Category Archives: science

Climate change and the food supply

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ResearchBlogging.orgOne of the most-cited effects of global warming is that of rising temperatures on crops – hotter average conditions should lead to warmer, drier conditions, reducing yields in the best growing areas and maybe eliminating them where conditions today are marginal. In this week’s Science, a new study puts some numbers behind that speculation [$-a], and the news is not good.


Photo by Josh Sommmers.

Assembling the results of 23 climate models, authors Battisti and Naylor compare projected temperature ranges for the coming century with the ranges observed in the previous one. By the final decade of the twenty-first century, they say, summertime high temperatures in most of the continental U.S. have a 70% probability of exceeding the hottest summer temperatures ever recorded; in Saharan Africa, much of the Middle East and central Asia, the probability is 90-100%.

To put these numbers into perspective, Battisti and Naylor go to the history books, citing an array of cases in which local high temperatures have disrupted food production, creating regional shortages that eventually impacted worldwide food markets:

By comparison, extremely high summer-averaged temperature in the former Soviet Union (USSR) in 1972 contributed to disruptions in world cereal markets and food security that remain a legacy in the minds of food policy analysts to this day. … Nominal prices for wheat — the crop most affected by the USSR weather shock — rose from $60 to $208 per metric ton in international markets between the first quarters of 1972 and 1974.

Battisti and Naylor end by calling for substantial investment in adaptation measures to prevent “a perpetual food crisis.” Increasingly, this looks like the only practical course of action – although reducing and eliminating man-made greenhouse gas emissions is critical, turning global climate around is going to be like steering an aircraft carrier, and it’s going to get pretty warm before we turn the corner.

Reference

D.S. Battisti, R.L. Naylor (2009). Historical warnings of future food insecurity with unprecedented seasonal heat Science, 323 (5911), 240-4 DOI: 10.1126/science.1164363

Evolution applied: Biological warfare against mosquito-borne disease

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ResearchBlogging.orgThis week’s issue of Science starts the new year with an exciting application of evolutionary dynamics: a sort of biological warfare agent to control disease-bearing mosquitoes.

Even in the twenty-first century, mosquito-borne diseases like malaria and Dengue fever remain major public health challenges, particularly in the developing world. When vaccines are not available, the only way to prevent these diseases is to control the mosquitoes that spread them. Yet mosquito populations have evolved resistance to commonly-used pesticides, and others, like DDT, have dangerous environmental side effects.


Aedes aegypti, a disease-bearing
mosquito species
Photo by dincordero.

It’s no wonder, then, that biologists are interested in ways to harness evolutionary population dynamics to reduce mosquito populations. McMeniman et al. take a big step toward this goal using the parasitic bacterium Wolbachia [$-a]. Wolbachia, which infects many other insect species, behaves like a “selfish gene” within its hosts. The bacterium is transmitted from females to their offspring, but not from males; so it induces infected females to lay more female eggs, and it kills the offspring of matings between infected males and uninfected females. This lets Wolbachia spread rapidly through populations, even if being infected is bad for the host.

Using Wolbachia against mosquitoes is not new; previously, people have discussed using genetically engineered forms of the bacterium to deliver agents that fight the diseases inside their carriers. But as McMeniman et al. describe, infection of the Dengue-bearing mosquito Aedes aegyptes actually already cuts the lifespan of the host in half. The Dengue pathogen needs time to incubate inside the mosquito host before it can be passed on to a human – longer, it turns out, than Wolbachia-infected mosquitoes typically live.

With this discovery, controlling Dengue or malaria could be as simple as introducing Wolbachia-infected female mosquitoes into at-risk areas, and monitoring the infection’s spread. Together with common-sense public health measures like distributing mosquito nets and reducing standing water sources, Wolbachia has the potential to save and improve millions of lives.

