Category: adaptation

Natural selection in real time: birds of a feather don’t evolve together

June 10, 2011 • 6:06 am

Want to see evolution in action—in “real time”? A new paper in Ecology by André Desrochers on songbirds (access free) might fill the bill. (Excuse the pun.)  It shows—I’d prefer to say “suggests strongly” since there are a few problems—that the shape of feathers in North American songbirds has evolved over the last century in response to changes in patterns of forestation.

Here’s the idea: in the last hundred years, North American forests have changed drastically.   The boreal (i.e. high-latitude subarctic) forests of eastern North America have been cut back heavily, replacing old coniferous stands with younger deciduous ones.  Temperate non-forest habitats have also become more fragmented.  Conversely, the temperate forests of eastern North American, severely deforested in the 19th century, have reversed this trend, undergoing “afforestation” in the 20th century.  Afforestation also characterizes boreal early-successional forest.   I can’t vouch for these generalizaitons as I’m not an ecologist, but the authors support them with references.

It’s also known that birds with “pointier” wings have more energy-efficient sustained flight, and that pointiness (we’ll define it below), can evolve rapidly in birds.  If your habitat becomes more fragmented, it would be advantageous to evolve pointier wings to travel more efficiently between distant foraging and resting places.  Conversely, if your habitat becomes less fragmented, you should lose those pointy wings, which impose energy costs in takeoff; and ounder wings are also better for foraging in thick vegetation or close to the ground.

From these observations Desrochers made the following hypothesis (from the paper):

I tested the following predictions: over the last century, species mostly found in boreal, mature, coniferous forests and temperate non-forest habitats evolved more pointed wings in response to increased fragmentation, whereas species associated with temperate mature forests and boreal early-successional forests evolved less pointed wings because of relaxed selection for mobility. Additionally, I examined whether the above predictions were better supported in nonmigratory species than in neotropical migrant species.

Migrants should show less changes since they have the constant selection (unchanged over the century) for having wings appropriate for their yearly long-distance round trips.

Desrochers measured 21 species of birds (average 40 specimens per species) collected between 1900 and 2008; all were from collections at Cornell University at the Canadian Museum of Nature.  This enabled him to test for any long-term changes feather shape that could reflect evolution.

How did he measure pointiness? Here’s a female scarlet tanager showing how the measure was made (on the right wing only):

(a) is the distance between the carpal joint of the right wing and the distal end of the outermost secondary feather.  (b) is the distance between the same joint and the wing tip.  “Pointiness” is the index 100 X (b – a)/a, in other words a measure of how much, relatively, the wing tips extend beyond the secondary feathers.  This is called the “primary projection” in bird argot.  Desrochers also took an unrelated measure (bill length) just to see if other morphological traits might also have changed, indicating perhaps other selective pressures besides flight.

The results?  Pretty convincing:

  • Of the 21 species (there were actually 22 sets of measurements, for the red-breasted nuthatch, Sitta canadensis, was measured from both boreal mature forest, expected to select for pointier wings, and temperate mature forest, selecting for rounder wings), nearly every one changed in the direction predicted from its habitat. Of 12 species from temperate mature forest and boreal open habitats, eleven “evolved” rounder wings, as expected.  Of the ten species in boreal mature forest and temperate open habitats, all ten evolved pointier wings over the last century—also as expected given the habitat fragmentation.  These directions of change alone, regardless of their magnitude, are statistically significant.  Desrochers’ analysis also shows that eleven of the 22 trends were also statistically significant unto themselves, though I actually count 12 from his table.  At any rate, this is a very strong confirmation of his hypothesis.
  • The one species tested in both types of habitats, the red-breasted nuthatch, showed divergent evolution, as expected, evolving pointier wings in boreal mature forest and rounder wings in temperate mature forest.
  • Migratory status wasn’t consistently correlated with evolution (we expect migrants to show less evolution), but it was in boreal forest birds, with pointiness increasing more in residents than in migrants.
  • There wasn’t anywhere near this degree of changes in beak shape, which changed in only five mature boreal species (getting longer); and that change was of borderline statistical significance.

