How it does a body good: The selective advantage of drinking milk depends where you drink it

ResearchBlogging.orgI am a super-powered mutant. For a given value of “super-powered” and “mutant,” anyway: I am an adult human who can drink milk. This is unusual among mammals, but as those (in retrospect, somewhat creepy) PSAs that used to run during my Saturday morning cartoons said, milk has a variety of nutritional benefits, if you can digest it. Which of these is behind the evolution of adult milk digestion in humans? According to a new paper in this week’s PLoS ONE, the benefit you get from drinking milk depends on where you live.

Originally, every human on Earth was lactose intolerant, like most mammals. That is, they lost the ability to digest lactose, the major sugar in milk, when their bodies stopped producing the necessary enzyme lactase after weaning. Then, some populations of humans domesticated milk-producing animals, and this seems to have generated strong natural selection [PDF] for a form of the lactase gene that remains active in adults.


Photo by bensonkua.

In fact, milk-drinking populations in Europe and Africa have evolved “lactase persistence” independently [$-a]. This parallel evolution of a single trait motivates the new study by Gerbault et al. — drinking milk might have different advantages for African pastoralists and Northern European farmers. Milk has two major dietary benefits:

  • It’s generally nutritious as a source of protein and calories, and
  • Lactose can aid in calcium uptake in lieu of Vitamin D.

The main source of Vitamin D, for humans, is sun exposure — the UVB rays in natural sunlight stimulate production of the vitamin. In Africa, close to the equator, it’s easy to get plenty of direct sunlight; but in northern Europe, sunlight is less direct — so it’s harder to produce enough Vitamin D. (This is actually thought to be one reason for geographic differences in human skin color [$-a]: under lots of direct sunlight, dark skin is favored to minimize cancer risk; but under indirect sunlight, light skin is favored to allow more Vitamin D production.)

If the benefit of milk is calcium, not protein, then we would expect adult-active forms of the lactase-producing gene to be common in northern populations, and to decrease in frequency with decreasing latitude. This has been observed in a survey across Europe [$-a] — but while the north-south pattern supports the calcium-benefit hypothesis, it is not conclusive evidence. This is because the same pattern could arise without any natural selection acting on the gene — populations generally tend to be less genetically similar if they’re farther away from each other, a phenomenon called isolation by distance, or IBD [PDF]. In fact, Gerbault et al. find that the north-south pattern of genetic similarity is replicated in genes that probably aren’t under selection arising from life at high latitudes, suggesting that IBD, not selection, is responsible for the pattern in the lactase gene.


Photo by tricky.

For a more conclusive test, Gerbault et al. developed computer simulations of the evolution of early European communities. By simulating populations’ evolution with different strengths of selection acting on the lactase gene, they could estimate how probable a particular value of selection was given the present-day frequency of lactase persistence in the real population — but also take into account the population genetic forces that create IBD. They found that in southern Europe, no natural selection was necessary to explain the present frequency of lactase persistence — but in the north, selection coefficients as high as 1.8% were needed. That is, in northern Europe, lactase persistence is so common that the simulations only produced the observed frequency when people who could not drink milk as adults had, on average, 1.8% fewer children than those who could.

In contrast to Europe, African communities don’t show the same gradual transition from frequent to rare lactase persistence, so IBD is less likely to explain the observed patterns. To explain the frequency of lactase persistence in African populations, the authors compared it to the frequency of pastoralism — and, finding a strong positive correlation, they concluded that lactase persistence evolved in Africa because it allowed shepherds to derive more nutrition from the animals they kept.

In short, widespread lactase persistence evolved in Africa because milk is a good source of protein; but it seems to have evolved in Europe because milk is a good source of bone-building calcium. Human populations on separate continents arrived at the same evolutionary solution, but for slightly different reasons.

Update, 18 October 2009: I’ve submitted this post to the NESCent competition for a travel award for the ScienceOnline 2010 conference in Durham, NC, January 14‐17th, 2010.

Update, 15 December 2009: Ye gads. I won!

