Category: adaptation

“Adaptive” hybridization in mice

July 22, 2011 • 5:30 am

One of the newer “expansions” of the modern synthetic theory of evolution is the idea that the genetic variation “used” by either natural selection or genetic drift can arise not just through mutations within a species, but also through hybridization with another species.  Hybridization between different species usually yields maladaptive offspring, but occasionally a fertile hybrid can be the source of a new gene that can spread through a species that didn’t originally have it.  (I’ve previously written about work showing that an adaptive color gene in aphids was acquired not by hybridization, but by ingestion.)

Such a case of “adaptive introgression” (“introgression is simply the acquisition of genes by one species or population from another by interbreeding) was just reported in Current Biology (reference below), and you can read a summary of the results in a piece by Kai Kupferschmidt in Science NOW (I’m quoted).

As reported by Song et al., the house mouse, Mus musculus domesticus, acquired a gene for rodenticide resistance by mating with a close relative, the wild mouse Mus spretus.  The rodenticide is the famous poison Warfarin, which is used widely to kill mice and rats.  It works by inhibiting the synthesis of blood clotting factors that themselves are dependent on vitamin K.  The mice then bleed to death internally. Warfarin is also used to inhibit the formation of blood clots in humans, but is called “Coumadin” for medical uses.  Since its introduction in the 1950s, many populations of mice have become resistant to the poison: this is a classic case of natural selection that isn’t as widely known as examples of bacterial resistance to antibiotics.

The poison.

Song et. al.  found through DNA sequencing that populations of the house mouse in Spain and Germany had one region of their genome, on chromosome 7, that actually came from Mus spretus. (The two species were formerly “allopatric”, i.e., lived in different places, until they become geographically contiguous when the house mouse moved with its human host.)  This region includes genes for Warfarin resistance, and experiments showed that house mice containing the introgressed spretus region lowered mortality from Warfarin and a similar poison from 84-100% to 9-20%.  The amino-acid sequence of the introgressed gene (vkorc1spr ) differed between the two species.  The authors haven’t done the definitive experiment—actually putting the spretus version of the gene into a house mouse genome and seeing whether it alone confers resistance to poison—but the mortality rates of introgressed house mus versus those lacking the gene are pretty good evidence.

Finally, the authors showed by population-genetic analysis that the spretus gene entered the house mouse population between 61 and 71 mouse generations ago, which corresponds to about 13-22 years in the wild, so the adaptive introgression occurred well after the poision was introduced. This again supports the notion that the spread of the spretus gene in house mice was promoted by natural selection for resistance to poison.

There are two questions to ask about this situation:

1. How common is adaptive introgression?  My guess (which you can see in the ScienceNOW link above), is probably that it isn’t very common.  Why? Because if it were, we would see it using DNA-based phylogenies.  Adaptive introgression would show up as a region of the genome that was much more similar to a region in a related species than could have occurred by simply genetic drift or natural selection in the first species.  We don’t see that kind of similarity very often. (It also could have other causes, like a variant in the common ancestor of the two species that simply was inherited by its descendants.)  So I suspect that while the capture of adaptive genes by hybridization occurs occasionally, it won’t be an important source of variation compared to mutation.

Further, most species (we are one) simply can’t form fertile hybrids with a related species, a condition that is necessary for adaptive introgression to occur. The genus Drosophila (the flies on which I work) contains about 1500 described species, but hybrids are known in only about a dozen cases, and most of these hybrids are sterile.

2.  Can species be selected to hybridize with other species?  Some biologists—especially botanists—have theorized that natural selection will foster those traits that facilitate one species mating with another one, so that it can capture alleles to facilitate its own evolution.  That doesn’t wash, because in the vast majority of cases the hybrids between different species are less fit than the parental species: hybrids can be sterile or inviable (the spretus/domesticus hybrids, for example, are largely sterile).  So, although hybridization may occasionally capture a “good” gene, most of the time it produces maladapted hybrid offspring.  Natural selection, then, would act to prevent rather than facilitate hybridization, because the beneficial effects of mating between species are far outweighed by the bad ones.

As the famous evolutionist Ronald Fisher wrote in 1930:

The grossest blunder in sexual preference, which we can conceive of an animal making, would be to mate with a species different from its own and with which hybrids are either infertile or, through the mixture of instincts and other attributes appropriate to different courses of life, at so serious a disadvantage as to leave no descendants. … it is no conjecture that a discriminative mechanism exists, variations in which will be capable of giving rise to a similar discrimination within its own species, should such a discrimination become at any time advantageous.