So, is this a good case of evolution in real time–in only three human generations?  I think so, but there are a few problems.  The most significant to me—and this is always the first thing that strikes me as a geneticist—is that there is no evidence that this change over time rests on changes in the frequencies of the birds’ genes.  Many ornithologists (and ecologists) often assume that if they see an animal change size or shape over a few generations, that change must automatically be genetic, and therefore the result of evolution via either natural selection or genetic drift.  But of course the change could be purely “developmental” or “phenotypic,” reflecting not genetic change but a purely developmental response to some unknown environmental change.

That’s not pure speculation, for there are plenty of examples.  The average height of Japanese, for example, has increased dramatically relative to Americans in the last generation.  Perhaps a Martian zoologist would, like some ecologists, attribute this change remarkably rapid evolution of increased height in the Japanese, probably due to natural selection.  But that’s wrong.  The height increase is not based on genes—it couldn’t be, for it’s happened way too fast. It resulted purely from an environmental change: the improved diet of the Japanese after the Second World War, which made them grow larger.

Many animals and plants have the ability to change their body shapes and appearances due to environmental circumstances (flamingos, remember, only become pink if they eat crustaceans and algae, incorporating the carotenoid pigments into their feathers).  I could point out other examples of ecologists making this fallacious “it’s all genetic” assumption, but I don’t want to embarrass my colleagues. Suffice it to say that without stronger evidence, seeing a trait change over generations leaves the question open if it really is genetic evolution.  The way to test this, I suppose, would be to release banded birds from single broods into diverse forest habitats, and see if living in those different environments could change the pointiness of their wings.

Desrochers tries to explain away this problem by invoking the concept of “heritability”: that is, the degree to which variation in a trait can be transmitted faithfully from parent to offspring within one population:

A second alternative explanation is that changes in primary projection may simply reflect phenotypic, as opposed to genetic, change (Gienapp et al. 2008). However, body measurements are highly heritable, with narrow-sense heritability (h2) generally between 0.6–0.7 in the case of wing length. . .

But this appeal to heritability is completely wrong, as has been pointed out for decades by the likes of Steve Gould, Richard Lewontin, and many other geneticists.  Just because a trait can be heritable within a population living in one environment (that is, a proportion of the variation in that population rests on variation in genes) says absolutely nothing about whether the difference in a trait among populations living in different environments (like Desrocher’s birds) has a genetic basis. The heritability of height is substantial in the population of North American humans, but one could not have used that to say that the difference in height between pre-war Japanese and Americans must have been largely genetic.  There was an important environmental difference there, too: diet.  All geneticists know that measurements of a trait’s heritability are confined to a single population in a single environment, and cannot be used to say anything about the genetic basis of differences in that trait between different populations in different environments. (This, of course, is the whole basis for the blow-up about differences in IQ between human “races,” who may inhabit different cultural and educational environments.)  Maybe, then, the differences in wing pointiness reflect some environmental modification of bird wings produced in different types of habitat.

Anyway, let me cease this rant and just let it serve as a lesson to ecologists to avoid assuming that changes over time are automatically genetic (or evolutionary)—and to not buttress this conclusion by specious appeals to “heritability.”

To my mind, that’s the biggest problem with this paper.  Desrochers mentions a few others—changes in food type, for example—but those seem unlikely based on the lack of changes of bill configuration.

I probably have been too hard on Desrochers.  To be fair, I think that he really has shown evolutionary changes in bird feathers in the predicted directions.  It is my gut feeling (nothing more) that there are probably not many environmental factors that could change feather pointiness, and so this could be genuine evolutionary change in a short period. In that case, it really would be a kind of landmark study—worthy of inclusion in textbooks along with the Grants’ work on Darwin’s finches.  But oh, how much stronger it would have been with some genetic data! (The Grants did, by the way, have that genetic data!).  I suppose I’m a bit peeved that elementary considerations of population genetics are being swept aside (or misused, in the case of heritability).  Nevertheless, I greatly admire Desrochers’ paper, and really hope he has some other evidence that wing pointiness cannot easily be changed by environmental factors alone.

I leave you with my admonition to ecologists:  DO NOT ASSUME THAT DIFFERENCES IN A TRAIT BETWEEN CURRENT POPULATIONS, OR BETWEEN POPULATIONS OVER TIME, REFLECT EVOLUTIONARY CHANGE UNLESS YOU GIVE SOME EVIDENCE THAT THOSE DIFFERENCES ARE BASED ON DIFFERENCES IN GENES.