References

Diamond, J. (2005). Evolutionary biology: Geography and skin colour Nature, 435 (7040), 283-4 DOI: 10.1038/435283a

Gerbault, P., Moret, C., Currat, M., & Sanchez-Mazas, A. (2009). Impact of selection and demography on the diffusion of lactase persistence PLoS ONE, 4 (7) DOI: 10.1371/journal.pone.0006369

Ingram, C., Mulcare, C., Itan, Y., Thomas, M., & Swallow, D. (2008). Lactose digestion and the evolutionary genetics of lactase persistence Human Genetics, 124 (6), 579-91 DOI: 10.1007/s00439-008-0593-6

Swallow, D. (2003). Genetics of lactase persistence and lactose intolerance Annual Review of Genetics, 37 (1), 197-219 DOI: 10.1146/annurev.genet.37.110801.143820

Tishkoff, S., Reed, F., Ranciaro, A., Voight, B., Babbitt, C., Silverman, J., Powell, K., Mortensen, H., Hirbo, J., Osman, M., Ibrahim, M., Omar, S., Lema, G., Nyambo, T., Ghori, J., Bumpstead, S., Pritchard, J., Wray, G., & Deloukas, P. (2006). Convergent adaptation of human lactase persistence in Africa and Europe Nature Genetics, 39 (1), 31-40 DOI: 10.1038/ng1946

Wright, S (1943). Isolation by distance Genetics, 28, 114-38 Other: http://www.genetics.org/cgi/reprint/28/2/114

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Kindle curmudgeonry

Via the Slog: Nicholson Baker reviews Amazon’s Kindle e-book reader for the New Yorker. He is, to say the least, skeptical.

Yes, you can definitely read things on the Kindle.

Damning with faint praise? Actually, the tone of the review is more just damning. Especially when you get to the list of books not yet available.

About the only thing I might want to do with an e-book reader is read PDF documents, which are my format of choice for journal articles. For that, I need note-taking support and the ability to rapidly zoom around on the page — and the confidence that figures will be as clear as they would be in color. I’ll stick with my MacBook for the time being.

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Here you see it, there you don’t: What determines species distributions?

ResearchBlogging.orgFor a year between undergrad and graduate school, I interned with the ecologists at the Western Pennsylvania Conservancy. A fair bit of the work revolved around documenting the locations of rare plants or animals. But “rare” can be relative: common ravens (for instance) were hard to find in Pennsylvania, but much more abundant just over the border in New York. In Pennsylvania, striped maples are so abundant that they can interfere with the regrowth of logged woodland, but in Ohio they’re rare enough to be considered endangered.


Striped maple: rare in Ohio, a pest in
Pennsylvania.

Photo by homeredwardprice.

This, of course, is because Pennsylvania’s human-drawn political boundaries straddle important ecological boundaries, like the transition from raven-friendly Circumboreal forests of New England to the raven-free forests to the south. And you might imagine that, even if there weren’t major topographic features within a particular set of human-created boundaries, there would still be enough climatic changes from one side to the other so that the species present at the southernmost edge of a flat, boring state like Kansas wouldn’t be entirely the same species present at the northernmost edge. That is, if you travel south to north across Kansas, you’ll probably pass a point where the lowest winter temperature becomes too low for some species.

Still, there are probably fewer species whose ranges end within the borders of Kansas than within Pennsylvania. As a paper in the latest Proceedings of the Royal Society attempts to show, this is for the intuitive reason I’ve tried to describe above: while Kansas is comparatively uniform, Pennsylvania is located at a transition between ecological regions. At such transitions, landscapes get complicated — and that complication, the paper’s authors say, is what helps create the boundaries of species distributions [$-a].

To make their point McInnes et al. examine the distribution of bird species across Africa. The broke the continent up into a grid, and scored each grid cell by the proportion of birds present in the square whose ranges had an edge within the cell, a measure they call “impermeability.” The impermeability of a given cell was strongly related to the number of habitat types represented in the cell — and these heterogeneous cells tended to be distributed along the boundaries of major ecological regions, like the Sahara Desert.

That is, species ranges tend to end in areas where desert is shading into grassland, or savanna into forest. Or, to put it another way, one group of species (birds) tend to have range edges that coincide with the boundaries of whole ecological communities. That’s an important observation — but it’s not exactly new. The entire concept of ecological communities arose because it’s obvious there are groups of living things that tend to occur together. McInnes et al. have quantified that observation, but they haven’t really explained it.

References

McInnes, L., Purvis, A., & Orme, C. (2009). Where do species’ geographic ranges stop and why? Landscape impermeability and the Afrotropical avifauna Proceedings of the Royal Society B: Biological Sciences, 276 (1670), 3063-70 DOI: 10.1098/rspb.2009.0656

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A brief history of airport architecture

Slate has a great photo essay tracing the evolution of airports from “fields” adjacent to runways, to train-station inspired terminals, to more modern concourses. The concluding thought, which I’m not sure I agree with, is that the best solution is basically a big shed.

