“Reinforcement” and the origin of species

December 8, 2010 • 8:13 am

The conventional definition of “a species” amongst evolutionary biologists is “a group of organisms whose members interbreed among themselves, but are separated from other groups by genetically-based barriers to gene flow.”  Under this view, the origin of a new species is the origin of those reproductive isolating barriers that keep a population distinct from other groups.

Genetic barriers aren’t thought to arise for the purpose of keeping species distinct.  Rather, they are usually thought to be evolutionary accidents:  geographically isolated populations diverge genetically under natural selection or other evolutionary forces like genetic drift, and that divergence leads to the evolution of genetic barriers (mate discrimination, the sterility of hybrids, ecological differences, etc.) as byproducts of evolutionary change.  For example, populations could adapt to different environments (one dry, one wet, for example), leading to them becoming genetically different.  When these populations meet each other again, this genetic divergence could result in hybrids that don’t develop properly because the parental genomes are sufficiently diverged that they can’t cooperate in building a single individual.

Under some conditions, however, natural selection might directly favor increasing the genetic barriers between newly-forming species. One of these processes is called “reinforcement,” and it works like this.  Suppose two populations have begun to differentiate when they are geographically isolated.  They differentiate to the point that there are some problems with the hybrids: hybrids might be partly sterile, for example, or only partly viable. Because these problems might not completely block gene flow (say, only 50% of the hybrids are sterile), the populations aren’t yet regarded as having become completely different species.

But suppose these isolated populations come back into contact with one another.  Individuals who mate with members of the “wrong” population produce some maladaptive hybrids.  Any individual that could discriminate, and mate only with members of its own population, would leave more copies of its genes than individuals who mate wrongly.

Under these conditions, natural selection could favor the evolution of mate discrimination, promoting those adaptations that allow you to selectively mate only with others of your type.  In this way genetic barriers could arise as the direct object of natural selection, and speciation might be completed.  This process—the evolution of reproductive barriers to prevent maldaptive hybridization between two populations that attained secondary contact—is called reinforcement.

Reinforcement was once a popular idea in evolution, for it gave natural selection a way to finish off the speciation process.  But does it work?  One problem is that if two populations come back together again, and can interbreed to some extent, then the evolution of high genetic barriers will be countered by the fact that hybrids keep forming, driving the populations to fuse at the same time selection “wants” them to separate.  Which will win?  This depends on the balance between selection, hybridization, and also migration of individuals into the “contact zone” from outside.  While there is some evidence of reinforcement in nature—seen in patterns of higher mate discrimination between species in areas where their ranges overlap than elsewhere—there hasn’t been much evidence from the lab that natural selection can increase barriers between “incipient” species that are allowed to hybridize.

In a new paper in Current Biology, my hotshot student Daniel Matute modeled the evolution of reinforcement in two species of Drosophila, and found that reproductive barriers could indeed arise—and arise very quickly—when populations were forced to coexist and hybridize, even if migration were allowed from the outside.

He used two of the groups we work on: Drosophila santomea and D. yakuba, two closely related species that coexist on the African island of São Tomé.  While these are designated as different species, they can still hybridize in the lab, and half of the hybrids (the females) are fertile, so gene exchange is possible between them.  (They also hybridize a bit in the wild where their ranges overlap on the island.)  But because there is a penalty associated with hybridization (half of hybrid offspring—all the males—are sterile), natural selection might be able to increase their genetic isolation if the species were forced to coexist.

Daniel produced this coexistence in the laboratory, forcing the species to cohabit in bottles where they could mate either with their own kind or with the other.  Further, he allowed different amounts of this hybridization by removing different numbers of the hybrids (easily distinguished by their intermediate pigmentation) from the bottles each generation.  Finally, he allowed different amounts of “migration” from the outside by introducing different numbers of flies into the “mixed” bottles.  This corresponds to individuals moving into an area of geographic overlap from the outside, a factor that works to overcome reinforcement.