________________

Desrochers, A. 2011.  Morphological response of songbirds to 100 years of landscape change in North America. Ecology 91:1577-1582.

h/t: Birds and Science, via Matthew Cobb

The Sphenisciform Shuffle: how penguins keep warm.

June 6, 2011 • 5:41 am

Read with me, if you will, the opening paragraphs of a swell new paper in PLoS One (access free; reference at bottom) about how emperor penguins, Aptenodytes forsteri, keep warm in the Antarctic winters.  Just by filming the birds for 4 hours on a single day (August 3, 2008), the four researchers found an amazing group behavior.  (I see no sense in rewriting the authors’ perfectly clear scientific prose):

Emperor penguins are the only vertebrates that breed during the austral winter where they have to endure temperatures below -45o C and winds of up to 50 m/s while fasting. From their arrival at the colony until the eggs hatch and the return of their mates, the males, who solely incubate the eggs, fast for about 110–120 days. To conserve energy and to maintain their body temperature, the penguins aggregate in huddles where ambient temperatures are above 0o C and can reach up to 37o C.

(37o C is, of course, human body temperature, so it’s nice and warm in the groups.)

Each colony consists of a group of huddles, and in each one the penguins are tightly packed, and all facing in the same direction. The density of penguins can reach—get this—21 animals per square meter! (And these are not small birds: they weigh between 50 and 100 pounds and can be up to 4 feet tall.)  The picture below shows several huddles within a larger group:

Several emperor penguin huddles. Photo by Robyn Mundy.

More from the paper:

Huddling poses an interesting physical problem. If the huddle density is too low, the penguins lose too much energy. If the huddle density is too high, internal rearrangement becomes impossible, and peripheral penguins are prevented to reach the warmer huddle center. This problem is reminiscent of colloidal jamming during a fluid-to-solid transition. In this paper we show that Emperor penguins prevent jamming by a recurring short-term coordination of their movements.

The authors filmed the penguins on a single day, using time-lapse photography at a rate of one image every 1.3 seconds.  As the figure from the paper (below) shows, individuals were tracked using the characteristic yellow and white face patch of the breed. Note that the males in these huddles are incubating eggs (nearly all of them had one), and when they move they do so by waddling with the egg balanced on their feet.

Here’s the amazing thing the authors found: penguins keep the huddle “fair”, and move from the periphery to the interior (and vice versa), by episodic but coordinated waves of penguin shuffling:

The jammed state of the huddle is interrupted every 30–60 s by small 5–10 cm steps of the penguins (Fig 1C,1E, Movie S2, S3), reminiscent of a temporary fluidization. These steps are also spatially coordinated and travel as a directed wave with a speed of about 12 cm/s through the entire huddle (Fig. 1E). After the wave has reached the end of it, the huddle re-enters the jammed state. Interestingly the propagation speed of the traveling wave is comparable to the speed of the individual penguins during the step. This is analogous to the propagation of sound waves in an elastic entropic medium (gas or fluid) where typical molecular velocities are comparable to the velocity of pressure waves.

You will of course want to see what this looks like.  The links below go to the three movies from the paper (each between 1.5 and 4 minutes long), along with the descriptions.  DO NOT MISS THESE STUNNING MOVIES.

Movie S1. Huddle formation and occurrence of coordinated traveling waves. Time lapse recordings (full field of view) over 2 h (resolution reduced from 10 MP to 480 p), showing about half of the penguin colony during the aggregation and huddling process. At the beginning of the movie (~12 p.m. with temperatures above −35°C), only few penguins aggregated in smaller huddles. As the temperatures gradually fell, larger and more stable huddles formed until nearly all the penguins aggregated in one large huddle.

To see the Sphenisciform Shuffle in the next two videos, keep your eye on the penguins’ white face patches. You’ll see them advancing in a jerky but coordinated way as the penguins step forward.

Movie S2. Huddle formation and occurrence of coordinated traveling waves (detail). Time-lapse recordings (detail of S2 over 1 h) showing multiple huddles. The penguins in a huddle mostly face in the same direction which defines a rear end and a front end of the huddle. When a penguin joins the huddle, it does so by aligning itself first in the direction in which the other penguins are facing, and then moving closer to the huddle. As a result, penguins tend to join a huddle at its rear (trailing) end and leave it at the front (leading) end. During the periodic traveling wave, the huddles move in the forward direction (in the direction in which the majority of the penguins are facing).

