Polymorphism in vertebrates

March 26, 2010 • 1:04 pm

by Greg Mayer

Darwin’s theory of evolution (and ours), unlike that of Lamarck, is variational, rather than transformational: the process of evolution is a change in frequency of different variants within a population, not a transformation of the individuals.  Darwin thus made the origin, nature, and inheritance of variation key problems for biology; indeed, for much of the 20th century, evolution and genetics were often taught as a single course at universities.

One of the most distinctive sorts of variation is polymorphism, in which two or more discontinuous forms are found in a single species (this is distinct from sexual or age related variation). Darwin himself pioneered the study of polymorphisms. Such discontinuous variation often has a simple genetic basis, with allelic variation at one genetic locus accounting for all (or most) of the variability.The color polymorphism in peppered moths (Biston betularia) is a well known and well studied case involving industrial melanism, in which light and dark forms are adapted to polluted and unpolluted environments, respectively. A well known case of polymorphism in vertebrates are the two color phases of Cuban sparrow hawk (Falco sparverius sparverioides). This case is not well studied, though, and we know nothing about the genetics, nor the adaptive significance (if any) of the polymorphism.

Light and rufous phase male Cuban sparrow hawks (Falco sparverius sparverioides).

A polymorphism in vertebrates that many Americans and Canadians are familiar with are the melanistic and gray forms of the gray squirrel (Sciurus carolinensis). The most frequent color form is gray, but blackish or dark brownish individuals are widely distributed, and in places quite frequent. I have seen them in Illinois (Cook County), Wisconsin (Racine and Kenosha Cos.) and Michigan (Ingham Co.), and also on the campus of Princeton University. (I was told at Princeton that, during football season, black squirrels are captured, and orange stripes applied to them, so that they resemble diminutive arboreal tigers, the tiger being Princeton’s mascot.)

A demonic gray squirrel (locally known as 'yard dogs'), Annapolis, MD, 23 June 2008.

A much less common color morph is the leucistic or albinistic form, which is whitish, cream or yellowish. They are famously common in Olney, Illinois (due to an introduction of two albinistic individuals to an area previously lacking any gray squirrels at all), and also occur regularly in Stevens Point, Wisconsin, but I had never seen one before my recent trip to Washington, DC, where I saw one on the tree right across from the steps on the Mall entrance to the USNM.  (The picture was taken through a bus window.)

Leucistic or albinistic gray squirrel, Washington, DC, 16 March 2010.

Vertebrate polymorphisms are often less well understood than those of invertebrates, because their generally greater size and longer generation times make experimental study more difficult. Melanism in squirrels, for example, has been related to thermoregulation and fire frequency, but no thoroughly compelling explanation has been found. One exception to this is coat color variation in mice of the genus Peromyscus, where coat color seems to be an adaptation for camouflage in varying environments.

Light and dark forms of Peromyscus polionotus from sandy and dark soils (P. p. leucocephalus on the left, P. p. polionotus on the right, I think).

In the 1930s, F.B. Sumner conducted classic field and lab studies on light colored mice living on sandy soils and dark mice on dark soils. Unlike the melanistic and albinistic squirrels, which are variant individuals within a populations, there is an element of geographic variation in the mice, which live in distinct, though adjacent, places. Sumner’s studies showed that there were several (not just one) genetic loci involved in coat color, and the color forms intergrade where their habitats meet and they interbreed. Hopi Hoekstra of the Museum of Comparative Zoology is currently conducting exciting studies of some of the same species studied by Sumner.

Although the mice occur in distinct modal forms (white vs. brown), the intergradation where they meet shows an underlying continuous variation. The frogs below show that although we can pick out distinctly different individuals, the range of pattern from plain to mottled to striped makes it difficult to recognize a small number of discrete color morphs, and the variation approaches a continuous dictribution. Such continuous variations were thought by Darwin, and most biologists today as well, to be important raw material for the evolutionary process.

Leotpdactylus albilabris from Isla Vieques.

New human evolution hall at the National Museum of Natural History

March 22, 2010 • 8:59 pm

by Greg Mayer

Last Wednesday, the National Museum of Natural History (known to biologists as the USNM, the initials of its former name and still the identifying code on its specimen tags and labels) opened its newest permanent exhibit, the Hall of Human Origins.

I was at the USNM much of last week, mostly doing research in the collections and meeting with colleagues, but I always like to take a look at the exhibits, and I’d in part planned my visit to be able to catch the opening on the last day of my visit. Unfortunately, it turns out the exhibit was only open from 12-3, the rest of the day being reserved for media and VIPs, so when I went to see it a bit after 3 all I could see was one skull through a crack in a barrier. I’m planning another go at it this summer, but some of the original specimens, loaned by foreign museums for the opening, are likely to have been replaced by casts by then.

