No room for group selection in disease evolution?

ResearchBlogging.orgParasites coexisting within a single host have been proposed as one of the best examples of individuals sacrificing their own reproductive fitness for the benefit of a group. A new theory paper in last week’s Nature suggests that the apparent effect of “group selection” in this case can be explained by individual-level selection instead [$-a].

Group selection posits that organisms sometimes evolve traits that hurt their individual fitness but benefit their social group. Charles Darwin originally proposed it to explain the evolution of human moral systems: in a tribal society, helping your neighbor might cost you, but it might still help your whole tribe to compete against other tribes. So natural selection on individuals within a tribe may act in one way, but be opposed by group selection arising from competition among tribes.

This process has also been proposed to explain a common phenomenon in the evolution of disease organisms: the trade-off between transmission and virulence [PDF]. Simply put, if it’s easy for a disease-causing critter to spread through a host population, it tends to do more damage to its hosts; and if it is less easy to spread, the disease tends to do less damage [$-a]. A classic case of this effect is documented in cholera, which has evolved lower virulence when good sanitation practices cut off the easy route of transmission through sewage-contaminated drinking water.

Proponents of group selection say that this occurs because, under difficult transmission conditions, disease organisms must throttle back their production of offspring lest they kill their shared host. But it’s also possible to describe a verbal model by which reduced transmission selects for lower virulence without invoking group selection, courtesy of kin selection.

Kin selection takes into account the effect of natural selection on not just the copies of an individual’s genes within that individual’s body, but also the copies borne by close relatives; if you’re a parasite that reproduces inside your host, making more offspring also means making more competitors for your offspring, and thereby reducing the fitness of the genes that you share with the next generation. So, unless it’s easy to disperse to new resources — uninfected hosts — natural selection can actually favor prudent reproduction by a parasite, which keeps the host alive longer.

The new paper in Nature puts some math behind this verbal model. The authors, Wild et al., building on a standard disease-modeling framework, assume a world of patchily-distributed hosts infected by a single parasite species. Parasites are transmitted by host-to-host contact; it’s assumed that the number of offspring a parasite produces is proportional to the chances that some of those offspring are transmitted to another host, so that more virulent parasites have a better chance of sending offspring to new, uninfected hosts.

Under this model, the authors show that the fitness of a mutant, more virulent parasite, differs from that of its less-virulent competitors in several important ways: A more virulent mutant has

  • an increased chance of killing its host;
  • an increased chance of sending offspring to another patch of hosts;
  • increased competition from the offspring it produces that do not disperse to another patch;
  • increased competition experienced by those offspring; and
  • a greater chance that, by killing its host, it will make way for an uninfected replacement host for its offspring.

When the parasites can disperse to new patches with maximum efficiency, they simply evolve to maximize their own fitness at the expense of the host — but as dispersal becomes more restricted, the costs of competition exert selection on individual parasites to evolve reduced virulence.

Wild et al. conclude that, because their model replicates the transmission-virulence trade-off without invoking group selection, they can reject the group selection hypothesis. In fact, they strongly suggest that group selection may not matter much in natural systems:

The multilevel (group) selection and kin selection (inclusive fitness) approaches to social evolution have long been known to be mathematically equivalent and, if the analyses are performed correctly, do not lead to conflicting predictions. Thus, irrespective of the relative strengths of within-group versus between-group selection, individuals are predicted to maximize their inclusive fitness. [In-text citations removed; emphasis added.]

Clearly this result shows that group selection isn’t necessary to create the transmission-virulence trade-off. On the other hand, it doesn’t provide a good comparison of group-level and individual-level selection, because (so far as I can see) it doesn’t explicitly contain an effect of group selection. It’s one thing to show that group selection isn’t necessary, but it’s another to show that its effects would be overwhelmed by individual-level selection.

References

Cochran G.M., Ewald P.W., & Cochran K.D. (2000). Infectious causation of disease: An evolutionary perspective Persp. Biol. Medecine, 43 (3), 406-48 DOI: 10.1353/pbm.2000.0016

Day, T., & Gandon, S. (2007). Applying population-genetic models in theoretical evolutionary epidemiology Ecology Letters, 10 (10), 876-88 DOI: 10.1111/j.1461-0248.2007.01091.x

Frank, S.A. (1996). Models of parasite virulence Quarterly Rev. Biol., 71 (1), 37-78 DOI: http://www.jstor.org/stable/3037829

Wild, G., Gardner, A., & West, S. (2009). Adaptation and the evolution of parasite virulence in a connected world Nature, 459 (7249), 983-6 DOI: 10.1038/nature08071

Wilson, D., & Wilson, E. (2007). Rethinking the theoretical foundation of sociobiology Quarterly Rev. Biol., 82 (4), 327-48 DOI: 10.1086/522809

For wasps’ pheromones, quantity predicts quality

ResearchBlogging.orgDon’t tell the people behind Axe body spray, but entomologists have shown that the fertility of male Nasonia vitripennis wasps is predicted by how much sex pheromone they produce [$-a].