The shuffling is most evident in this video:

Movie S3. Coordinated traveling waves in a densely packed huddle. 21 min sequence from S2 (detail corresponding to Fig. 1B) at reduced speed. The movie shows the travelling wave of small steps every 30–60 sec.

The authors show that this type of movement is not unique to penguins, but has been seen in locusts and fish schools, as well as in tissue-cultured cells. They also show that it resembles “fluid-to-solid gelation of short-ranged attractive colloids.”  But returning to the biology, the behavior raises two interesting questions:

  • Is this behavior evolved or learned, or a combination of the two?  Natural selection could of course favor this “altruistic” behavior since it’s to each penguin’s advantage to participate in a shuffle.  The time you lose being cold on the periphery is more than compensated for by the larger amount of time you spend inside the warm huddle.  (The behavior is not pure altruism, of course, for individuals gain rather than lose fitness by participating in the shuffle).  But what about cheaters, who don’t move along, or didn’t when the behavior evolved? They would benefit by never having to be on the periphery, but they could of course have been punished for such cheating by the other penguins. It would be very hard to test whether this behavior was hard-wired, since it would involve creating large huddles of naive, hand-reared penguins under artificial conditions, and then subjecting them to an artificial winter.
  • Mechanically, how does it work?  The authors note, “It is also unclear whether the traveling wave in a huddle is triggered by a single or few leading penguins and follows a well-defined hierarchy among group members, similar to the collective behavior in pigeon flocks. Modeling attempts with self-driven agents have explained collective behavior such as temporal and long-range spatial synchronization in bird flocks, fish schools or traffic congestion by evolutionary strategies and a small set of simple interaction rules between neighboring agents. Similar mechanisms may also apply to the collective behavior of penguins in a huddle.

You don’t need fancy machinery or DNA sequencers to discover amazing things about our world.  This senational behavior was revealed by four researchers armed only with a question and a video camera.

________

Zitterbart D.P., B. Wienecke, J. P. Butler JP, and B. Fabry. 2011. Coordinated movements prevent jamming in an emperor penguin huddle. PLoS ONE 6(6): e20260. doi:10.1371/journal.pone.0020260

Do sloths dump in the trees?

May 23, 2011 • 10:23 am

The answer is no.  This delicate issue arose in our video and subsequent discussion of sloths:  the beast has the odd habit of defecating on the ground, at the base of the tree it inhabits. That means a looooong, slow climb down from the branches.  And, of course, you’ve seen from that video how awkward sloths are on the ground, so the descent exposes it to all sorts of predatory dangers.  Fortunately, sloths digest their food (leaves) as slowly as they move, so they have to make the round-trip only about once a week.

Here’s an Attenborough video showing sloths at the loo, with His Holiness noting that the behavior is a complete mystery:

Given the difficulty and time involved in the trip, and the helplessness of sloths on the ground, why do they do it? By “why”, of course, I mean what were the advantages of any genes that produced this behavior?  I’m assuming here that this behavior is genetically based rather than simply learned, which seems a reasonable supposition.  I can think of four evolutionary explanations:

  1. It reduces predation from above.  Eagles and other aerial predators are said to detect the sound or appearance of droppings, using them to cue in on the sloth as prey.  By hiding its toilet at the base of the tree, it makes itself less liable to aerial attack.  I don’t find this very plausible given that aerial predators hunt visually, and I don’t see how they could detect a sloth more readily when it’s defecating from a branch.  Indeed, it seems like it would be more likely to detect a sloth climbing down the tree.
  2. It reduces predation from below.  This seems more likely to me than alternative #1.  Terrestrial predators like jaguars can hear droppings striking the forest floor, and seem likely to be able to associate them with prey above.  Cats, for example, are known to climb trees to take sloths (if you’re not squeamish you can see a video of that here.)  On the other hand, couldn’t a cat smell sloth droppings and use them as a way to hunt?
  3. It fertilizes the sloth’s tree.  I’ve heard this bandied about, but it doesn’t seem credible.  Sloths hang out in big trees, and I’m not sure that a sloth with genes to move to the base and deposit fertilizer would really gain a reproductive advantage.  That presumes that that such fertilizing would make such a substantial difference in the tree’s output of leaves that the sloth would wind up better fed and have more offspring.  In addition, I’m not sure (though perhaps a reader can tell me) whether sloths remain in the same tree for years, as is required by this hypothesis.
  4. It’s a way to attract mates.  Creating your own personal dung pile may be the equivalent of expelling pheromones, alerting sloths of the opposite sex that you’re up above.  I know nothing about sloth mating, but given their lassitude and site-fidelity, surely locating a mate—the most important behavior in outcrossing species—is subject to strong selection.  How do you find another sloth two trees over? By sniffing the base of the tree.