This is a subject with which I already have an unhealthy obsession: see my rant about airports here, and my Flickr collection of airport imagery below.

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Finding Joshua tree’s niche

ResearchBlogging.orgA new Joshua tree study is just out in the current issue of New Phytologist, presenting an analysis of the environments occupied by the two different types of Joshua tree. The results demonstrate that the two tree types mostly grow in similar climatic conditions [PDF], which suggests that coevolution with its pollinators, not natural selection from differing environments, is responsible for the evolution of the two different tree types.

The Pellmyr Lab has been studying the two types of Joshua tree, which are pollinated by two separate, highly specialized moths, for several years now. Previous papers have shown that the two types of Joshua tree, first described in the 1970s based only on their vegetative features, are most strongly differentiated by the shape of their flowers [$-a]; and that, although the two moths are separate species, the two tree types are not fully genetically differentiated [PDF].


Hey, those Joshua trees look kinda
different.

Photo by jby.

The latest paper is a chapter from the dissertation of Will Godsoe, who just received his doctorate last week. It presents an analysis that sidesteps a fundamental problem with studying long-lived, specialized organisms — they’re hard use in fully controlled experiments. To determine whether the two types of Joshua tree really evolved as a result of coevolution with their pollinators, we’d like to be able to eliminate the alternative hypothesis that the two types evolved in response to different environmental conditions. Except for a small contact zone in central Nevada, each tree type occurs in a different part of the Mojave desert, and the two regions do have some broad-scale differences in when they receive precipitation.

Ideally, to determine whether two plants have different environmental needs, you just perform an experimental transplant, growing each plant in the other’s environment to see whether it fares as well as it does at home. This isn’t really possible with Joshua trees, which are pretty tricky to sprout from seeds (I’ve tried), and which, in any event, take something like twenty years to mature. So Will proposed to use niche modeling methods instead. Niche models are statistical descriptions of environments where an organism is known to live, often used to predict where it could live. To build niche models for each type of Joshua tree, Will assembled location data we’d collected over several field seasons in the Mojave, then spent another field trip driving around the desert some more to fill in the gaps — he wanted locations where Joshua trees were definitely growing and where they definitely weren’t, to fully “inform” the models.

Using the location data, it was possible to determine what kinds of climates each Joshua tree type tended to occupy by cross-referencing with existing climate databases, then fitting statistical models to the results. The models produced for each tree type could then be compared — and, for the most part, they’re similar. That is, if you collected seeds from one tree type, planted them where the other type grows, and waited around for a few decades to check the result, you’d probably find that it grew as well as it did in its home range.

So, if differing climates don’t explain the origin of the two types of Joshua tree, does that leave no other possibility but the pollinating moths? Not exactly — there are lots of environmental variables that weren’t available for Will’s niche models, for instance, or there could be a third, completely unknown factor. But this does make coevolution with the moths a more plausible explanation. In light of some of our very latest results — which should be going to press fairly soon — coevolution is looking like a better and better possibility.

Reference

Godsoe, W., Strand, E., Smith, C.I., Yoder, J.B., Esque, T., & Pellmyr, O. (2009). Divergence in an obligate mutualism is not explained by divergent climatic factors New Phytologist, 183 (3), 589-99 DOI: 10.1111/j.1469-8137.2009.02942.x

Godsoe, W., Yoder, J.B., Smith, C.I., & Pellmyr, O. (2008). Coevolution and divergence in the Joshua tree/yucca moth mutualism The American Naturalist, 171 (6), 816-23 DOI: 10.1086/587757

Smith, C.I., Godsoe, W., Tank, S., Yoder, J.B., & Pellmyr, O. (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

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An order of service for the first congregational church of Carl Kassel

Introit: “All Things Considered Theme” (arr. for organ)

Litany
Leader: The time is now seven minutes past the hour. I’m Terry Gross. From whence cometh our news?
Congregation: Shallow, adversarial coverage holds sway, on broadcast as it is on cable.
L: From whence cometh our commentary?
C: Humorless, unreasoned opinions are a swelling tide all around us.
L: Yet behold! A gust of Fresh Air for those living in the shadow of Fox.
C: What is this Fresh Air?
L: Our news cometh from Public Radio.
C: Thoughtful, in-depth reporting is the salvation of our days.
L: Our commentary cometh from Public Radio.
C: Measured discussions of relevant issues make our minds easy.
L & C together: Thanks be to listeners like you.

