Some years, they don’t bloom. I’m just back from a week and a half of attempted fieldwork in Nevada, with a hiatus to Southern California for a lecture to a Desert Institute class. Very few Joshua trees were in flower, so the trip was kind of a bust. But it was still good to get out into the desert. The weather was only really cold a couple nights, and almost too warm in Palm Springs. When I drove back into Moscow this afternoon, it was snowing.
Category Archives: science
Giant’s Shoulders #9 now online
The ninth edition of the Giant’s Shoulders, a monthly blog carnival celebrating classic research, is now online at The Evilutionary Biologist. Topics range way beyond my own foci, from Einstein’s other groundbreaking equation to a really cool nineteenth century attempt to define death in purely materialistic terms. Check it out!
The environmental impacts of war
Last year Bioscience published a review article proposing a new discipline in conservation ecology: warfare ecology [PDF]. It’s now making the rounds in the science blogosphere, with good coverage at Conservation Blog and Deep Sea News, where I first happened upon it – and it deserves all the attention it can get.
In the U.S., at any rate, war and preparation for war tend to get priority over everything – especially tree-hugging environmental concerns. Exhibit A is last year’s Supreme Court decision that the Navy’s need to practice with sonar trumps the damage sonar can do to whale populations, to the extent that the Navy could not be required to do an environmental impact assessment before beginning the exercise. War is treated as an emergency, and who worries about environmental impacts during emergencies?
Yet environmental damage caused in the course of war has direct impact on the human aftermath of conflict. Refugees provided with nowhere else to go will often set up camp in protected lands. Materials used in warfare – Agent Orange defoliant used in Southeast Asia, depleted uranium in Iraq – can continue to kill people long after the fighting ends. On the other hand, the review’s authors, Machlis and Hanson, point out that demilitarized zones and military training grounds often serve as (perhaps overly-well protected) accidental preserves.
This is a subject I’ve thought about quite a bit before – way back in my undergraduate days, I won a Mennonite Central Committee oratorical contest with a speech that connected peace theology to environmental concerns. That speech now looks to me like slightly embarrassing juvenalia, but the central idea still holds, and it’s great to see that working ecologists are thinking along similar lines. By laying out a framework for thinking about the environmental impacts of war, Machlis and Hanson’s paper can hopefully help push governments to consider the longer-term environmental, economic, and social consequences of ecological decisions made in the course of preparing for and prosecuting war.
Reference
G. Machlis, & T. Hanson (2008). Warfare ecology BioScience, 58 (8), 729-36 DOI: 10.1641/B580809
Pollinator isolation and divergence in floral shape
This post was written for The Giant’s Shoulders, a monthly blog carnival focusing on classic research.
Since Darwin, evolutionary biologists have thought that interactions between species cause diversification. However, it wasn’t until the second half of the Twentieth Century that scientists began to draw a connection between species interactions and speciation. One of the earliest of these studies was Verne Grant’s 1949 discovery of cleverly indirect evidence that pollinator isolation shapes the evolution of flowers [$-a].
A bee at work. (Flickr: jby)
Pollinator isolation is reproductive isolation created when animal pollinators don’t transfer pollen between plants of two different species. This could be because of pollinator behavior – say, because pollinator species tend to prefer a single plant. Or it could be because of the mechanics of pollen presentation by a flower, with each plant species applying pollen to a different part of a pollinator’s body so that foreign pollen is less likely to come into contact with the female floral parts. In either case, flowers are the key to the isolation – either to guide pollinators to their preferred target, or to make sure that the wrong pollen isn’t delivered.
Grant reasoned that pollinator isolation should have a real effect on how plant species are classified. Pollinator-isolated species probably have very different flowers; taxonomists, who look for characteristics that easily differentiate between related organisms, might therefore be more likely to use floral characteristics to tell pollinator-isolated plant species apart. To test this, Grant collected published classifications of plants pollinated by specialized animals (birds, bees, and long-tongued flies) and plants pollinated either by non-specialized animals, by water, or by wind.
Figure 1 from Grant (1959), showing the effect of pollinator isolation.
The result is presented in the tidy graph seen here. In plants pollinated by birds (A) and bees or long-tongued flies (CD), a much larger of the characteristics used by taxonomists to identify species were floral traits, compared to plants with non-specialized pollinators (E) or wind- and water-pollinated plants (FG). To follow up this result, Grant took systematic observations of pollinators’ movements through an experimental garden planted with three subspecies of Gilia capitata, each of which had differently colored flowers. The bees seemed to forage mainly among plants with similar flowers, and when Grant raised seeds from the experimental plants in the greenhouse, he found that there were fewer hybrids between subspecies than would be expected from random pollinator movement.
Generally, today, we wouldn’t assume that species classifications are an unbiased proxy for biological diversity – to some degree, they’re human constructs. But the basic idea that Grant develops, that speciation is an accidental consequence of plants’ interactions with pollinators, is still very important to how we understand the history of life. Together, flowering plants and insects make up the majority of the diversity of life on Earth, and it seems reasonable to think that this may be because the two groups interact so intimately.
More than fifty years after Grant’s study, pollinator isolation is a well-established mechanism for speciation. And the principle that Grant proposed, that increased divergence in floral traits is a sign of pollinator isolation, is still very useful. My lab, for instance, recently found that two forms of Joshua trees pollinated by different moth species are more different in certain floral dimensions than in non-floral traits [PDF]. That’s only the first step in what promises to be a long program of research (including my dissertation), seeking answers to some of the same questions that motivated Grant’s study.