The last explanation seems the best to me, though of course the reproductive advantage of sloth toiletry could have involved more than one of these factors.  And in principle these theories are testable.  We could see, for example, how sloths manage to find each other at mating time.  We could also do mock-defecation studies from branches, using model sloths, to see if the noise attracts predators.

I’m pretty sure, however, that nobody is doing these studies . .

Feel free to offer your own explanations: it’s sloth evolutionary psychology!

David Brooks and the evolution of human altruism

May 18, 2011 • 10:05 am

David Brooks, New York Times columnist and author of The Social Animal, has never met an evolutionary psychology argument he didn’t like.  I haven’t read his book, but I did read a long excerpt in The New Yorker and found it credulous, tedious, and lame.  P.Z. Myers, who reviewed the book, had the same opinion.  So did philosopher Thomas Nagel, who, reviewing the book in the New York Times, pretty much ripped it apart, noting that “Brooks seems willing to take seriously any claim by a cognitive scientist, however idiotic. . ” (It’s quite unusual for the Times to publish bad reviews of books by their own columnists.)

In yesterday’s New York Times, Brooks writes about recent scientific “advances” in the understanding of human altruism.  And he signs on to the idea that altruism evolved by group selection.

I disagree, and see Brooks as ignorant about the true scientific issues.  If true altruism (which I define here) is indeed a trait that’s deleterious to an individual’s reproductive fitness, then it could, as Brooks envisions, evolve only by the differential survival and reproduction of groups.

That form of evolution would work like this: although genes for altruistic behavior would be constantly weeded out of populations (for altruists, by definition, sacrifice their own genetic heritage for others), those genes might survive if groups that contained higher proportions of altruists were the groups that persisted, giving rise to descendant groups more often than groups lacking altruists.  (The idea here is that groups without altruists wouldn’t flourish very well.)  That’s group selection, and it’s how Brooks sees altruism as evolving:

In his book, “The Righteous Mind,” to be published early next year, Jonathan Haidt joins Edward O. Wilson, David Sloan Wilson, and others who argue that natural selection takes place not only when individuals compete with other individuals, but also when groups compete with other groups. Both competitions are examples of the survival of the fittest, but when groups compete, it’s the cohesive, cooperative, internally altruistic groups that win and pass on their genes. The idea of “group selection” was heresy a few years ago, but there is momentum behind it now.

Let’s be clear about what biologists really know about group selection and altruism.  If true human altruism has a genetic basis, it is individually disadvantageous and could have evolved only by differential propagation of groups. That’s very unlikely, since it requires that the rate at which altruist-containing groups reproduce themselves must be high enough to counteract the substantial rate at which altruism genes disappear within groups.  It’s unlikely because groups reproduce much less often than do individuals!  Further, once a group consists entirely of altruists, any non-altruistic genes would rapidly invade it, as their carriers reap the benefits of altruism without sacrificing their reproduction.

Now if we’re talking about apparent altruism, in which individuals appear to sacrifice their reproductive interests but actually reap hidden genetic benefits, then we don’t need group selection to explain it.  As I’ve written in a longer post on this topic, kin selection (“inclusive fitness”) can do it, as can simple individual selection based on reciprocity or, simply. selection for the advantages of cooperation, as in hunting lions.

Humans, after all, evolved in small social groups, which provide the ideal environment for the evolution of “reciprocal altruism” (“I scratch your back and you scratch mine”).  That kind of altruism, which isn’t “true” altruism in the sense of hurting one’s reproductive prospects, evolves most readily in small groups where individuals know and recognize each other, and have a big brain for remembering and reciprocating good deeds.  Living in groups, particularly of kin, facilitates the evolution of apparent altruism, but that is not group selection since it doesn’t require differential propagation of groups.  Genes that are selected in groups based on relatedness or individual advantage will spread throughout a species without requiring differential reproduction of groups.  That is, selection occurs in the context of groups, but doesn’t occur through selection among groups.

