Category Archives: evolution

The joy of sex (well, one, anyway): Fewer parasites

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Natural selection does not necessarily love sex. Photo by xcode.

Hey, don’t knock [selfing]! It’s sex with someone I love.
—Woody Allen, in Annie Hall

Sex is a puzzle to evolutionary biologists. I don’t mean that we’re socially awkward—I mean that sexual reproduction, which involves mixing your genes with someone else’s to produce one or more children, seems to be at odds with natural selection. Every child produced by sexual reproduction carries only half the genetic material of each of her parents; but parents who can make children without sex pass on all their genes to every child.

Over time, individuals who can make babies without sex should become more common in the population than individuals who have to have sex to reproduce, simply because every baby produced without sex “counts” twice as much for its parent. We know of cases (for instance, stick insects) where asexual reproduction has apparently evolved and spread multiple times.

And yet, not only is sexual reproduction widespread in the natural world, there are many species of living things in which some individuals reproduce sexually and some reproduce without sex, and the two types coexist more-or-less stably. This is particularly common in plants, but it’s also seen in lots of other taxa. That suggests there must be something useful about sexual reproduction that offsets the cost associated with making only half a copy of your genome for every child you have.

One popular hypothesis is that sexual reproduction helps generate new combinations of genes to fight parasites and diseases—this is called the Red Queen Hypothesis, after the character in Through the Looking-Glass who tells Alice that “… it takes all the running you can do, to keep in the same place.” Sex, the thinking goes, means that your children are more likely to have new parasite-fighting gene combinations, and that populations can “run faster” in the coevolutionary race against parasites. And now, a new study in a population of peculiar little fish provides some reasonably direct evidence [$a] for that proposed benefit of sex.

A mangrove killifish. Photo via USGS, used under fair use rationale.

The mangrove killifish, Rivulus marmoratus, leads a pretty remarkable life even before you consider its reproductive strategy. Mangrove killifish live in coastal mangrove swamps, where they must contend with changes in water salinity and water level—and they deal with dry spells by packing into hollows in mangrove tree trunks. Jammed together in a hollow log, the killifish can survive up to two months entirely out of water.

They’re also one of very few vertebrate species known to be able to reproduce asexually. Most mangrove killifish are hermaphrodites, capable of making both eggs and sperm and combining them—or “selfing”—to lay fertilized eggs. A few killifish develop as “pure” males instead, capable of producing only sperm, and therefore only capable of sexual reproduction. Why that small fraction of males persists in killifish populations is probably related to the selective costs and benefits of sex, both for mangrove killifish and for living things in general.

The Red Queen hypothesis predicts that sex is beneficial because it creates new combinations of genes, which in turn lead to greater parasite resistance. Therefore, if killifish produced by sexual reproduction should have more diverse genomes, and are better able to resist parasites than killifish who only have one hermaphroditic parent, then the Red Queen may be the reason why male killifish haven’t gone the way of the dodo.

This is what Amy Ellison and her coauthors found in a population of mangrove killifish from four sites in Belize. They collected killifish and took their genetic fingerprints to identify individuals that were most likely descended from a single selfing lineage, or those that carried genes from multiple lineages. They also checked each fish for infection by three major groups of parasites—bacteria, a common protozoan parasite of killifish, and parasitic worms.

Their total sample size is a bit small, but the team found a pattern generally quite consistent with the Red Queen. Fish descended from sexually-reproducing parents were more likely to be heterozygous—to carry two different forms of a gene—than fish descended from asexual lines. More importantly, fish descended from sexually-reproducing parents also generally had fewer parasites of all three classes, and were generally less likely to carry any protozoans or worms, than those descended from hermaphrodites. That’s consistent with the Red Queen, and it shows the perfectly good selective “reason” for a hermaphrodite to mate with a “pure” male—even though the hermaphrodite is giving up half the selective benefit of the offspring thus produced, those offspring are more likely to be healthy.