Edward Rothstein, the New York Times’ museum reviewer, whose reviews I always find interesting, did get to see it.  He gives it a mixed review. A hall worth “repeated, close viewing” suggests an exhibit rich in the diversity and number of its specimens, a characteristic of the “cabinet” style in museum exhibits, but he laments the poor execution of the computers and touch screens of the “interactive” style:

The hall bears repeated, close viewing, though children will also find amusements here, including the opportunity to come face to face with floor-level bronze models of their ancestors. But the two computer simulations at the exhibition’s end — one a simplified Sims-type game of cultural and environmental choice, the other a cartoonish vision of possible future evolutionary change — should be far more subtle. More wall text summarizing themes would have also helped: too much is left to the text of touch screens, buried inside menus of choices.

He also raises an issue that concerned me when the opening was announced last fall: that the exhibit might adopt some theological viewpoint:

There are times too when it seems as if the Smithsonian has almost gone too far in humanizing evolution, as if it were answering those who, on religious grounds, object to the evolutionary universe and its inhuman brutality. (A touch-screen F.A.Q. suggests simply that such visitors use the show to “explore new scientific findings and decide how these findings complement their ideas about the natural world.”)

At any rate, the exhibition’s focus doesn’t really give us a feel for the daring of the evolutionary vision, which is a tale not of progress but of accident, frightening in the moment, fortuitous only in retrospect.

At the exhibit website, I found the page for the Broader Social Impacts Committee. The committee consists of 14 people, all but one of whom are identified by their religion (including one “Humanist”). This is a rather odd composition and set of descriptors for a group concerned with broader social impacts– no historians, sociologists, political scientists. But as the website makes clear, the charge of the committee is to deal with religious issues. The following statement from the website, while straightforward in acknowledging the diversity of views, seems to prefer the last view (“interaction or engagement”), but its not clear to me what exactly this view entails:

There are a number of different approaches to the science-religion relationship. One approach is to see science and religion as separate domains that ask different questions focusing on separate interests in human life – for example, about the natural world in science and about God in religion. This approach depends on respecting and maintaining the distinctions but can sometimes overlook the ways in which scientific interpretations may have an effect on religious beliefs. Conflict is seen to arise when efforts are made to eliminate the separation that the first approach assumes. The strongest conflicts develop when either science or religion asserts a standard of truth to which the other must adhere or otherwise be dismissed. An alternative approach sees interaction or engagement as positive. Engagement takes many forms, including personal efforts by individuals to integrate scientific and religious understandings, statements by religious organizations that affirm and even celebrate the scientific findings, and constructive interactions between theologians and scientists seeking common ground, respect, and shared insight into how the science of human evolution contributes to an awareness of what it means to be human.

My full opinion will have to wait till I get to see the exhibit myself. One thing I’m looking forward to are the new reconstructions.  John Gurche, the renowned scientific artist, has made a set of incredibly detailed life reconstructions for the exhibit (seen here; check out the rest of his website for more paintings and sculptures), and Smithsonian Magazine has had two pieces on them.

Edward Rothstein’s final word:

But the retrospective vistas provided here are, nevertheless, compelling and illuminating. This was conceived as a permanent exhibition, meant to serve a generation of visitors, but it was also designed to be easily adaptable to the pressures of scientific advances and visitor tastes. The evolution continues.

A set of skull images released by the USNM.

(PS: On the way in to the museum that morning, Greenpeace protesters, dressed like law enforcement agents from the “Climate Crimes” unit, handed me a flyer denouncing David Koch, who contributed most of the funding for the hall (and whose name is on it). More on this angle at the Wonk Room and USA Today.)

New human evolution exhibit to open at the USNM

October 14, 2009 • 8:06 pm

by Greg Mayer

The Smithsonian‘s National Museum of Natural History (known to biologists as the USNM) has announced that its new Hall of Human Origins, first announced in 2006 and originally slated to open this year,will open in March of next year. Normally this would be unadulterated good news; WEIT readers will recall the ‘thumbs up’ I gave the USNM’s exhibits in August. But a New York Times article [update 20.iii.2010: link now dead] says that

The museum also is establishing an advisory group called the Broader Social Impacts Committee to foster discussion on how scientific and religious perspectives on human origins can be compatible.