What do we know about human altruism? First of all, we don’t know whether true altruism, in which individuals behave in ways that help others by hurting their own reproductive prospects (firemen are one example), has any genetic basis in human society.  True altruism like that isn’t known in any other species, and I suspect that, to the extent it occurs in ours, it’s an epiphenomenon: a byproduct of our general social cooperativeness.  As far as whether we are genetically cooperative (rather than truly altruistic), that seems quite likely, but it doesn’t require group selection.  It requires selection that occurred in groups, which is different.  And we almost certainly have some behaviors that evolved by kin selection, parental care being the most obvious.

So Brooks misrepresents the views of biologists in his piece.  There really isn’t much momentum in the evolution community behind the idea of “group selection.” There is increasing realization that selection can occur in groups, that being in groups can affect how selection operates on genes, and that there can be group effects (“multilevel selection”) that influence the evolution of genes.  But there is no general feeling that “group selection” is widespread or important.  And there is no widespread agreement that true altruism, or even apparent altruism, evolved by the differential propagation of groups.

In short, we know nothing about the evolution of true human altruism except that it probably didn’t evolve.  And we don’t know much more about the evolution of human cooperation.  It almost certainly has a genetic basis—we’re social animals, after all—but we’re ignorant about the form of natural selection that favored such cooperation, and about the social and environmental circumstances that promoted that selection.

Brooks makes one more biological error, asserting that the evolution of cooperation necessarily entails evolved morality.

But the big upshot is this: For decades, people tried to devise a rigorous “scientific” system to analyze behavior that would be divorced from morality. But if cooperation permeates our nature, then so does morality, and there is no escaping ethics, emotion and religion in our quest to understand who we are and how we got this way.

Cooperation also permeates the nature of honeybees, termites, naked mole rats, and lions, but they don’t have morality.  Morality is the result of having a big brain that, in a social species, can remember other individuals and make calculations about their intentions.  Whether that “result” is genetic, so that our moral feelings are encoded in our DNA, or simply an epiphenomenon, in which we’re taught rules that enable us to function, is an open question. I suspect that some of it is genetic, but we just don’t know.

Myrmecomorphs

May 12, 2011 • 7:16 am

Ants are one of the most abundant groups on earth, but, curiously, not a lot of things eat them.  Yes, there are anteaters (who also eat a lot of termites), and some lizards specialize on ants, but the little critters are full of noxious chemicals and pheromones that put them way down on the list of predators’ preferred foodstuffs.

Because of this, many other insects and arthropods have evolved to mimic ants, taking advantage of the aversion of predators to anything antlike.  These mimics are called myrmecomorphs, and they’re the subject of a really nice eponymous feature in this week’s Current Biology  (access is free, too).  The name comes from the Greek “myrmecos”, for ant, and “morph” for form. The authors are Florian Maderspacher, the journal’s senior reviews editor, and Marcus Stensmyr, a researcher at the Max Planck Institute for Chemical Ecology at Jena.

I won’t summarize the text, which talks about the history of work on these beasts; you should read that for yourself.  But I do want to show some of the amazing photographs of ant mimics.

When a perfectly edible species evolves to resemble a noxious one that is avoided by predators, thereby gaining protection from being eaten, it’s called Batesian mimicry, after the English naturalist and explorer Henry Walter Bates, who described the phenomenon.

We’ve read a lot lately about the amazing shapes of treehoppers (membracids). Here are some photographs of  the treehopper Cyphonia clavata, whose helmet (pronotum) has evolved to resemble an ant.

The picture below shows the hopper with a sympatric (living in the same place) noxious ant, Cephalotes atratus.

As the authors note:

Notably, the ant-mimicking structure seems to be inverted, with the imitated head facing towards the back of the treehopper. That way, as the treehopper moves forward, it probably creates a rather good impression of a reversing and agitated ant in erect defensive posture, deterring any would-be predators. To complete the illusion, the terminal segments of the treehopper’s hindlegs, coloured like the ‘ant’, most likely serve as the ‘ant’s’ forelegs, which provides the static protrusion with the illusion of movement. Too bad our specimen was dead.

Of course, for this mimicry to evolve (and work), the noxious ant “model” and its edible mimic have to live in the same area, and be encountered by the same potential predators.