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
V. Grant (1949). Pollination systems as isolating mechanisms in angiosperms. Evolution, 3, 82-97
Parasites like their hosts clustered
In epidemiology the importance of ecological and evolutionary processes comes into sharp relief: questions about the networks of interactions between species in a community, or about the evolution of parasite specificity, virulence, and contagiousness have immediate implications for human health, as well as in animal husbandry and conservation. One of the most basic of these questions is, what determines the community of parasites that infect a species? One answer is in this month’s issue of The American Naturalist, where a neat meta-analysis shows that the size of mammals’ home ranges shapes the number of parasite species they attract [$-a].
For mammals, we already know that parasite communities are shaped by the host’s body size, geographical range, and population density. In this new study, Bordes et al. propose another factor: the host’s home range, the area that a single individual occupies. There are two major ways that home range might shape the diversity of parasites infecting host. Greater home range could mean that the host encounters a broader array of habitats, and opportunities for infection, so that home range and parasite diversity are positively correlated. Alternatively, hosts with smaller home ranges effectively live at higher density, which should create more opportunities for parasite transmission between hosts, generating a negative correlation between home range and parasite diversity.
Bordes et al. test these hypotheses by collecting published studies of the number of parasitic worm (helminth) species infecting mammals, and then performing regressions (corrected for phylogenetic relationships between host species) of parasite species richness on a variety of possible causal factors, including home range. They find that host home range is a stronger predictor of parasite species diversity than host body size, and that home range is negatively correlated with parasite diversity.
In a way, then, this result confirms the importance of host density in host-parasite interactions. But it’s not an obvious outcome – it is intuitive that more densely populated hosts should be more susceptible to parasitism in general, but not that they should also be attacked by a wider array of parasites. Maybe dense host populations are more productive habitat to parasites, so that there’s ecological “space” to support a greater diversity of parasites. Or maybe these dynamics are a result of the specific biology of helminth parasites, many of which have different hosts for different parts of their life cycle.
Reference
F. Bordes, S. Morand, D.A. Kelt, D.H. Van Vuren (2009). Home range and parasite diversity in mammals The American Naturalist, 173 (4), 467-74 DOI: 10.1086/597227
Open access on the line: H.R.801
A bill presently under consideration by the House Judiciary Committee would end the National Institutes of Health open access policy – which requires NIH-funded research to be made freely available to the public 12 months after publication – and ban other federal funding agencies from enacting similar measures.
This is, of course, primarily for the benefit of scientific publishers, who rely on subscription and online access fees as a major source of income. But it means that taxpayer-funded research would be inaccessible to members of the public who don’t benefit from institutional subscriptions. How we fund scientific publishing in the Internet Age is a tricky question – but legal fiat is not a good way to negotiate that question. Contact your representatives, and tell them to vote “no” on H.R. 801.
Via OpenCongress. See also coverage on Greg Laden’s blog.
Milkweed’s bitter arms race against herbivores
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.
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.
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.
Reference
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
Cooperation from selfishness?
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.
Reference
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
Perspective
Want to speciate? Stay home.
I’ve said it before, and I’ll say it again: the formation of new species is almost always an accident. There you are, adapting to changes in your obligate pollinator, or the local environment – and suddenly, you can’t mate with the folks on the other side of the mountain. That’s the lesson I take from an article in last week’s PNAS, which suggests that a diverse group of birds got that way by being homebodies [$-a]
The authors (including Jared Diamond, who communicated the paper to PNAS), set out to determine why there are so many species of white-eyes, a group of songbirds distributed across Africa, Southeast Asia, and the southern Pacific. They built a phylogeny for the group, calibrated it to real time using the geological dates of origin for Pacific islands occupied by white-eyes, and then estimated the rate at which the group produced new species. They found, as reported by Wired.com, that the largest group of white-eyes have one of the fastest species-accumulation rates recorded in vertebrates, about 1.6 new species every million years.
That’s a weird result, when you think about it – we’re talking about birds, and widely-distributed birds, here. All things being equal, speciation is facilitated by lack of movement – Appalachian salamanders, for instance, diversified largely because they’re too gimpy to move between stream drainages very often [$-a]. Furthermore, the authors say, white-eyes don’t display a lot of ecological differences that might contribute to isolation. So how did they speciate at a record-setting pace?
The solution? The authors propose that white-eyes are prone to rapid changes in their dispersal ability. As evidence, they cite numerous cases in which white-eyes must have crossed great distances to colonize one island, then failed to make it across much smaller distances to colonize others nearby. Nodding to Diamond’s groundbreaking work on human history and cultural evolution, they compare this to the colonization of Polynesia, in which people stopped traveling long distances as the chance of discovering an uninhabited island decreased.
References
K.H. Kozak, D.W. Weisrock, A. Larson (2006). Rapid lineage accumulation in a non-adaptive radiation: phylogenetic analysis of diversification rates in eastern North American woodland salamanders (Plethodontidae: Plethodon) Proceedings of the Royal Society B: Biological Sciences, 273 (1586), 539-46 DOI: 10.1098/rspb.2005.3326
R.G. Moyle, C.E. Filardi, C.E. Smith, J. Diamond (2009). Explosive Pleistocene diversification and hemispheric expansion of a “great speciator” Proceedings of the National Academy of Sciences, 106 (6), 1863-8 DOI: 10.1073/pnas.0809861105