus Pseudonocardia. Pseudonocardia grow on leafcutter ants’ exoskeletons [$-a], and the ants seem to regulate the bacteria’s growth depending on how much they need the antibiotics it produces. This seems like an obvious case of coevolution – the ants and their bacteria symbiotes live in close proximity to each other, and seem to rely on each other for their respective ways of life.

But the new paper, by Mueller et al., indicates that appearances can be deceiving. The authors present a new phylogeny of the bacterial family containing Pseudonocardia, which suggests that leafcutter ants frequently “recruit” new strains of Pseudonocardia from their environment. Coevolution, the authors argue, would mean that a single lineage of bacteria has been associated with the ants from the beginning of the relationship – but the phylogeny shows, instead, that ant-associated Pseudonocardia are often more closely related to free-living bacteria than to other strains of ant-associated Pseudonocardia. That is, the ant-associated strains are not monophyletic. Furthermore, individual ant species often carry Pseudonocardia from multiple different evolutionary lineages, which suggests that “recruitment” events don’t happen one after another, but continuously.

This result makes good biological sense. Ants using Pseudonocardia to control disease probably stimulate the evolution of resistant disease organisms, just the same problem that humans have found after less than a century of antibiotic use. Recruiting new strains of antibiotic-producing bacteria is just the way to deal with resistant disease organisms. It therefore makes about as much sense to say that ants are coevolving with Pseudonocardia as it would to say that humans are coevolving with penicillin – both the ant-associated bacterium and the mass-produced antibiotic are tools to be cast away when no longer useful.

Reference

C.R. Currie, J.A. Scott, R.C. Summerbell, D. Malloch (1999). Fungus-growing ants use antibiotic-producing bacteria to control garden parasites. Nature, 398 (6729), 701-4 DOI: 10.1038/19519

DH Janzen (1980). When is it coevolution? Evolution, 34 (3), 611-2 http://www.jstor.org/pss/2408229

U.G. Mueller, D. Dash, C. Rabeling, A. Rodrigues (2008). Coevolution between Attine ants and Actinomycete bacteria: A reevaluation. Evolution, 62 (11), 2894-912 DOI: 10.1111/j.1558-5646.2008.00501.x

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