Some mimics imitate the ants only during part of the life cycle.  Here’s a nymph of the Texas bow-legged bug (a true bug), Hyalymenus tarsatus (left) imitating an ant of the genus Ectatomma (right).

Some of the most remarkable cases of ant mimicry involve spiders. To pull off the trick, the spiders have to make their extra pair of legs look like antennae, put a constriction in their cephalothorax to resemble the separate head and thorax of ants, thin out their body, and, often, evolve fake eyespots to look like the large eyes of ants.  Here’s a spider Sphecotypus niger (left) looking like the ant Pachychondela villosa (right), which the authors describe as a an “aggressive and predatory ant.” Note how the spider extends its first pair of legs forward to look like antennae:

There are several types of ant mimicry.  Besides Batesian mimicry, we have “aggressive mimicry,” in which an animal will evolve to resemble another animal so that it can deceive it into thinking it’s one of its fellows, who then unwittingly allows it to approach.  (There are other types of aggressive mimicry as well: some mantids imitate orchids, hanging from trees and waiting to eat the hapless insects who come to pollinate it.)

Here’s a remarkable case of aggressive ant mimicy.  The animal on the left is actually the crab spider Aphantochilus rogersi, which resembles ants of the genus Cephalotes (right). It’s hard to tell them apart!  The spider’s head is biting the ant’s neck, so the ant’s head is bent down. It’s a goner.

This type of mimicry implies that the “model” ants must have pretty good vision, for otherwise there would be no selection on the spider to resemble an ant so closely.  And indeed, ants of this group do see pretty well: you can see that its eyes are quite large.  From this you’d predict that mymecomorphs who prey on ants that don’t see very well might be less perfect mimics.

I love cases of mimicry, for they truly show the power of natural selection.  The degree to which mimics resemble models—and it’s often spot-on—shows that there is lots of genetic variation in the model that can be used by natural selection, and that the selection is strong enough to affect many features of the mimic. It’s one of the few cases—sex ratio is another—in which biologists know a priori what the optimum result of selection should be, and how closely selection can achieve that target. As you see from the photos above, it comes damn close!

The power of selection acting on pervasive genetic variation is, of course, also responsible for the power of artificial selection, something that Darwin highlighted in The Origin:

“Breeders frequently speak of an animal’s organization as something plastic, which they can model almost as they please.”

I won’t use the word “spiritual” to describe my feelings when I see the remarkable forms that have resulted from blind, materialistic processes acting on DNA molecules, but they certainly evoke considerable wonder.

The strange origin of the treehopper “helmet”

May 5, 2011 • 5:38 am

Last November I posted pictures of some bizarre treehoppers (membracids, a family in the order of true bugs, Hemiptera) that had strange structures branching from the top of their bodies. Because these insects were so incredibly weird, it proved to be the most linked-to post I’ve ever written.  At any rate, go back for a minute and look at some of those things.   And here are some more, taken from a new paper in Nature by Benjamin Prud’homme et al. (click to enlarge):

We’ve seen the one at the upper left before: it’s Bocydium globulare.  The function of these “helmets” isn’t always clear: some of them may deter predators (e.g. left images, rows 2 and 3), others may serve as camouflage (last row, left and middle), and still others may mimic distasteful or dangerous insects like ants (lower right).  Regardless of their function, the Prud’homme paper has made a significant contribution to understanding where these “helmets” come from.

They are highly modified homologs of the insect’s wings: an ancient winglike structure that was long repressed but appeared again to take on a variety of new functions as a helmet.

The “helmet” appears on the first segment of the insect’s thorax (“T1”), and is the only known dorsal appendage on that segment in insects (wings of modern insects are always on the last two segments, T2 and T3).  Prud’homme et al. suggest, with good evidence, that the helmet is a pair of fused wing primordia: remnants of the early structures that gave rise to modern wings.

How do we know these are wing homologs?  Prud’homme gives several lines of evidence:

  • The helmet is not simply an outgrowth of the thorax, but is connected to it, as are the wings and other appendages, by a “complex articulation”: a flexible joint.   Here’s a cross section of a dissected treehopper: the joint between thorax and helmet (top, red box) is similar to that between the wings and thorax (bottom, blue box):
  • The development of the helmet is similar to that of the wing:  both “unfold” on emergence (many times I’ve watched the wings of newly eclosed flies unfurl from nubbins to full wings, a remarkable process that takes only a minute or two).
  • There are other morphological similarities between wings and helmets:  both consist of two layers of cells connected by columns, and both are suffused with a complex network of veins.
  • The genes that are expressed in the developing helmet are the same as crucial genes expressed in the developing wing, including Nubbin, Distal-less, and homothorax.