How many sperm a male wasp can produce turns out to be a big deal for female Nasonia wasps, because the species is haplodiploid — fertilized eggs become females, and unfertilized eggs become males. Because females are the only sex that can fly off to lay more eggs, the number of female offspring a wasp produces strongly determines her reproductive fitness. She wants, therefore, to mate with a male who can fertilize a lot of eggs, and determines who that is by smelling prospective mates.

Reference

Ruther, J., Matschke, M., Garbe, L., & Steiner, S. (2009). Quantity matters: male sex pheromone signals mate quality in the parasitic wasp Nasonia vitripennis. Proc. R. Soc. B DOI: 10.1098/rspb.2009.0738

Attention: followup

Following Matt’s comment that my earlier analysis of my note-taking during the Evolution 2009 meetings, I’ve counted up characters as well as lines of notes taken, with the following result: although I think it was an open question, it turns out that there’s a pretty tight correlation between lines of text taken as notes, and the total number of characters.


(Click image to see larger version.)

Hello, my name is Jeremy, and I am a nerd.

John Hodgman on Obama the nerd

Via harpersnotes: Hodg-Man himself assesses President Obama’s nerd credentials at the Radio and Television Correspondents’ Dinner. And it is awesome.

Essential companion reading: Sarah Vowell’s essay “The Nerd Voice” (in The Partly Cloudy Patriot, Simon and Schuster: 2003), which captures exactly the jock-ness of the Bush Administration, and the very nerdy reasons why Al Gore is not our first modern nerd president.

Evolution 2009: Day four

Evolution 2009
ResearchBlogging.org
Today was the final day of symposia and paper presentations for Evolution 2009, and everyone was ready for it. At least, I knew I was when I came suddenly awake in the middle of a talk I’d really wanted to see, about the use of Bayesian clustering methods to detect interspecific hybrids in natural populations. This was in the middle of the third talk session for the day; I took it as a sign from above (or wherever) and bailed for a bike ride.

Before I got to that point, though, there were some great last-day talks. Opening a symposium of retrospectives on The Origin of Species, Doug Schemske discussed the book The Origin, before Alfred Russell Wallace’s independent discovery of natural selection forced Darwin to publish “an abstract” to defend his priority. The writings Darwin intended for the much larger Natural Selection weren’t assembled for publication until 1975, but Schemske explained that they clarify a number of points commonly said to be missing or underrepresented in The Origin, such as role of geographic isolation in speciation.

In the “Systematics and Adaptive Radiation” session, Joel Cracraft argued that we don’t really know what mechanisms shape rates of speciation and extinction over macroevolutionary time. Incredibly, he was not mobbed afterward by irate students of adaptive radiation theory, which posits some very specific mechanisms for exactly those phenomena.

In the spirit of my posts from last year’s meetings, I’ll conclude by testing the hypothesis that I gave equal attention the talks I attended all week long. Following previously-described methods, I counted up the lines of text in my notes for each talk (talks attended = 16 on day 1, 10 on day 2, 15 on day 3, and 9 on day 4), which was easier this year because I took notes in text files on my laptop.

The means and variation in note length for each day are summarized in the boxplot below. A one-factor ANOVA finds a strongly significant effect of the day of the proceedings on the volume of notes I took (p = 0.00016), but this is mostly due to the shorter notes taken on the first day: there is no significant effect of the day of the proceedings on volume of notes taken if this day is excluded (p = 0.22768). I’m not sure why I took fewer notes on that first day — maybe I was trying for more bloggable detail after writing my first post on the meeting proceedings.

If I have some time tomorrow, I’m going to follow up on the use of Twitter and FriendFeed by meeting attendees. But I’m done for tonight.

Evolution 2009: Day three

Evolution 2009
ResearchBlogging.org
On the third day of Evolution 2009, things are winding down already. I’ve been up late saying goodbye to folks leaving tomorrow.


A bog turtle
Photo by Wall Tea.

The most entertaining talk of the day was more about physics than evolution as such: specifically, an analysis of turtle shell architecture. C.T. Stayton discussed work he published in the May issue of Evolution, showing that turtle shell shapes are a compromise between streamlining for efficient swimming and ability to resist crushing attacks from predators [$-a]. He referenced Terry Pratchett in his introductory slides, but I didn’t have the presence of mind to ask afterward what the optimal shell shape is to support the weight of four elephants and a Discworld.

Other highlights:

  • Simone Des Roches presented more results from experiments that showed how the adaptive divergence of sticklebacks can alter ecosystem dynamics. (I discussed the original publication back in April.)
  • The amoeba Dictostelium discoideum responds to stress by forming spore-making fruiting bodies. Some cells “cheat” by taking the beneficial spots in the fruiting body and leaving others to form its non-reproductive stalk — and it seems that the cheaters do this by getting there first.
  • Live-bearing guppies are able to compensate for a reduced food supply by restricting the size of their developing babies.
  • Although whaling nations argue that Minke whales have become much more abundant due to lack of competition from species hunted to near extinction in the early 20th century, population genetic data suggest that Minke whales are about as numerous as they were prior to that time.
  • It’s actually proving pretty tricky to determine the evolutionary relationships of chipmunks in Western North America, both because they hybridize frequently and because they speciated rapidly.
  • Boundaries between related species’s geographic distributions may be maintained by locally-adapted pathogens.