A broader prediction that follows from these results is that mangrove killifish populations with higher rates of parasite attack should have more males, or at least more individuals with two parents. What would really be cool, though, is if hermaphroditic killifish can respond to parasite infections by choosing to reproduce sexually—self-medicating, like monarch butterflies, but with sex instead of a toxic host plant. It’s been observed that the hermaphroditic nematode worm Caenorhabditis elegans responds to environmental stress by giving birth to more male offspring, but I know of no such result in a vertebrate. ◼

Reference

Ellison, A., Cable, J., & Consuegra, S. (2011). Best of both worlds? Association between outcrossing and parasite loads in a selfing fish. Evolution, 65 (10), 3021-6 DOI: 10.1111/j.1558-5646.2011.01354.x

CreatureCast: Strangler figs

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Kevin Zelnio’s post about the need for evolutionary biologists to approach outreach like viral marketing reminded me about CreatureCast, a frickin’ awesome project by the Dunn Lab at Brown University, which has scientists talking about their work in bite-sized videos illustrated with whimsical animation. Here’s one on strangler figs:

If this isn’t a good argument for adding a little money to your next grant to support an undergrad video production or communication major as a “broader impact,” I don’t know what is. ◼

It’s that time of year again!

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The Portland Marathon two years ago. Looks fun, right?.

This weekend I’m flying out to Portland for the 2011 Portland Marathon, my third. It’s been a bit tricky keeping up with my training on top of moving to a new town and starting up a postdoc with a whole new study system, but I think I’ll be ready. While I pack, why not check out my post on the occasion of last year’s Seattle Marathon, in which I discuss what I’ve learned over a couple years of long runs and leg cramps. It all still applies.

I can make it through even a half-marathon on a good breakfast and carefully-judged pre-race hydration, but to go much longer I need more food (and water) mid-run. The long-term exercise involved in a long race is fueled by a combination of fat reserves and glycogen stored in the liver and muscle tissue. Glycogen is the more efficient fuel, so as exercise intensity increases, muscles draw on it more heavily.

For far more detail on evidence-based endurance training approaches, I suggest Dave Munger’s great science-based running. See you in 26.2 miles! ◼

What does evolution have to do with the cost of police in Switzerland? Probably not much.

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Lucerne, Switzerland. Photo by Jamie McHale.

ResearchBlogging.orgSo a little while ago, I was perusing the latest from PLoS ONE while doing some low-attention-requiring lab work Monday afternoon, and a title caught my eye: “A test of evolutionary policing theory with data from human societies.” Oh, hey. That looked interesting.

The paper’s author, Rolf Kümmerli, claims to have found evidence for a particular kind of evolutionary model of cooperation in recent economic data from Switzerland. The problem Kümmerli addresses is a classic one: from the perspective of natural selection, individuals (apparently) have little evolutionary incentive to cooperate, unless they’re relatives. And yet, we see cooperation in human societies.

One solution to this quandary has been group-level selection, which is a whole ‘nother kettle of worms. Another is that policing behavior could evolve to help keep groups of less-closely-related people cooperative. Of course, modern human societies are a long way past the days when most of us lived in villages that were also basically big extended families. Kümmerli proposes that we might use some sort of rule of behavioral thumb to (unconsciously) assess how likely it is we’re interacting with close relatives—which is less likely in bigger communities, and communities with larger immigrant populations.

Er, what?

Kümmerli compiled data from the Swiss national government, comparing crime rates and police expenditures to the population size percentage of foreign nationals living in every Swiss canton, or administrative region. Rather than just use the raw population size or percentage of foreigners in each canton, he constructed an index that combines the two. And he did, indeed, find that as this index of “dissimilarity” increases, so do crime rates and expenditures on police.

Kümmerli concludes that his data support the “evolutionary policing theory.” But what has he actually shown? Crime happens for lots of reasons, not necessarily because people somehow “know” to behave more cooperatively in small towns. Most glaringly, Kümmerli’s data set includes no data on poverty, which seem like an obvious alternative explanation for the pattern—bigger communities with more immigrants also often have more poor people, and poverty is certainly related to crime rates.