This is a cause of concern. Scientific institutions should not have an official theology. This sure sounds like they want to endorse Catholicism (at least of the John Paul II kind) or mainline Protestantism or some other religion that doesn’t object to science (much). I don’t see any problem in having it pointed out that, “Look, there’s a scientist who’s religious” (e.g. Ken Miller). [Added later: or pointing out, “Look, there’s a religious leader who accepts evolution” (e.g. John Paul II).] But to foster the claim they are “compatible” beyond that simple empirical point is not a scientific endeavor.

Curator Rick Potts is a little more reassuring as quoted in the Times, stressing the evidence of human evolution, saying that the exhibit will be a

place to look at the fossil evidence, to explore the fossil evidence and the archaeological evidence that informs about human evolution.

A day at the museum

August 30, 2009 • 11:08 am

by Greg Mayer

Last Monday I spent a few hours at the Smithsonian‘s National Museum of Natural History (originally known as the United States National Museum, and still known as the USNM to scientists, even after the Smithsonian began collecting art, airplanes, tchotchkes, etc., and thus gave its museums more specific names), and came across WEIT in their gift shop.


I moved this copy about two feet to the right to photograph it, because I thought Jerry would appreciate the surroundings: it’s almost a diorama.

The great natural history museums, such as the USNM, are among the world’s premier scientific institutions.  The mission of the Smithsonian Institution, Constitution Ave. entrance to the USNM, August 2009bequeathed to it by its founding benefactor, James Smithson, is the “increase and diffusion of knowledge”, and it is a fitting mission, encompassing both research and education, for not only museums, but also universities. Having been a postdoctoral fellow at the USNM twenty years ago, but not having had a chance for many years to go through the exhibits (which are a chief way in which the USNM diffuses knowledge), I wanted to see how the exhibits had changed.

The old evolution exhibit had been taken down some years ago, and a new Ocean Hall stood in its place; I was anxious to see how the newer exhibits covered evolution and the history of life, which had always been a theme throughout exhibits such as Dinosaurs and Life in the Ancient Seas, and of course are the major themes of the work of the USNM’s researchers. I didn’t think that the Smithsonian would have succumbed to any external pressures: they successfully fought off a creationist legal attack in 1980, eliciting from the DC Circuit Court of Appeals the memorable ruling that the balance between freedom of religion and learning

…was long ago struck in favor of diffusion of knowledge based on responsible scientific foundations, and against special constitutional protection of religious believers from the competition generated by such knowledge diffusion.

I’m happy to report that the presentation of evolution is even more forthright than I’d anticipated.  This sign greets visitors in the Rotunda, at the feet of the USNM’s iconic elephant:Sign in USNM Rotunda

Note particularly the line “Evolution is at the heart of this museum.”

Detail of Evolution Trail sign

Stromatolite in Ocean Hall at USNM In the Ocean Hall, one of the main galleries is devoted to the origin and history of life on earth, including everything from stromatolites, to a very nice exhibit on the origin of whales (see here, too), a story highlighted in chapter 2 of WEIT. Hung from the ceiling are skeletons of  the sea lion-like Maiacetus, and the fully marine, yet hind-legged, Dorudon and Basilosaurus.

Maiacetus at USNMDorudon at USNMBasilosaurus at USNM

There are also two temporary evolution exhibits opening next month. Darwin’s Legacy will be about Darwin and how his “theory soon found supporters at the Smithsonian, including Joseph Henry, the first Secretary of the Smithsonian Institution, and it continues to guide research at the National Museum of Natural History.”  The exhibit Since Darwin: the Evolution of Evolution

focuses on the significant role that Darwin’s theories have played in explaining and unifying all the biological sciences.  Specimens from the Museum’s diverse collections, along with documentation from our ongoing research, illustrate the importance of evolution as a scientific foundation, and how our knowledge of evolution has evolved over the last 150 years.

I was also interested to see how the USNM had fared with regard to two other issues facing modern museums: commercialization and exhibit design.  Museums are under increasing financial pressure, and even publicly supported institutions have had to seek at least supplementary private funding. There are potential pitfalls with such funding though: private funding  might distort the mission or the message of the museum. The USNM had experienced a controversy over this while I was there, when the insect extermination company Orkin sponsored, and had its name put on, the Insect Zoo. There have been other controversies concerning donors since. I found that the named-by-sponsor exhibits were still there, but had not increased substantially.  The new Ocean Hall was named for Roger Sant, a member of the Smithsonian’s Board of Regents, and a major donor. And there was a temporary exhibit on forensic anthropology co-sponsored by the History Channel, but the exhibit contents largely allayed my fears that the exhibit might be made to cable TV standards of quality. Most annoying, but not at the USNM, but rather across the mall in The Castle, was a heavily hawked tie-in to the movie Night at the Museum: Battle of the Smithsonian.