Mus spretus. Wild mice are incredibly cute, and I could never bring myself to kill them in traps.


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Song, Y. et al. 2011.  Adaptive introgression of anticoagulant rodent poison resistance by hybridization between Old World mice.  Current Biology: doi:10.1016/j.cub.2011.06.043

The biology of Mauritius: part 2

July 2, 2011 • 9:09 am

Yesterday I presented some photographs and descriptions by biologist Dennis Hansen of his work on the isolated island of Mauritius.  That only scratched the surface of the amazing biology of the endemic species on this island, and I want to finish up this brief lesson with some more show-and-tell.

Dennis also sent me an reallly nice paper written by him and Christine Müller in The International Journal of Plant Science (reference below).  It describes how one species of very rare Mauritian plant seems to be both pollinated and have its seeds dispersed solely by a lovely endemic gecko. Maybe I’m wrong, but I think this is the only case of a plant that depends entirely on a lizard for reproduction.  But I’ll let Dennis tell the tale (his photographs are below, and I suspect that if you leave your email address in a comment, he’ll send you a pdf of the paper, which is well worth reading):

Anyway, here’s a handful of shots from the field—one of my favourite Mauritian endemic plants, Roussea simplex, being both pollinated (photo 1 &2) AND having its seeds dispersed (photo 3) by the same gecko species, the blue-tailed day-gecko, Phelsuma cepediana. It’s a cool system, but under threat from invasive ants that chase off the geckos (!), and invasive plants that outcompete Roussea simplex. There are likely less than 150 plants left of this species. Don’t worry. In Mauritius, that’s plenty (like Douglas Adams recounted for the Rodrigues fruit bat in ‘Last Chance to See‘ — “don’t worry, there are HUNDREDS of bats left!”). Yes, many species—and even more interactions—are in trouble indeed. The paper about this interaction even made it to the front cover of a journal from your university press (Int J Plant Sci). Who cares that this paper probably will pick up less than 10 citations in its lifetime—it made me feel immensely privileged to have studied this interaction. If nothing else, then because it means that it will not—unlike Quammen puts it for The Song of the Dodo—forever remain unbeknownst to us, because no one took the time to sit down in the forest and listen (or watch, in this case).

Photo 1:

Photo 2:

Photo 3:

Here’s a photo I’ve taken from Hansen and Müller’s paper showing the lizard with a smear of pollen on its head (arrow). The pollen is mixed with a sticky, viscid residue, and so is unsuitable for disperal by insects, but fits nicely on the lizard’s head.  And when the lizard goes to the next plant for nectar, the pollen is transferred to the stigma.


Curiously, though seed dispersal appears to rely entirely on the gecko, seeds that were recovered after passing through geckos, or taken directly from the plant, were never seen to germinate.  They were all attacked by a fungus that killed them. Perhaps the specific microclimate of the forest floor somehow facilitates seed germination.

But wait—there’s more.  Again, I defer to Dennis’s descriptions:

Colea colei: an amazing little understory plant from the rainforest in Mauritius, sort of like a thin, upright liana up to a few meters high, with a tuft of green leaves at the top. It is cauliflorous, with the gorgeous flowers emerging straight from the stem, all the way down to the ground (don’t get me started on the cool evolutionary ecology of that trait).

Telfair’s skink [Leiolopisma telfairii] with Pandanus fruit: from Round Island, north of Mauritius. Heaven on Earth, that island is. Home to many of the surviving endemic reptiles of Mauritius. I think of Churchill with a cigar whenever I look at this photo. [JAC: this island is the only place known to harbor this reptile.]

One of my favourite geckos, the Ornate Day Gecko, Phelsuma ornata:

Two of the huge orb-weaving spider Nephila inaurata & its gecko-victim: sometimes male geckos will jump off the surface they are on, if threatened by a larger male. In this case, the poor fella jumped right into the web of the spider! The second photo shows the gecko two hours after the spider’s injection of digestive enzymes (several points of injection). Kinda looks like it’s been hit by a flame thrower.


One of a Dombeya flower—notice the secondary pollen presentation on the tip of the petals, and the yellow nectar at the base of the petals.  Coloured nectar is an incredible subject on its own, but I will refrain from harping on at great length here.

One of Telfair’s skink, Leiolopisma telfairii, on a palm inflorescence. All the yellow spots in its face are pollen grains, too.