Swiss police. Photo by Kecko.

Fortunately, the data Kümmerli uses, and many more variables, are all freely available online through the Swiss Statistical Encyclopedia. So I took a couple hours to play around with the raw numbers. I did all my statistical work in good old R.

For each of the 26 cantons, I compiled the number of reported crimes in 2009, the number of citizens (in thousands) in 2009, the percentage of foreign residents in 2009, the percentage of unemployed residents in 2010, annual expenditures on police in 2008, and—just for the heck of it—the percentage of commuters using public transit in 2000. As in Kümmerli’s data set, each statistic is the most recent value available. I didn’t try to replicate Kümmerli’s “dissimilarity” index because it’s not clearly explained in the paper; but I did log-transform the crime rate, the number of citizens, police expenditures, the unemployment rate, and the transit use rate to make them better conform to a normal distribution.

Here’s what the simple linear relationships among all those variables look like. Apologies for the complicated graphic, but this is a complex data set.

Linear relationships (upper triangle) and correlation coefficients (lower triangle) among variables from the Swiss Statistical Encyclopedia. Grapic by jby.

In the upper triangle of this matrix, you can see scatter plots with linear regression lines estimated from the data. Regression lines are colored according to statistical significance, corrected for multiple testing: red lines are “very” significant, orange just significant; grey lines indicate relationships no stronger than expected by chance. The bottom triangle gives the raw correlation coefficient between the variables, on a scale where 1 means a perfect relationship and 0 means no relationship.

What you should notice first is that top row of scatterplots, which show that crime rates have strong linear relationships with every other variable in the dataset, from population size to mass transit use. But that makes a certain amount of sense—all these variables are interrelated. Larger communities tend to attract more immigrants and tend to have better public transit systems that support more use. Communities with more unemployed people might have higher mass transit use, since cars are expensive. So, lots of correlation—but is there any causation in there?

There are a number of ways to tackle that question. A relatively easy one is to use multiple regression and a “model comparison” approach. This essentially builds a statistical model in which multiple variables—population, foreign residents, unemployment, mass transit use—are used to predict a single variable, crime rates. The procedure then compares the model’s AIC score, an index of the model’s ability to predict crime rates from the other variables, to models with each of the individual variables removed. If removing a variable makes a “significant” reduction in AIC—which is typically understood to be a difference of at least 2 AIC points, then that variable contributes significantly to predicting crime rates.

A Swiss public transit police car. Photo by Kecko.

It turns out that all the variables I considered make a significant difference in a multiple linear regression model trying to predict Swiss crime rates. But they aren’t equally important. Removing unemployment from consideration made a difference of 4.9 AIC points, removing the percentage of foreigners made a difference of 5.9, and removing the percentage of people using mass transit made a difference of 13.6. But removing the number of citizens made a difference of 92.9 points—an order of magnitude bigger difference than the other variables.

So it looks like the strongest pattern in Kümmerli’s data is just the effect of larger communities—they have more crime.

This is not what we scientists call a “surprise.”

Moreover, it’s not particularly informative for the purpose of the question Kümmerli sets out to answer—we don’t really know how population size actually relates to humans’ tendency to be less “cooperative,” or to need police to make them cooperative. Larger population does seem to be related to more crime, but it’s also related to more mass transit use—and mass transit use strikes me as a pretty cooperative behavior.

Admittedly, that’s a pretty off-the-cuff assessment based on a couple hours of fiddling around with simple statistical analysis of an easy-to-access public data set. But I strongly suspect that you could say exactly the same thing of Kümmerli’s paper. ◼

Reference

Kümmerli, R. (2011). A test of evolutionary policing theory with data from human societies. PLoS ONE, 6 (9) DOI: 10.1371/journal.pone.0024350

Passwords and eviction notices: How do plants keep their bacterial partners honest?