If the helmet is a re-expression of a repressed wing structure, does that mean that early insects had more than two wings? Well, even some modern insects have more than two wings: dragonflies and bees, for example.  But these wings are always, as I mentioned, on the second and third thoracic segment.  We know that insect flight evolved about 350 million years ago (insects are the only flying invertebrates), but fossils are scanty. One theory, espoused by Jarmilla Kukalova-Peck and supported by more recent genetic work, suggests that wings evolved from gill plates in early insects (I’ve written before about the likelihood that the ancestors of modern insects were closely associated with water).  In one of her papers, Kukalova-Peck gives a reconstruction of a Paleozoic mayfly nymph (an aquatic life stage), showing winglike structures—gill plates—on every segment of the thorax and abdomen:

From Kukalova-Peck, J., J. Morphology 156:53-125 (1978)

Another theory is that wings arose from branches of the ancestral insect limb.   Both theories, though, posit that there were more than one or two ancestral structures that were winglike, and that the evolution of the two or four wings in modern insects involved genetic repression of these ancestral features.  The gene Scr (sex combs reduced) seems to be involved in this repression: when it’s inactivated in some insects, extra wing primordia form on T1.

The authors thus posited that somehow, in the last 100 million years (the time when membracids arose), the Scr gene lost its ability to repress wing promordia in the membracid lineage, allowing the helmet to evolve.  To test this, they actually inserted the membracid Scr gene into Drosophila (which has a normal, “repressive” Scr), expecting that perhaps wing primordia would then arise on the first segment of the fly thorax. They didn’t.  They thus suggest that other genes—genes normally repressed by Scr in membracids—have lost their ability to be repressed, and these genes are involved in making the helmet.

Here are the authors’ conclusions from the paper. Even if you’re not a biologist you should be able to understand them, for if you haven’t, I have not written clearly enough!

Our results show that treehoppers have evolved a T1 dorsal appendage, thereby departing from the typical winged-insect body plan, by expressing a developmental potential that had beenmaintained under the repression of a Hox gene for 250Myr. This argues that the constraint preventing extra dorsal appendage formation in insects is not developmental but rather selective. We submit that morphological innovations can arise from the deployment of existing but silenced developmental potentials, therefore requiring not so much the evolution of new genetic material but instead the expression of these potentials.

The breadth of morphological diversity in helmets that has evolved in less than 40 Myr (ref. 27 and C. Dietrich, personal communication) is unusual for an appendage. The pace of appendage evolution is generally slow, probably because of the strong selective pressure associated with their role in locomotion. This is particularly true for the wings, and we speculate that, initially alleviated from functional requirements, the recently evolved helmet was free to explore the morphological space through changes in its developmental program.

As your reward for reading this far, here’s another really weird membracid (Heteronotus sp., from Ecuador), photographed by Alex Wild and taken from his wonderful website, Myrmecos.  You can see both the helmet and the wings.

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Prud’homme, B. et al. 2011.  Body plan innovations in treehoppers through the evolution of an extra wing-like appendage.  Nature 473: 83-86.

Monster flower blooming in Switzerland

April 25, 2011 • 4:45 am

Several readers have informed me of a rare botanical event happening in Basel, Switzerland.  You should go immediately to have a gander at a very rare—and ephemeral—flower shown on a webcam at the University of Basel.

It’s an Amorphophallus titanum from Sumatra, loosely translated as “giant misshapen penis,” a concupiscent but accurate descriptor.  Isn’t it lovely?

This species has the world’s tallest flower structure, reaching up to 3 meters.  It flowers only very rarely, though, so this event in Switzerland is a must-see.

Unfortunately, like the world’s largest flower, Rafflesia arnoldii, it smells like a decaying corpse.  That’s to attract flies and beetles which, thinking it’s a dead mammal, come to feed—and pollinate it as a byproduct.

The flower remains open for only a day or two, though it takes several weeks to grow. The Basel site gives time-lapse photographs, showing the flower beginning to appear at the end of March.

You can see other pictures of a 2009 bloom here, from which these pictures were taken.