What he found is that, under many conditions, reinforcement did work: the species became more genetically isolated when forced to coexist.   And this evolutionary change happened quickly: within five to ten generations (a generation in the lab is about two weeks).  Two types of barriers were strengthened:  sexual isolation (the species forced to coexist became less willing to mate with each other) and gametic isolation (females who mated with the “wrong” males evolved the ability to get rid of the foreign sperm more quickly, giving them a chance to mate with the “right” males again).  Predictably, when hybridization was too strong, or migration from the outside too pervasive, these forms of reinforcement did not evolve.  But what is surprising is that under “reasonable” levels of hybridization—meaning conditions likely to be met in wild populations—reinforcement evolved fairly quickly.

The upshot is that these experiments establish reinforcement as a viable process that can “polish off” speciation in the wild.  And I should add that in populations of these species on São Tomé, reproductive isolation is indeed higher between populations taken from areas where they coexist than from areas where the species live separately, so perhaps reinforcement in nature explains this.

Curiously, though, the “reinforcement” seen in the wild applies to gametic isolation but not sexual isolation.  While sexual isolation (mate discrimination) quickly became stronger in forcibly-coexisting lab populations, it’s no stronger in nature in areas where the species coexist than elsewhere.  It’s a mystery to us why both forms of isolation evolve so quickly in the lab but only one is seen in co-occurring populations in nature.

____________

Matute, D. R. 2010. Reinforcement can overcome gene flow during speciation in Drosophila. Curr. Biol. 20:doi:10.1016/j.cub.2010.11.036.

Dawkins’s “hero of 2010”

December 7, 2010 • 11:19 am

It’s Christopher Hitchens.

I must say that if I had a mortal illness, I wouldn’t have the intestinal fortitude—much less the mental concentration—to engage in public debates on matters of faith and reason.  The man is amazing.
Hitchens’ latest piece from Slate: “Turn yourself in, Julian Assange: the WikiLeaks founder is an unscrupulous megalmomaniac with a political agenda.”

Cat contest update

December 6, 2010 • 4:31 pm

We have a grand prize winner and two runners-up, all of them awesome.  I’ll announce them all this Caturday: runners-up in the a.m., first place in the afternoon.

I’m glad I didn’t have to judge: this was a tough call.  There were terrific photographs and lovely stories—some of them hilarious, others heartbreaking.

Even if your cat didn’t win a prize, there’s a good chance it will appear on a Caturday to come.  There’s no question but that many entries deserve a post.

kthxbye

Some frogs from Colombia, including the world’s most poisonous vertebrate

December 6, 2010 • 8:52 am

Here are some lovely frogs I photographed at the Universidad de los Andes in Bogotá.  Click photos to enlarge (you can do this twice in succession).

Hemiphractus fasciatus (you can see the same frog’s remarkable gape here):

Phyllomedusa venusta. This frog becomes more camouflaged in the second picture, when it’s sitting amidst leaves:

Some of the gorgeous jeweled “harlequin frogs”.  Phyllobates bicolor:

Phyllobates aurotaenia:

Phyllobates terribilis, the famous “golden poison frog”:

This frog is the world’s most poisonous vertebrate.  It has in its skin lethal quantities of alkaloid toxins (obtained from its diet) that are used purely as defense against predators.  Indigenous peoples make poison darts by heating the frogs in a fire and dipping points into the exudate.

Amphiweb notes:

The combination of batrachotoxin and homobatrachotoxin is produced in quantities up to 1900 micrograms per frog, which is at least 20-fold more than other toxic species in the family Dendrobatidae. The range of batrachotoxin-homobatrachotoxin produced by individual frogs was 700-1900 micrograms, with an average of 1100 micrograms per frog. The lethal dose of batrachotoxin-homobatrachotoxin for a 20 gram white laboratory mouse is .05 micrograms when injected subcutaneously. Thus one P. terribilis frog skin contains enough toxin to kill about 22,000 mice. The lethal dose of batrachotoxin for humans is not known but has been estimated at 200 micrograms, with a single frog thus potentially holding enough poison to kill about 10 humans.