Some of the largest surviving Phelsuma [genus of “day geckos”] gecko, Guenther’s gecko [Phelsuma guentheri]:

Some more of the Blue-tailed Day Gecko, P. cepediana, pollinating flowers of Trochetia blackburniana (notice the size difference between the large male hanging off the flower, and the small female/juvenile almost disappearing into the flower)


THE END

Thanks to Dennis for sharing his work and photos with us.

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Hansen, D. M. and C. B. Müller.  2009.  Reproductive ecology of the endangered enigmatic Mauritian endemic Roussea simplex (Rousseaceae).  Int. J. Plant Sci 170:42-52.

World’s loudest creature (for its size)

June 30, 2011 • 5:05 am

Alert readers Diane G. and Ray P. called my attention to a new paper in PLoS ONE by Soeur et al. that reveals the loudest known creature for its size on Earth.

What is it?  It’s a water boatman of the species Micronecta scholtzi. These are in the Hemiptera (“true bugs,” remember?).  It’s tiny—only 2 mm long (the size of Lou Jost’s orchid)—and inhabits streams in Europe. Here’s one:

What’s the big deal?  These insects produce the loudest sound per unit length of any known species on Earth. Scientists captured some in a river in Paris and recorded them underwater in the lab using a hydrophone.  Only the males make noise, a clue that the song serves a sexual function.

How loud are they? The authors note: “SPL [sound pressure level] values of M. scholtzi were compared with the values reported for 227 other species (2 reptiles, 3 fishes, 24 mammals, 29 birds, 46 amphibians and 123 arthropods) collected from the literature (Table S1).

The song consists of three parts, and the third, the loudest, can be as large as 105 decibels! That’s as loud as a power mower from three feet away, and a level that, if inflicted constantly, could cause permanent hearing loss.

Is it really the loudest animal on earth?  Yes, if the volume is scaled by a measure of body size (length). As the article notes:

. . .  the most striking feature of the song is its intensity. The song can be heard by a human ear from the side of a pond or river, propagating across the water-air interface. Estimating the sound intensity at a distance of one metre reveals a value of ~79 dB SPL rms. When considering peak values, i.e. the loudest part of signal, the intensity can reach 100 dB SPL. Whilst these values are far below those estimated for large mammals such as dolphins, whales, elephants, hippos, or bison, when scaled to body size, M. scholtzi has the highest ratio dB/body size. Even if such comparison might need to be adjusted with corrections taking into account different recording methods and conditions, M. scholtzi is clearly an extreme outlier with a dB/body size ratio of 31.5 while the mean is at 6.9 and the second highest value is estimated at 19.63 for the snapping shrimp S. parneomeris. This water bug might be the exception that proves the rule that stipulates that the size and the intensity of a source are positively related. This departure from the rule is apparent within the group of stridulating animals. In this sub-sample, M. scholtzi is identified as an extreme outlier. No other recorded animals rival M. scholtzi. Two other arthropods were also identified as outliers; the Australian miniature cricket C. canariensis [27] and the Praying Mantis M. religiosa [28].

Well, what’s the real loudest animal on earth? If you don’t scale by body size, it is a whale and a monkey in the aquatic and terrestrial realms, respectively. The National Zoo says this:

Blue whales’ low-frequency pulses are as loud as 188 decibels—louder than a jet engine—and can be detected more than 500 miles away. On land, the loudest animals are howler monkeys, whose howl can be heard three miles away.

How and why do they make this noise? The males rub their penis against their abdomen. The authors theorize that “runaway sexual selection” due to competition by males for females has made the song so loud. But there are other forms of sexual selection not mentioned by the authors, like “sensory exploitation” (females pre-adapted to respond to such songs), that could also explain the evolution of such loud songs. There are many forms of sexual selection and it’s very hard in a given case to figure out which has operated.

You can hear the sounds of these insects by clicking on a BBC Nature piece, “‘Singing penis” sets noise reord for water insect.

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Sueur J, Mackie D, Windmill JFC, 2011. So small, so loud: Extremely high sound pressure level from a pygmy aquatic insect (Corixidae, Micronectinae). PLoS ONE 6(6): e21089. doi:10.1371/journal.pone.0021089

Look away if you’re an earthworm

June 26, 2011 • 6:22 am

This post,  by Matthew Cobb, comes from his Zoology Z-letter, and he’s given me permission to repost it.