References

C.J. McMeniman, R.V. Lane, B.N. Cass, A.W.C. Fong, M. Sidhu, Y.-F. Wang, S.L. O’Neill (2009). Stable introduction of a life-shortening Wolbachia infection into the mosquito Aedes aegypti. Science, 323 (5910), 141-4 DOI: 10.1126/science.1165326

A.F. Read, M.B. Thomas (2009). MICROBIOLOGY: Mosquitoes cut short Science, 323 (5910), 51-2 DOI: 10.1126/science.1168659

Shrikes take their cues from the competition

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ResearchBlogging.orgOver evolutionary time, the easiest way to deal with a competitor is to do something different – if your competitor eats big seeds, say, it may be easier to start eating small seeds than to fight for the big ones. This idea goes all back to the Origin, wherein Darwin proposed that competition drives evolutionary diversification, with living things dividing up available resources into ever-finer slices as they scramble for shares:

Lighten any check, mitigate the destruction [of offspring] ever so little, and the number of the species will almost instantaneously increase to any amount. The face of Nature may be compared to a yielding surface, with ten thousand sharp wedges packed close together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force.

But what if competition can sometimes make competitors more like each other? A new study, published through PLoS ONE this week, shows that red-backed shrikes prefer to set up hunting territories in places where their competitors have already been hunting.


Photo by phenolog.

Shrikes are cute but vicious predators – they capture small prey and spear them on thorns or twigs for storage, or to indicate to a prospective mate what great hunters they are. Red-backed shrikes migrate from Africa to Eastern Europe for the summer mating season. When they arrive, male red-backed shrikes must establish a hunting territory with a nesting site, but they have to contend with the established territories of great gray shrikes, which live in the same area year-round, and eat the same kind of prey.

You might expect, then, that red-backed shrikes would establish nest sites well away from the impaled victims of great gray shrikes. In fact, as the paper’s authors show, red-backed shrikes are more likely to nest near great gray shrike caches. They don’t raid the competitors’ larders, but, the authors argue, understand the presence of a great gray shrike’s cache to mean there is plenty of prey nearby.

This could mean a number of things: perhaps great gray shrikes and red-backed shrikes prey on critters that are so abundant, it’s arguable that they’re not really competing. If that’s the case, it makes plenty of sense for red-backed shrikes to use great gray shrike caches as cues to find particularly good hunting grounds. Alternatively, red-backed shrikes settling near great gray shrike caches might shift their prey preferences to avoid competition – the presence of one type of prey may very well correlate with the abundance of many other types, so that the great gray shrike caches are only indirect indicators of prey abundance. Unfortunately, the current paper has no data comparing prey preferences of red-backed shrikes nesting nearby and away from great gray shrike caches, so there’s no way to test this hypothesis.

Still, this observation has significant implications for the way we think about species interactions across evolutionary time. If competitors can be drawn together as well as driven apart, maybe competition doesn’t contribute to diversification as much as we think it does.

Reference

M. Hromada, M. Antczak, T.J. Valone, P. Tryjanowski (2008). Settling decisions and heterospecific social information use in shrikes PLoS ONE, 3 (12) DOI: 10.1371/journal.pone.0003930

Vikings brought violence, destruction – and mice

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ResearchBlogging.orgTraveling groups of humans are really mobile ecosystems, as we bring with us a whole collection of species we find useful, and not-so-useful: domestic animals, crop plants, pests, diseases, and parasites. Even if we fumigated our clothes and our vehicles, we’d still bring with us a whole collection of intestinal microbes. If you knew nothing more about humans than this, you could reconstruct our historical movement from the changes we’ve made to the living communities around us.


Photo by Pehpsii.

This is one thesis of a new paper in Proceedings of the Royal Society, which shows that the population genetics of house mice in the British Isles still bear the mark of medieval Viking raids. It’s an extremely simple result: in sites especially subject to regular Viking depredations, the northwestern coasts of Scotland and Ireland, the house mice are more closely related to house mice in Norway than they are to mice from other parts of Britain. It’s not clear whether this is because the Vikings brought the first house mice to these areas, or whether stowaway mice from Norway interbred with an already-established population. House mice were in Britain well before the Vikings came along, but human settlements along the northwestern coasts apparently weren’t established much before the Vikings started raiding them.

