Category Archives: science

Invasive plants turn out to be native

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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

The smallest possible eye

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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

What puts the “co” in coevolution?

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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

Why do butterflies have four wings?

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In this week’s PNAS is a tidy result that demonstrates what you ca get away with when you study invertebrates: butterflies and moths can still fly if their hindwings are amputated, but they can’t take evasive action [$-a]. That summary tells you just about all you need to about the reported experimental result; but the rest of the article has some interesting speculation.

A monarch butterfly, collecting nectar and probably transferring pollen (Flickr, jby)

The authors, Jantzen and Eisner, start from the premise that the large, showy wings of butterflies should (very generally) make them major targets of predation. They note, however, that butterflies are also marked by highly erratic flight – an extreme maneuverability that makes it difficult for a predator to guess where a butterfly is going to be in the future based on its current trajectory. Maybe showy wings actually act as a kind of inverse protective coloration:

A bird, we suggest, could learn or inherently know that brightly colored airborne prey, discernible from afar, is not worth the chase. Too elusive to catch and, because of their [wing] scales, too slippery to hold … Birds might simply write butterflies off, and … relegate them all to the category of the undesirable, treating them as they treat noxious insects that they disregard.

Jantzen and Eisner’s experiment, in which they amputated butterflies’ hindwings, confirms that butterflies can still fly without the second pair of wings, but fly less erratically. Does the result confirm the authors’ major hypothesis, though? I’m not so sure. There are plenty of other reasons to have big, showy wings – mate attraction, or as placement for eye spots to fool predators – and plenty of butterflies and moths are comparatively small and non-showy. Jantzen and Eisner’s hypothesis smells more than a bit like adaptive story-telling, though it provides some food for thought.

Reference

B. Jantzen, T. Eisner (2008). Hindwings are unnecessary for flight but essential for execution of normal evasive flight in Lepidoptera PNAS, 105 (43), 16636-40 DOI: 10.1073/pnas.0807223105

Towards an empirical morality

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ResearchBlogging.orgAndrew Sullivan links to a thought-provoking 1998 essay by E.O. Wilson, in which the champion of sociobiology delves into the question of whether morality arises from divine revelation or natural selection. Wilson takes an interesting position, attempting to turn the question around by ninety degrees:

But the split is not, as popularly supposed, between religious believers and secularists. It is between transcendentalists, who think that moral guidelines exist outside the human mind, and empiricists, who think them contrivances of the mind. In simplest terms, the options are as follows: I believe in the independence of moral values, whether from God or not, and I believe that moral values come from human beings alone, whether or not God exists. [Italics sic.]


Photo by lumierefl.

Although this perspective pulls back from the God-vs.-Science dilemma, it doesn’t quite eliminate it. Science tends to lean towards the “moral values come from human beings alone” position, and not just because any “transcendent” source of morality is probably beyond the scope of scientific inquiry. Exhibit A is the “trolley dilemma” dissected eloquently in a 2006 episode of Radio Lab: To prevent a runaway trolley from hitting a group of bystanders, most people judge it moral to pull a lever to divert the trolley onto a side track, even if doing so kills one person standing on the side track. But ask them to push that single person into the path of the trolley to stop it hitting the crowd, and most people balk.

In the experiment at the focus of that Radio Lab episode, Joshua Greene and his coauthors used functional magnetic resonance imaging to look at brain activity in people considering the two variants of the trolley dilemma, and found evidence that the dilemma creates a conflict between rational and emotional responses [PDF]. “Rational” parts of the brain were active in the decision to pull the lever, but “emotional” ones were involved in unwillingness to push a person into the trolley’s path. As Greene et al. write:

The thought of pushing someone to his death is, we propose, more emotionally salient than the thought of hitting a switch that will cause a trolley to produced similar consequences, and it is this emotional response that accounts for people’s tendency to treat these cases differently.

This result suggests that there isn’t some universal, transcendent standard of morality by which people are making decisions – in either pushing or lever-pulling, the choice is whether or not to sacrifice one life for the sake of many. But something in the fundamental architecture of the human brain determines that sometimes morality is judged in purely utilitarian terms and sometimes it isn’t. This is the kind of data that bears on the transcendence versus empiricism debate that Wilson outlines.