Spotted on Lucas Brouwers’ Twitter feed (@lucasbrouwers), this great video of a Powelliphanta snail from New Zealand snarfing an earthworm. Keep your eye on the video – it all happens incredibly quickly! Odd thing to say about a snail, but true.

According to this PDF from the NZ Department of Conservation, Powelliphanta snails can grow up to 9 cm across and are nocturnal. They are also endangered, primarily because of human activity, although a recent survey suggested they were making a slight recovery. According to Wikipedia, “There are 21 species and 51 subspecies within the genus. The relationship between the species is complex, and it has been suggested that the group Powelliphanta gilliesi-traversi-hochstetteri-rossiana-lignaria-superba forms a ring species.”

There are other carnivorous snails on NZ, including the Rhytididae, which seem to be particularly vicious, according the NZ Dept of Conservation:

“They can eat other snails by biting their heads off and then they carry them to a quiet spot on the back of their foot where they insert their tails up into the prey’s shell. The tail secretes a liquid that slowly dissolves the prey’s flesh and the calcium from its shell. The Rhytida snail then absorbs the dissolved nutrients. It can take the snail several days to actually complete such a meal.”

One rhytidid snail, Wainuia urnula urnula, seems to use a similar rapid action to that seen in Powelliphanta and probably has the same basis. According to Murray Efford in The Journal of Molluscan Studies, “In the laboratory, W. urnula urnula captured landhoppers by rapidly everting the TVU-section odontophore beneath the prey and immediately drawing it into the mouth in a single action.”

So that’s how they (probably) do it. No sucking, just incredibly rapid movement, using that odontophore…

Sloth defecation is for mating

June 24, 2011 • 10:50 am

A while back I put up a post and a video showing the bizarre behavior of sloths when they have to defecate: once a week they make the long, slow climb down to the base of their tree, dump their load, and then retrace their steps up. I suggested four hypotheses for this behavior, and decided that the most likely one was to identify their location to potential mates.  There were lots of comments, and most people disagreed with the “mating” idea because, after all, how would one sloth ever find another if they had to climb down a tree and sniff around other trees? It seemed inefficient.  Now, Becky Cliffe from the University of Manchester claims she has some support for the mating hypothesis (I am putting below a comment she just added to the earlier post):

I am doing the study and I can shed some light on this matter! To cut a long project short, I have been monitoring sloth behaviour, reproductive activity, thermoregulation and metabolic activity for the last 12 months and can tell you that the defecating at the base of the tree is almost certainly linked to finding a mate.

You won’t find any literature on this at the moment but I can tell you female sloths have a very regular 28 day reproductive cycle, during which, they are in heat for 10 days. During these 10 days,they actually let out regular high pitched screams that attract males from up to 700m away, and they descend to the same spot at the base of the same tree daily to go poo/pee (even if it is just the tiniest amount!)

It also takes them an average of 31 days for food to pass from ingestion to excretion … amazing! And yes, their body temperature CAN fluctuate up 8 degrees over the course of a day. And the respiratory rate can range between 6 breaths per min to 120 !

Oh and the stuff about them being half deaf and half blind… completely false and I have no idea who spouted that one originally!! Very interesting animals 🙂

Just to show this is legit, here’s a YouTube video of Becky with baby sloths:

The gyrfalcon is a seabird and uses ancient nests

June 20, 2011 • 4:49 am

Reader Dominic has called my attention to two new reports from the BBC about the gyrfalcon (Falco rusticolis), pronounced “JER-falcon.”

First, a bit about the bird. It’s the world’s largest falcon, with some specimens reaching three pounds with a four-foot wingspan. They’re magnificent birds:


Photo by Doug Backlund; go to his page for many more great photos of the bird

Here’s the range map from Cornell’s All About Birds; they breed on the North American tundra but range widely south.  A map of its worldwide distribution, including Greenland, northern Europe and Asia can be found here.

They’re bird eaters; the Cornell site reports that they “eat mostly ptarmigan, but many other prey species have been recorded, including fulmars, gulls, jaegers, ducks, geese, Rough-legged Hawk, Short-eared Owl, sparrows, buntings, and redpolls.”   Here’s an amazing video of one taking a ptarmigan on the wing: the strike occurs at about 2:15 (I find the music annoying; you might want to turn it off).

So what’s new about the bird?  Two discoveries, both made by a team headed by Kurt Burnham, a Ph.D. student at Oxford’s Edward Grey Institute of Field Ornithology.