And, finally, video of Eugenie Scott’s Gould Award lecture is now online for streaming in Real Video format here.

References

Harmon, L., Matthews, B., Des Roches, S., Chase, J., Shurin, J., & Schluter, D. (2009). Evolutionary diversification in stickleback affects ecosystem functioning Nature, 458 (7242), 1167-70 DOI: 10.1038/nature07974

Stayton, C. (2009). Application of thin-plate spline transformations to finite element models, or, how to turn a bog turtle into a spotted turtle to analyze both. Evolution, 63 (5), 1348-55 DOI: 10.1111/j.1558-5646.2009.00655.x

Evolution 2009: Day two

Evolution 2009
ResearchBlogging.org
After a late (early) night yesterday, I started my day at the R.A. Fisher Award talk, a presentation of results from “an outstanding Ph.D. dissertation paper published in the journal Evolution.” This year’s winner turned out to be a paper I remember reading when it was first published, in which Megan Higgie and Mark Blows showed that sexual selection for mate-signaling hydrocarbons in Drosophila serrata is opposed by selection to avoid mating with the closely related D. birchii. Populations of D. serrata that occur with D. birchii have been selected for different hydrocarbon profiles [$-a] than populations that don’t occur with D. birchii — so that adaptive speciation could result from the opposing selective regimes.


Photo by Jo Mur.

On a more natural history-oriented note, today I learned that honeyguides, the African birds known for their habit of guiding badgers (or humans) to bees’ nests, are also particularly vicious brood parasites. Like cuckoos and cowbirds, honeyguides lay their eggs in the nests of other bird species, and let the parasitized parents raise ’em. Apparently the Greater Honeyguide (the chick in the figure provided) has had its eggs selected to more closely match a variety of host species.

Other highlights:

  • Flowers of the genus Pedicularis are more different in co-occuring species than would be expected by chance, possibly to minimize the chance that their shared pollinators, bumblebees, transfer pollen between different species.
  • When female Hadena bircuris moths pollinate their host plant Silene latifolia, they lay eggs on the flower so that their larvae can eat some of the seeds produced — much like yucca moths — but male moths of the same species also pollinate, and this may help offset the cost of female pollination.
  • Host-parasite coevolution may actually drive the evolution of mutation rates in the host and the parasite, much like yesterday’s demonstration that coevolution can alter migration rates.

Finished the day much more quietly than yesterday, with a handful of folks at my place for burgers and beer. And, hey, I’m getting to bed before 4 a.m.!

References

Higgie, M., & Blows, M. (2008). The evolution of reproductive character displacement conflicts with how sexual selection operates within a species. Evolution, 62 (5), 1192-203 DOI: 10.1111/j.1558-5646.2008.00357.x

Evolution 2009: Day one

Evolution 2009
ResearchBlogging.org
In this morning’s session on species interactions and coevolution, everyone was talking about these videos of snakes attacking snails. Turns out that snail shell chirality (the direction the shell spirals) can determine how easy it is for a snake to attack. Very, very cool. Detailed discussion by John Dennehy is here. [Edit, 14 June 2009: link to the video originally found by Matt Labrum.]

I presented today, and survived another twelve-minute talk. Immediately after I finished describing my preliminary conclusion that coevolution between species only generates evolutionary diversity if it exerts disruptive selection on one or the other interactor — the best example of which may be competitive exclusion — Jeremy Fox described a model in which competitor species converge on a single set of traits [$-a]. It’s a cool result, and one I’ll need to consider carefully.

I also learned today that

  • A bacterial endosymbiont helps fruit flies fight off parasitic worms;
  • It might not “cost” anything for some specialist herbivores to sequester the toxins produced by the plants they eat;
  • Coevolution can actually change the migration rates of interacting species; and
  • Bacteria and phage living inside horse chestnut leaves are locally adapted within individual trees, but not within individual leaves.

Was out way too late, as the timing of this post may indicate. Don’t think I’ll make the morning sessions. Not that the love-in-a-canoe coffee provided by campus catering will help. Ugh.

References

Fox, J., & Vasseur, D. (2008). Character Convergence under Competition for Nutritionally Essential Resources The American Naturalist, 172 (5), 667-80 DOI: 10.1086/591689

Evolution 2009: Underway

Evolution 2009After months of preparation, the Evolution meetings start this evening, with a reception followed by Eugenie Scott’s lecture as the inaugural recipient of the Gould Award. I’ve already checked in and got my swag bag. (Not too shabby if I say so myself: a nice water bottle, a cool t-shirt, and a sample of Cowgirl Chocolates.) I’m presenting my talk Saturday afternoon. Should be a great, science-y extra-long weekend.

Watch the meetings’ blogging page and FriendFeed for updates.


Our venue, the biggest barn in Idaho.
Photo by Allen Dale Thompson.