Passing the Peace
The Congregation are invited to stand and congratulate each other on their intelligence and good taste.

Congregational hymns (G. Keillor, song leader)
# 142, “Tell my why”
# 576, “Swedish folksong”
# 606, “Gimme that old-time religion”

Communion
All those attending are invited to participate; follow the usher’s cue to walk up the aisle, accept a piece of bagel from the host, and dip it in coffee in the Morning Edition logo mug.

Reading of the Headlines (L. Hansen)
National: Justice Department begins investigation into Congressional fundraising scandal
International: Chinese government cracks down on provincial uprising
Science: Researchers create first human-sheep hybrid embryo
Quirky: Missoula man bicycles from Anchorage to Lima to promote alternative fuels

Our Top Story (S. Simon, special correspondents K. Levine, R. Krulwich)
Global warming: close to home (part three of five-part series)

Meditation (D. Schorr)
This President might be pretty lousy, but he’s no Richard Nixon.

Congregational response: Thanks be to our correspondents!

Hymn of response (I. Glass, song leader)
# 337, “James K. Polk” (They Might Be Giants)

Pledge Drive
Offertory: Selections from The Peter, Paul, and Mary Reunion Tour (available on CD as a thank-you gift for persons pledging $75 or more)
Please place your pledge cards in the tote bag as the ushers pass it down each row. If you are already a member, consider increasing your membership level, or giving a gift membership to a friend or loved one. The ushers will hand out thank-you gifts in the narthex after the service; all new members will receive a subscription to Local Arts Magazine.

Benediction
Leaders: Behold, you’ve wasted another perfectly good hour.
Congregation: Don’t drive like my brother!
Leaders: Don’t drive like my brother!

Recessional: “Little Deuce Coupe” (Beach Boys)

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Sonar gives whales the bends

ResearchBlogging.orgThe New York Times Magazine has a cover article on human-whale interactions, with special attention to whales’ cognitive, communicative, and social abilities. It’s pretty neat stuff, and I started reading it with the intention of posting something about it with a title along the lines of “So long, and thanks for all the fish.” But, rather than all the whales-as-fellow-sentients stuff, this early aside about the effects of navigational sonar on whales is what actually caught my attention:

The results of the examinations performed on the [beached] Canary Islands whales, however, added a whole other, darker dimension to the whale-stranding mystery. In addition to bleeding around the whales’ brains and ears, scientists found lesions in their livers, lungs and kidneys, as well as nitrogen bubbles in their organs and tissue, all classic symptoms of a sickness that scientists had naturally assumed whales would be immune to: the bends.

That’s right — the bends. As in the harmful effects of a rapid decrease in atmospheric pressure associated with rising too rapidly from deep water to the surface, which can cause gasses dissolved in the bloodstream to come out of solution and form bubbles. Human divers take precautions to avoid “decompression sickness,” but it’s surprising to find that mammals who spend their lives underwater should run the same risk. The idea is that navigational sonar is so irritating or disorienting that it drives whales to the surface faster than is safe, and often kills them.

Naturally, I went to Google Scholar: although the original report of “gas-bubble lesions” in beached whales [$-a] generated some controversy [PDF] at first, the most recent review I could find seems to accept the diagnosis [PDF]:

Although no potential mechanisms can be eliminated at this stage, we highlight gas bubble formation mediated through a behavioural response as plausible and in need of intensive study.

The review cites a number of documented whale strandings closely associated with offshore naval maneuvers, and calls for, among other things, re-evaluating past records of strandings with an eye to whether sonar use may have been involved.

This, of course, is what prompted conservation groups to sue the U.S. Navy for investigation of the environmental impact of navigational sonar; the Supreme Court eventually ruled, regrettably, that the Navy’s need for sonar use in training exercises trumps the requirements of federal environmental law. I’d followed the story when it originally unfolded, but never really understood exactly how whales were hurt by sonar — I think I assumed they were just sort of driven onto beaches. So, my totally non-marine biologist, non-mammalogist reaction: wow. And, ugh.