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The nodule-y roots of a soybean plant. (Flickr: Pro-Soil AG Solutions)

ResearchBlogging.orgNitrogen is one of the elemental building blocks of life as we know it—it’s a basic component of amino acids, which are in turn the building blocks of proteins, which form the building blocks and moving parts of every living cell. The nitrogen interwoven in our tissues originated as part of the atmosphere we breathe, but the path from atmosphere to living flesh is far less direct than drawing a breath. Atmospheric nitrogen becomes useful to us animals only via an intimate relationship between a plant and bacterial growing in its roots.

The bacteria, called rhizobia, have the rare ability to “fix” free-floating nitrogen into biologically useable form. In return for this nitrogen source, the host plant allows the rhizobia to infect a specialized knob of root tissue, a root nodule, which it supplies with sugar for the benefit of its nitrogen-fixing guests. The plant uses the fixed nitrogen to make proteins for its own use, and anything that eats the plant afterwards benefits.

If all this sounds familiar, it’s because the interaction between plants and rhizobia is the focus of my developing postdoctoral research, and I’ve been writing about it as I’ve done more reading about it. Specifically, I’ve been interested in how plants might be able to make sure their root nodules house helpful bacteria rather than freeloaders, who enjoy the sugar supply inside the nodule without fixing nitrogen in return.

I’ve discussed a couple of different mathematical models that suggest some options. However, models are really just formal ways to follow through the implications of a particular idea, not necessarily descriptions of what actually transpires between a plant and the rhizobia inside its roots. So I thought it might make sense to step back and survey what we presently know about what goes on inside those root nodules.

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Carnival of Evolution, September 2011

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Photo by joiseyshowaa.

The September issue of the Carnival of Evolution is online now at The End of the Pier Show. Okay, it’s not as glamorous as the better known September issue, but it’s still the number one spot for a month’s worth of online writing about descent with modification and all its scientific, cultural, and political ramifications. A few highlights:

Carnival of Evolution, August 2011

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Grizzly bear. Photo by Alaska Dude.

The latest edition of the Carnival of Evolution, a monthly collection of online writing about evolution and all its ramifications, is online at Sandwalk. Check it out to learn why genetic testing for grizzly bears is important, what new fossil may have taken the place of Archeopteryx in the evolutionary history of birds, and what pioneer of evolutionary biology will soon be on a U.S. postage stamp.

Of mice and men, making a living in rarefied air

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(A)

High-elevation populations of deer mice have evolved “stickier” hemoglobin to cope with the thin atmosphere. (Animal Diversity Web)

ResearchBlogging.orgIt’s easy to walk through the woods and fields of North America and never spot Peromyscus maniculatus, the deer mouse, but you’ve probably heard them scampering off through the leaf litter or under cover of tall grass. They’re exceptionally widespread little rodents, found in forest undergrowth and fields from central Mexico all the way north to the Arctic treeline. In all this range, they look about the same: small and brown, with white underparts and big, sensitive ears.

That apparent sameness is deceptive, however.

A big, varied range presents lots of different environmental conditions to which a widespread species must adapt. And when that big, varied range includes the Rocky Mountains, one of those environmental conditions is as basic as the air itself. At high altitudes, atmospheric pressure is lower, which means lower partial pressure of oxygen, the gas that makes life as we know it work.

The fundamental problem at high altitude is to pull more oxygen from thinner air. Natural selection is good at solving problems, and it has multiple options for adapting a mammal to thinner air at high altitudes, to the extent that these traits are heritable. Selection could favor individuals who more readily respond to thin air by breathing faster and deeper, pulling in more air to make up for its lower oxygen content. Or selection could favor individuals who produce more red blood cells, so that a given volume of blood pumped through their lungs picks up more oxygen. Or, at the most basic level, selection could favor individuals whose individual red blood cells are better at picking up oxygen, via a new form of hemoglobin, the oxygen-binding molecule that packs every red blood cell.

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