Night at the Museum prop at the Smithsonian Castle

In its exhibit halls, the USNM seems to have avoided the worst of corporate sponsorship, such as the garish corporate-advertising-masquerading-as-sponsorship I’ve seen in some zoos and even national parks.

As regards design, there has for many years been a tension between exhibits that are rich in the number and diversity of labeled specimens they contain, and those that are “interactive”.  The modern trend in design, as Steve Gould (Natural History, January 1994; see also his “Dinomania” in the NYRB) put it, has been

…fewer specimens, more emphasis on overt pedagogy, and increasing focus on “interactive” display (meaning good and thoughtful rapport of visitor and object when done well, and glitzy, noisy, push-button-activated nonsense when done poorly)…

The former notion of what makes for a good exhibit, which Gould admired and called “cabinet museums”, still informs, if not dominates, most of the USNM’s exhibits, including the newer ones such as Ocean Hall. While not consisting of row after row of specimens in the Victorian model, the newer exhibits continue to be rich in the diversity and number of specimens exhibited, and in the excellence of their labeling. A small exhibit in The Castle harks back to the Victorian model, but the best example of this approach I know of today is the marvelous Sense of Wonder at the Milwaukee Public Museum, a deliberate recreation of the older style. The juxtaposition of specimens is a major intellectual benefit of a such a style: it practically mandates that viewers begin composing “compare and conrast” essays in their heads. Such comparisons are a source of wonder, and an inspiration for systematics, comparative anatomy, and , indeed, much of the whole of biology. As a graduate student, I spent many hours “animalizing” (as we called it) studying the skeletons in the Museum of Comparative Zoology‘s Hall of Mammals (which is also close to the Victorian model). The USNM’s Osteology Hall, with its many vertebrate skeletons, has something of the same flavor.


My major criticism of the newer exhibits, especially the Hall of Mammals, is that they have largely left out dioramas, which, to me, have always been among the most compelling components of a natural history museum. A late Victorian development of the cabinet style pioneered by Carl Akeley at the Milwaukee Public Museum, and reaching perhaps its apogee at the American Museum of Natural History, a diorama associates animals and plants that live together in a natural setting.  Most suited for exhibits with a biogeographic arrangement, their richness and frequency serve as  a means to distinguish the great, from the merely good, public exhibit museums.

Alligator diorama at the USNM

In the mammal hall, the mammals have been removed from the environmental context provided by the diorama, and presented against stark backgrounds painted in neutral colors, with a minimal or stylized attempt to portray the environment. Glass-panel walls and overhanging girders dominate the main exhibit hall. In part, this separation of organism from environment harks back to the early Victorian cabinet museum, with its unadorned skeletons and taxidermy mounts. These early exhibits are redeemed by the richness and density of the specimens, but this only partly mitigates the present exhibit. I think there are advantages to both the diorama and the starker style evident here, but to give up the one almost entirely is a mistake.  This is seen most clearly in a segment of the mammal hall entitled “Savanna Waterhole“, where a giraffe and zebra stretch to drink, but there is no waterhole, and another zebra looks back at a glass wall.


Despite this criticism, there is much to admire in the mammal hall, and much to learn in it. This hall, and the museum as a whole, is a wonder, and one of the nations, and the world’s, scientific and educational treasures. To walk through the USNM’s exhibit halls is a marvelously enjoyable, richly rewarding, and deeply edifying intellectual and aesthetic experience.

Addendum. In response to some of the comments, here are a couple more items.

I just miss hundreds of different birds in a long, long hallway.

It’s a curved hallway, and somewhat obscurely placed outside the Baird Auditorium, but the Birds of DC exhibit is hundreds of birds in a longish hallway.  Here’s the cabinet that contains mostly owls, but also a passenger pigeon and a Carolina parakeet.Pigeons, parakeet, and owls in Birds of DC at USNM

Is the life-size fiberglass blue whale still there??

No, but a life-size northern right whale is, right in the middle of the new Ocean Hall.  It’s modeled after an actual individual whale, named Phoenix, studied by scientists from the New England Aquarium.  The story of its creation is here.Northern right whale in Ocean hall at USNM

I’m not sure where the blue whale is now. It was part of the Life in the Sea exhibit, which was in what is now the Hall of Mammals (I think– I’m trying to recall the hall layout from years ago, and don’t have an old floorplan). It’s story, up to 1996, is told here in Smithsonian magazine.