But empirical morality seems to run directly into the “naturalistic fallacy,” conflating that which is with that which ought to be. Wilson argues that empirical morality does not assume that the innate moral judgments of the human brain are also the judgments we ought to make – instead, it requires constant introspection and re-examination of the consequences produced by society’s moral code:

The empiricist view recognizes that the strength of commitment can wane as a result of new knowledge and experience, with the result that certain rules may be desacralized, old laws rescinded, and formerly prohibited behavior set free. It also recognizes that for the same reason new moral codes may need to be devised, with the potential of being made sacred in time.

That seems an inherently progressive point of view, one not far removed from the way Jesus described a morality that built on and universalized the old Jewish law, as with revenge: “You have heard that it was said, ‘Eye for eye, and tooth for tooth.’ But I tell you, Do not resist an evil person. If someone strikes you on the right cheek, turn to him the other also.” (Matt. 5:38-9, italics mine) And Jesus also tells his disciples to judge prophets not by their appeal to some special (transcendent?) revelation, but “by their fruit,” the consequences of their teachings (Matt. 7:15-22).

Yet – how do we judge what is a good outcome and what is a bad one? Science is good for predicting the consequences of actions and moral positions, but it is unable to determine which ones are good. Ultimately, empirical morality must proceed from some basic ethical framework, some agreed-upon prior definitions of “good” and “bad.” But that’s not really a victory for the transcendentalists. Even a perfectly articulated Platonic morality needs data from which to proceed – how many people are in the trolley’s way, and how much mass it would take to stop the trolley. Morality without reference to the empirical world is worse than meaningless. And the only access we have to the empirical world and its mechanisms is the scientific method.

Reference

J.D. Greene, R.B. Sommerville, L.E. Nystrom, J.M. Darley, J.D. Cohen (2001). An fMRI Investigation of Emotional Engagement in Moral Judgment Science, 293 (5537), 2105-8 DOI: 10.1126/science.1062872

Old vials of chemical residue published in Science

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ResearchBlogging.orgThe chief lesson from a new article in this week’s Science is, never, ever throw out out your samples. Most people are probably familiar with Stanley Miller’s classic biochemistry experiment, in which he produced amino acids in a simulation of Earth’s early atmosphere [PDF]. That experiment was groundbreaking, but since it was published in 1953 geochemsists have questioned whether it accurately reflected conditions on ancient Earth. But another of Miller’s experimental results, which went unpublished until now, may be the response to that criticism.


Volcanic steam: the origin of
life on Earth?
Photo by vtveen.

After Miller’s death in 2007, one of his former graduate students inherited a bunch of boxes full of Miller’s experimental products. The box included products from an experiment simulating steamy conditions around a volcanic vent. The student, Jeffrey Bada, decided to re-analyze the preserved product using (among other approaches) high-performance liquid chromatography, a method of identifying organic compounds that wasn’t available when Miller did his original work in the 1950s. It turns out that the volcano experiment produced an even richer array of amino acids than Miller knew [$-a]. Enough, maybe, to lay the groundwork for life. That’s what Bada and his coauthors argue:

Reduced gases and lightning associated with volcanic eruptions in hot spots or island arc–type systems could have been prevalent on the early Earth before extensive continents formed. … Amino acids formed in volcanic island systems could have accumulated in tidal areas, where they could be polymerized by carbonyl sulfide, a simple volcanic gas that has been shown to form peptides under mild conditions.

Naturally, this is only a jumping-off point for further work, starting with replication of Miller’s original experiment. But it’s a useful discovery, and a cautionary tale to any grad student who’s careless about record-keeping – you never know when that throwaway result will turn out to be useful.

References

A.P. Johnson, H.J. Cleaves, J.P. Dworkin, D.P. Glavin, A. Lazcano, J.L. Bada (2008). The Miller volcanic spark discharge experiment Science, 322 (5900), 404-404 DOI: 10.1126/science.1161527

S.L. Miller (1953). A production of amino acids under possible primitive Earth conditions Science, 117, 528-9 Full text (PDF)

Sympatric skepticism

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ResearchBlogging.orgThe new issue of The Journal of Evolutionary Biology has a great article on a question that dates back to Darwin: sympatric speciation[$-a].