Note that this frog is small, too: about 45 mm (ca. 1 and 3/4 inches) long.

Oophaga histrionica:

Atelopus, apparently undescribed species:

Another shot of P. terribilis, showing its inky nose and feet:

These frogs were photographed in the lab of Vicky Flechas at UdlA, who, with her husband Andrew Crawford, hosted me. Many thanks for the hospitality and photography!  Here’s Vicky in her meticulously ordered frog lab:


A remarkable “flying” snake

December 5, 2010 • 12:30 pm

An article in this week’s New York Times Science Observer column highlights the paradise tree snake (Chrysopelea paradisi) of Asia, long known to escape predators by hurling itself from a tree and sailing through the canopy to alight on a new tree.  Studies suggest it can travel as far as 300 feet in this way.  A new study published in the oddly-titled and hard-to-find journal Bioinspiration and Biomimetics analyzes the mechanics of this “flight”:

. . . a study in which scientists threw the snakes from a 50-foot tower and recorded their descent on video suggests that the snakes are active fliers, manipulating their bodies to aerodynamic effect.

“It essentially looks like they are slithering in the air, like a whip moving left and right,” said Jake Socha, the study’s lead author and a biomechanist at Virginia Tech. “The body itself moves up and down as well.”

Dr. Socha and his colleagues found that the paradise tree snake tilts its body about 25 to 30 degrees relative to the airflow to stay as aerodynamic as possible. The farthest a snake was able to travel from the tower was about 79 feet.

This is far better seen than described; here’s a nice video from PBS:

And Wikipedia says a bit more:

Upon reaching the end of a tree’s branch, the snake continues moving until its tail dangles from the branch’s end. It then makes a J-shape bend,[7] leans forward to select the level of inclination it wishes to travel to control its flight path, as well as selecting a desired landing area. Once it decides on a destination, it propels itself by thrusting its body up and away from the tree, sucking in its stomach, flaring out its ribs to turn its body in a “pseudo concave wing”[8] all the while making a continual serpentine motion of lateral undulation[9] parallel to the ground[10] to stabilise its direction in midair in order to land safely.[11]

The combination of sucking in its stomach and making a motion of lateral undulation in the air makes it possible for the snake to glide in the air, where it also manages to save energy compared to travel on the ground and dodge terrestrial bounded predators.[7] The concave wing that a snake creates in sucking its stomach, flattens its body to up to twice its width from back of the head to the anal vent, which is close to the end of the snake’s tail, causes the cross section of the snake’s body to resemble the cross section of a frisbee or flying disc.[10] When a flying disc spins in the air, the designed cross sectional concavity causes increased air pressure under the centre of the disc, causing lift for the disc to fly.[12] A snake continuously moves in lateral undulation to create the same effect of increased air pressure underneath its arched body to glide.[10] Flying snakes are able to glide better than flying squirrels and other gliding animals, despite the lack of limbs, wings, or any other wing-like projections, gliding through the forest and jungle it inhabits with the distance being as great as 100 m.[10][13] Their destination is mostly predicted by ballistics; however, they can exercise some in-flight attitude control by “slithering” in the air.[1]

Redundant parts

December 5, 2010 • 9:51 am

My Ph.D. advisor Dick Lewontin recently had a triple-bypass operation (it went well and he’s fine), and during my check-in call he described the procedure.  Like all bypass operations, veins or arteries from other parts of the body (usually the arm, leg, or chest) are harvested to circumvent the blocked coronary arteries.  Dick noted that the body is full of superfluous blood vessels.

“Superfluous?” I said.  “Maybe we can live without them, but couldn’t they have evolved as ‘backups’—in case something went wrong with a partner vessel?”

“Oh, Jerry,” said Dick.  “Don’t be such a selectionist.  Maybe they’re truly redundant: they didn’t have to evolve as backups.”  (Dick, remember, was the second author of the famous “Spandrels of San Marco” paper, arguing that much of biological form was not directly adaptive, but present for other reasons.)