The authors propose expanding a survey of mouse genetics in Europe to better document the extent of Viking travel. It’s one more biological tool for archaeologists, reconstructing the past based on what we leave behind.

Reference

J.B. Searle, C.S. Jones, İ. Gündüz, M. Scascitelli, E.P. Jones, J.S. Herman, R.V. Rambau, L.R. Noble, R.J. Berry, M.D. Giménez, F. Jóhannesdóttir (2009). Of mice and (Viking?) men: phylogeography of British and Irish house mice. Proc. R. Soc. B, 276 (1655), 201-7 DOI: 10.1098/rspb.2008.0958

If it’s online, it must be for real

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The website for the Evolution 2009 meetings, to be held right here at the University of Idaho this spring, is officially live, although issues remain with our domain registration (eventually, evolutionmeetings09.org is supposed to forward to this page). Graphic design for the conference logo is by Christian Blackman, a UI Art and Design student; HTML coding and layout by yours truly.

Natural selection at work

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Roger Alsing has written a genetic algorithm – a computer simulation of evolution via random mutation and “natural” selection – that recreates the Mona Lisa. It achieved a pretty good replica layering only 50 semi-transparent polygons of various colors, in just shy of a million generations. And it got pretty close in the first hundred thousand generations; a neat example of R. A. Fisher’s “geometric model” of evolution toward an optimum, in which evolutionary change slows as the distance to the optimum decreases.

Via kottke.org and BoingBoing.

(Considerable debate on the BoingBoing thread about whether this is “really” evolution, since there’s a preordained optimum – I’m going to to say that it is, in fact, evolution. Specifically, a single bout of adaptive, directional evolution towards “Mona Lisa”-ness. The equivalent of which happens all the time in nature, except that usually the selective optimum shifts from “Mona Lisa” to “Les Demoiselles D’Avignon” after a million years or so.)

Snail trails lead toward speciation

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ResearchBlogging.orgFinding a mate is at the top of just about every to-do list in the animal kingdom. This might involve following the smell of pheromones or triangulating the source of a mating call; in the snail Littorina saxatilis, it turns out to require tracking your beloved by the trail of her slime [$-a].

That’s according to a paper in the latest issue of Evolution, in which Kerstin Johannesson and coauthors took video of male and female snails to catch slime trail-following in action. And it occurred to them that slime-following could be a component of speciation in L. saxatilis. This particular snail comes in two forms, or “ecotypes”: a small one that lives in the crevices of exposed rock faces and a larger one that lives in quieter, sheltered pools. When Johannesson et al. presented male snails with slime trails from each ecotype, the males preferred to follow trails made by females of their own ecotype.

This is what’s called assortative mating – preferentially mating with similar individuals – and it’s usually thought of as a first step towards speciation. Whether L. saxatilis ever eventually evolves into two species is another question, though. The world is full of experiments in speciation, where adaptation to local conditions or difficulty moving between populations can cause a species to begin diverging. But it’s just as likely that the forces pushing a species apart will change or disappear, and diverging groups re-merge into a single interbreeding population. Part of the fun of studying the natural world is finding things like snail’s slime trail discrimination, and trying to figure out what will happen next.

Reference

K. Johannesson, J.N. Havenhand, P.R. Jonsson, M. Lindegarth, A. Sundin, J. Hollander (2008). Male discrimintation of female mucous trails permits assortative mating in a marine snail species Evolution, 62 (12), 3178-84 DOI: 10.1111/j.1558-5646.2008.00510.x

Acorn shortage? Maybe …

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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

Earliest-known turtle had only half a shell

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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

Poison dart frogs can’t get too creative

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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