Sympatric speciation is simply speciation that occurs when a species splits into two reproductively isolated groups without any physical barrier arising between those groups. It’s often treated as the opposite of allopatric speciation, in which a species is split by some external barrier (a new mountain range, a river, &c) and the separated populations evolve on different trajectories until they’re unable to exchange genes even if the barrier is removed. There are a number of ways biologists think this can happen – for instance, a population of insects using two different, co-occurring host plants, might split into two species on the different hosts – but not many good cases where we’re pretty sure that it has happened.

In the new article, Kirkpatrick and his coauthors argue that the problem is one of definition. Sympatic speciation, as a concept, is set up to be impossible to test conclusively: Although it’s easy to show that two closely-related species occupy the same territory in the present, it’s rarely possible to show that they became different species while they were occupying the same territory at some point in the past, much less that they were “panmictic,” or freely interbreeding.

The problem for empiricists is that biogeographical sympatry is relatively straightforward to diagnose, but the initial condition of panmixia specified by population genetic models is virtually impossible to test.

As an alternative, the authors suggest ignoring the Platonic ideals of “allopatric” versus “sympatric” speciation, and instead concentrating on the interaction between divergent natural selection and gene flow in causing or preventing speciation. Not only is this sensible, it’s what most evolutionary ecologists are doing right now, anyway.

Reference

B.M. Fitzpatrick, J.A. Fordyce, S. Gavrilets (2008). What, if anything, is sympatric speciation? Journal of Evolutionary Biology, 1452-9 DOI: 10.1111/j.1420-9101.2008.01611.x

Nowhere to go but uphill

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ResearchBlogging.orgHistorical data sets are invaluable in assessing the impact of climate change on natural systems. Case in point: in today’s issue of Science, a new paper uses a century-old survey of small mammals in Yosemite National Park to see how the park’s community has shifted as climate warmed [$-a].


Belding’s ground squirrels contracted their
high-altitude range as climate warmed.
Photo by infinite wilderness.

Moritz et al. repeated a survey of small mammals – chipmunks, shrews, ground squirrels, and the like – originally conducted by the biologist Joseph Grinnell between 1914 and 1920. Since that time, average minimum monthly temperatures in the Yosemite area have increased approximately three degrees Centigrade (five and a half degrees Fahrenheit), and Moritz et al. found significant changes in the distributions of small mammals associated with that warming.

In the face of warming temperatures, the easiest thing for animals to do is move up to the cooler climes at higher elevations, and this is what many species did. Those at lower elevations expanded their ranges uphill. But small mammals already living at high elevations, like the aptly named Alpine Chipmunk (Tamias alpinus) have nowhere cooler to go – so their ranges contracted over the last century. As the globe warms up, this pattern is likely being repeated in ecosystems everywhere – not a happy prospect for critters that live at high elevations.

Reference

C Moritz, JL Patton, CJ Conroy, JL Parra, GC White, SR Beissinger (2008). Impact of a Century of Climate Change on Small-Mammal Communities in Yosemite National Park, USA. Science, 322, 261-4 DOI: 10.1126/science.1163428

Joshua tree genetics suggest coevolutionary divergence

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ResearchBlogging.orgThe latest results from the Pellmyr Lab’s ongoing study of Joshua tree and its pollinators are online as part of the new October issue of Evolution. It’s the cover article, no less. The study, whose lead author is Chris Smith (now on the faculty at Willamette University) compares patterns in the population genetics of Joshua trees and the moths that pollinate them, and shows that although the moths have become two separate species, the trees may not have followed suit [PDF].

Evolution cover
Photo by Chris Smith.

Female yucca moths carry pollen between Joshua tree flowers in special mouthparts. When she arrives at a new flower, the female moth lays her eggs inside it, then deliberately applies pollen to the flower’s receptive surface. When the fertilized flower develops into a fruit, the moth eggs hatch, and the larvae eat some of the seeds inside the fruit.