References

Cox, T.M., T.J. Ragen, A.J. Read, E. Vos, R.W. Baird, K. Balcomb, J. Barlow, J. Caldwell, T. Cranford, & L. Crum (2005). Understanding the impacts of anthropogenic sound on beaked whales. J. Cetacean Res., 7 (3), 177-87

Jepson, P., Arbelo, M., Deaville, R., Patterson, I., Castro, P., Baker, J., Degollada, E., Ross, H., Herráez, P., Pocknell, A., Rodríguez, F., Howie, F., Espinosa, A., Reid, R., Jaber, J., Martin, V., Cunningham, A., & Fernández, A. (2003). Gas-bubble lesions in stranded cetaceans Nature, 425 (6958), 575-576 DOI: 10.1038/425575a

Piantadosi, C., & Thalmann, E. (2004). Pathology: Whales, sonar and decompression sickness Nature, 428 (6984) DOI: 10.1038/nature02527a

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Correlation and causation: Why are there so many flowering plants?

ResearchBlogging.orgAmong the flowering plants, groups with flowers adapted to a narrower range of pollinators — the more specialized ones, like orchids or mintstend to contain more species. Why? The classic hypothesis is that coevolution between plants and their pollinators leads to more pollinator-specialized plants, which are then more likely to become reproductively isolated, and eventually form separate species. However, I’ve just finished reading a review article that suggests an interesting alternative: that angiosperms may not be diverse because they’re specialized, but specialize because they’re diverse [$-a].

The review’s authors, Armbruster and Muchhala, first lay out a list of possible mechanisms connecting diversity and specialization. Three of them have specialization creating diversity, by (1) creating reproductive isolation, (2) enhancing isolation created by other forces, or (3) reducing extinction rates. Finally, there’s the possibility that diversity creates specialization, by (4) essentially forcing plants to divvy up the available pollinator community more and more finely.


Collinsia heterophylla, a
member of a plant genus
probably shaped by competition.

Photo by Ken-Ichi.

The first two mechanisms are, as far as I’m concerned, contained within the classic specialization-creates-diversity hypothesis classically advanced by Verne Grant, that increased floral specialization makes it easier to form new species [$-a]. The third is a bit odd — generally, ecologists think that increased specialization means an increased, not a decreased, risk of extinction [$-a]. It’s intuitive that if you rely on fewer pollinator species, you can afford to lose fewer of them, and you have fewer opportunities to colonize new sites; so on the one hand, you’re at greater risk of local extinction, and on the other, you have difficulty establishing new populations. However, as Armbruster and Muchhala point out, this process should make more-specialized plant groups less diverse, which is the opposite of what we see.

The fourth hypothesis, that competition for pollinators causes greater to create greater specialization, leads to predictions that nicely differentiate it from the classic hypothesis: that hybridization between related flowering plants should be rare, and that plants should rarely occur in the same community as their closest evolutionary relatives. The first is important because it gives a reason to specialize on one or a few available pollinators — if a plant can’t reproduce with nearby relatives, all the pollen it exchanges with them represents wasted effort, and may actually interfere with pollen transfer from members of its own species. The second is a consequence of that process; plants are most likely to be able to hybridize with their evolutionary sisters, so successful speciation will usually require geographic or ecological isolation.

The authors then evaluate the evidence for these predictions in four plant genera with which they have prior experience: Dalechampia, Collinsia (pictured above), Burmeistera, and Stylidium. For these four groups, they find good support for the diversity-causes-specialization hypothesis — few natural, or even artificial hybrids, and few co-occurring sister species. To some degree, then, the new hypothesis is an effect of a researcher’s favorite study systems influencing their perspective on the broader picture of evolution. Armbruster and Muchhala give the same treatment to orchids, and find that for the most diverse angiosperm family, natural hybrids and co-occuring sister species are not rare. This ambiguity makes the review more interesting — it overturns the causation commonly inferred from the correlation between diversity and specialization, but it doesn’t make the mistake of sweepingly assuming the opposite instead.

Correlation, and causation.
xkcd.

References

Armbruster, W., & Muchhala, N. (2008). Associations between floral specialization and species diversity: Cause, effect, or correlation? Evolutionary Ecology, 23 (1), 159-79 DOI: 10.1007/s10682-008-9259-z

V. Grant (1949). Pollination systems as isolating mechanisms in angiosperms. Evolution, 3, 82-97

Johnson, S.D., & Steiner, K.E. (2000). Generalization versus specialization in plant pollination systems Trends in Ecology & Evolution, 15 (4), 140-3 DOI: 10.1016/S0169-5347(99)01811-X

Sargent, R. (2004). Floral symmetry affects speciation rates in angiosperms Proc. R. Soc. B, 271 (1539), 603-608 DOI: 10.1098/rspb.2003.2644

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