1.  The gyrfalcon is basically a seabird.  As the BBC reports, up to now its hunting habits during the nonbreeding season—the winter—were largely unknown.  But tracking the birds with radiocollars shows gyrfalcons to be “secret seabirds”:

Gyrfalcons living in the high Arctic overwinter out at sea, spending long periods living and hunting on pack ice.

It is the first time any falcon species has been found regularly living at sea.

The birds likely rest on the ice and hunt other seabirds such as gulls and guillemots, over what appears to be one of the largest winter ranges yet documented for any raptor.

“I was very surprised by this finding,” said ornithologist Kurt Burnham who made the discovery. “These birds are not moving between land masses, but actually using the ice floes or pack ice as winter habitat for extended periods of time.”

“Previously, all species of falcon were considered to be land-based birds.”

. . . Those on the east coast ranged far more widely, covering between 27,000-64,000 square kilometres. Some of these had no obvious winter home ranges and travelled continuously during the non-breeding period, spending up to 40 consecutive days at sea.

During the winter one juvenile female travelled more than 4,500km over 200 days, spending over half that time over the ocean between Greenland and Iceland.

2.  Some gyrfalcon nest sites are ancient. Another report, from BBC EarthNews, shows that the nesting areas used by these birds can be several thousand years old.  Like other falcons, gyrfalcons don’t build nests, but simply scrape out an area on a rock ledge.  New research shows this:

Carbon dating revealed that one nest in Kangerlussuaq in central-west Greenland is between 2,360 and 2,740 years old, the researchers report in Ibis.

Three other nests in the area are older than 1,000 years, with the youngest nest site first being occupied 520 to 650 years ago.

These ancient nests are still being regularly used by gyrfalcons.

“While I know many falcon species re-use nest sites year after year, I never imagined we would be talking about nests that have been used on and off for over 2,000 years,” says Burnham.

They also carbon-dated some gyrfalcon feathers to over 600 years old. But these aren’t the oldest continuously used nesting sites by birds, not by far:

By carbon dating solidified stomach contents, peat moss deposits and bone and feather samples from various moulting sites, researchers have in the past shown that colonies of snow petrel have returned to the same sites for 34,000 years and adelie penguins for 44,000 years.

Why are there no insects in the sea?

June 19, 2011 • 3:05 am

by Matthew Cobb

One of my favourite questions relating to evolution is ‘Why are there no insects in the sea?’ Arthropods came onto the land around 380 MY ago, and crustaceans and insects separated soon afterwards, probably because of that ecological shift. More or less, you get crustaceans in the sea, and insects on land. So why didn’t the insects go back into the sea? It’s very hard to be certain of the answer to this – doing an experiment would be pretty tricky, after all. But we can get towards what might be the answer by thinking about some of the possible answers we might give:

– Insects can’t live in water. Although no insect species lives its whole life-cycle in water without access to air, many insect species pass their nymphal stage in freshwater, breathing with gills. Mayflies (more on this in an upcoming post) and dragonflies are two obvious examples. Last year Daniel Rubinoff, an entomologist at the University of Hawaii, discovered a number of moth species that have caterpillars (= larvae) that are equally at home on land or in freshwater. You can see a great video of one of these caterpillars moving between water and land here.

– Insects can’t cope physiologically with salt water. Not true. There are a large number of species of insect that have a larval stage that lives in brackish salt water, so living in the sea is not impossible. (Indeed, this fact shows that the real question should be ‘Why are there no insects that have their full life-cycle in the sea?’)

– The sea is full. I think this is probably the right answer – the niches that insects would occupy in the sea are already full. The insects’ cousins, the crustaceans, are already there. This is what ecologists call ‘competitive exclusion’. Any insect that started going back into the sea would either starve or be eaten, I reckon. Proving this, however, is tricky.

One implication of this is that evolution finds it difficult to go ‘backwards’ because life shapes and changes the ecosystem over time. When environmental conditions changed to make it possible for terrestrial life to evolve, crustacean-like arthropods moved onto the land and rapidly made a series of adaptations that led to their incredible success as insects (reducing the number of appendages, evolving first wings and then highly specialized larval stages, and so on). Retracing their steps back into the sea is no longer possible, for ecological reasons. Were something terrible to happen to some or all of the crustaceans, however, it seems pretty likely that those adaptable insects would be back in the sea in the blink of a geological eye.