Among the yuccas, Joshua trees are unique because they’re pollinated by two species of moths, which are each other’s closest evolutionary relative. One species is found in the eastern part of Joshua tree’s range, the other in the west. Joshua trees from the east and west have differently-shaped flowers [PDF], which is consistent with the hypothesis that coevolution between moths and trees has driven both toward an evolutionary split.

“Western” Joshua trees at Joshua Tree National Park. Photo by me.

The new study goes deeper to look at genetic relationships between different populations of the moths and the trees, and what it finds isn’t as tidy as the earlier work might suggest: While Joshua trees’ morphology corresponds nicely to the split in the pollinators, the patterns visible in their chloroplast DNA does not. In some populations, trees look “eastern,” but have chloroplast DNA more closely related to “western” populations. This suggests that, although the moths have become separate species, they’re still moving between the two kinds of Joshua tree frequently enough that the trees haven’t quite split. Why do the two tree types look different, then? One possibility is coevolution with the two moth species, which might exert selection the trees in different ways.

There’s still a lot of work to do before we fully understand what’s going on here. Will Godsoe, the other doctoral student in our lab, is doing some intensive niche modeling to see how much environmental differences might be contributing to the patterns we see here. My own dissertation will look at whether the same incongruities turn up in nuclear DNA, which can have a different evolutionary history than that in the chloroplast.

References

W. Godsoe, J.B. Yoder, C.I. Smith, O. Pellmyr (2008). Coevolution and Divergence in the Joshua Tree/Yucca Moth Mutualism The American Naturalist, 171 (6), 816-23 DOI: 10.1086/587757

C.I. Smith, W.K.W. Godsoe, S. Tank, J.B. Yoder, O. Pellmyr (2008). Distinguishing coevolution from covicariance in an obligate pollination mutualism: asynchronous divergence in Joshua tree and its pollinators. Evolution, 62 (10), 2676-87 DOI: 10.1111/j.1558-5646.2008.00500.x

Ecological differences divide Mimulus guttatus

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ResearchBlogging.orgReproductive isolation is the engine of evolutionary diversification. When two populations become unable to exchange genes, they’re effectively separate species, free to evolve on independent trajectories.

Biologists have documented many examples of reproductive isolation arising from all sorts of different interactions between organisms and their environments, including incompatibilities between gametes, adaptation to different pollinators (in plants), or the evolution of different sexual characteristics. The cover article for this month’s issue of Evolution describes another way reproductive isolation can arise – adaptation to different environments.


Mimulus guttatus
Photo by Dawn Endico.

The new paper, by Lowry et al., describes how different ecological conditions create reproductive isolation where there would otherwise be none [$-a]. The wildflower Mimulus guttatus grows all along the U.S. Pacific Coast. Some Mimulus populations grow inland, in coastal mountains, where the summers are hot and dry; others grow right on the coast, where fog provides moisture but plants have to tolerate salt spray from the sea. Plants from inland and coastal populations look quite different (inland = tall with big flowers; coastal = short with small flowers), and have previously been separated out into different subspecies. But are they actually isolated?

Lowry et al. found that inland and coastal plants perform poorly when transplanted to the others’ habitats, and that they flower at significantly different times. A population genetic analysis shows that the coastal and inland populations don’t exchange genes very often. But it’s possible to hybridize the two types in the greenhouse. In short, it looks like Mimulus is a case of what Nosil et al. called “immigrant inviability” [$-a]. Immigration between inland and coastal sites may be possible, and immigrants would (theoretically) be able to reproduce if they mated with plants from the local population – but before they get a chance, they’re nailed by summer drought (at inland sites) or salt spray (at coastal sites). So even before they’ve evolved fundamental incompatibilities, these two types of Mimulus are well on their way to being separate species.

References

D.B. Lowry, R.C. Rockwood, J.H. Willis (2008). Ecological reproductive isolation of coast and inland races of Mimulus guttatus. Evolution, 62 (9), 2196-214 DOI: 10.1111/j.1558-5646.2008.00457.x

P. Nosil, T.H. Vines, D.J. Funk (2005). Reproductive isolation caused by natural selection against immigrants from divergent habitats. Evolution, 59 (4), 705-19 DOI: 10.1554/04-428