Thinking about it, I think there’s a lot of truth in what he said.  First of all, it’s unlikely that we have superfluous veins because our ancestors had vein problems and needed backup vessels.  What killed them was not thrombosis, but disease, predation, infection, and hard living.

(Caveat: because modern humans can do “fine” without a major artery in the leg doesn’t mean that not having it in our ancestors might have conferred some marginal disadvantage.)

And it’s not just blood vessels that seem superfluous.  We have two kidneys, and can do very well without one.  Do we have two because one could serve as a backup for the other? And if that’s why we have two kidneys and two lungs and two eyes and two testes/ovaries, why only one liver and one heart?

We are bilaterally symmetrical, bipedal organisms descended from bilaterally symmetrical fishes. In some cases having two of something is useful.  Our two eyes give us binocular vision, but we have two not because of that facility, but because our fishy ancestors had two eyes that enabled them to see, nonbinocularly, on both sides of their bodies.  It may be the case that when some new organs evolve, like a lung or kidney or ovary, the genes that make it act bilaterally, giving us two organs instead of one. That, and not an evolved redundancy, may explain why we have “superfluous” bits.  The fact that one can act as a backup is simply a spandrel–a fortuitous but nonselected byproduct of a developmental imperative.

When we ask, then, why some bits are present in pairs and others not, the answer, then, may not always be because it’s better to have two than one.

 

Coda: The bypass operation has been one of medicine’s great advances, giving many people many years of added, productive life. I remember the first one, which was only in 1960; and for years afterwards it was a dicey and dangerous procedure.  Now it’s more or less routine with a high expectation of success: as Dick said, his hospital was running a “bypass factory.”

Republicans: the party of the rich, privileged, and obstructionist

December 4, 2010 • 12:20 pm

As we know, President Obama, with the support of most Democrats in Congress, has proposed extending the Bush-era tax cuts to all but wealthy taxpayers.  The cutoff was at $200,000 yearly income for individuals ($250,000 for couples), meant to give tax relief to lower and middle-class (and, I suppose upper-middle-class) Americans as part of the stimulus relief package.

Republicans oppose this: they want everyone, even billionaires, to keep their breaks.

As The New York Times reports, Obama’s plan has failed in the Senate: the vote was 53-36 in favor of the Obama bill, seven votes short of the 60 it needed to advance.  The bill had already been approved in the House.

Democrats, including Mr. Obama, had long questioned the economic basis for lower tax rates on the wealthiest Americans, particularly at a time of deep concern over the nation’s rising debt. White House officials said the revenue lost to tax cuts for the rich would be better spent on tax breaks for the middle class and businesses to help spur growth.

Republicans insisted that allowing the tax rates to expire for the top two income brackets would amount to a big tax increase on small businesses, which generate many of the nation’s jobs — an assertion many economic and tax analysts say is largely baseless.

The assertion is based on the number of taxpayers who report nonwage income on their tax returns, but most such income does not come from what are generally regarded as small businesses.

(Note: it would be nice to have this kind of fact-checking in science journalism!)

Republicans just have to have their tax breaks for the rich, despite any evidence that extending the previous cuts for to the wealthiest would have any salutary effect on poorer Americans or the economy as a whole. They just want the wealthy to be able to keep forever what they got before.

It’s worse:

The drubbing Democrats took in the elections, as Republicans won a majority in the House and picked up six seats in the Senate, further undermined the Democrats’ negotiating position. Republicans have since viewed an extension of the lower income tax rates as a foregone conclusion.

To speed up what they viewed as the Democrats’ inevitable capitulation, Senate Republicans said they would block virtually all legislative business on the Senate floor until the tax debate was resolved and a temporary spending measure had been adopted to finance the government.

Yes, by all means, let’s just shut down the  whole goddamned government until Obama and the Democrats give in on this totally symbolic issue. Really responsible governance. (Republicans, of course, have proposed no viable solutions for our economic difficulties).

This is only the beginning of the political hell we’re going to have for at least the next two years.