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.

27 thoughts on “Polymorphism in vertebrates

  1. When I moved to a small town in Ontario in the 1970s nearly all of the squirrels in the town were black or dark brown. By the time I left, 2 decades later, black squirrels were a rarity and most were gray. I was impressed with how fast the squirrel population changed.

  2. Interesting piece, thanks.
    FYI: Raptors occur in “morphs” not “phases.” That change in terminology, for hawks & owls, has been in use for about 30 years.

    1. Wanting to know the difference, I stumbled across this:


      Which seems to be a good accounting of the issue but still leaves me confused, although the issue seems to be less settled than you suggest but also probably isn’t an issue specific to raptors?

      Towards the bottom of the page the author seems to favor applying the term “morph” to change in structure but not color. However, I’m not sure I’ve even got that right.

      1. Nice discussion you found there– thanks for the link. Interestigly, the link provides no citations for advocating the idea that ‘phase’ implies seasonal or ontogenetic change, only authors who criticized the word because they thought it meant that. As someone whose ornithological interests are primarily scientific, I read the older systematic and zoogeographic literature (not just the latest field guides for identification), and have never encountered ambiguity about the meaning of ‘phase’– the definition given in your link is a good one.


  3. Brevard, North Carolina has a population of white squirrels. There’s even a White Squirrel festival each spring (which is, in reality, just an excuse to get out of the house, eat some fried dough, and listen to some over-amped amateur musicians.

    To actually see the squirrels during the White Squirrel Fest, you have to go to nearby Brevard College. Nice place.

  4. I just thought I’d add that catching the black squirrels in Princeton would be ridiculously easy – those critters are freakishly tame, and will take any offering right out of your hand.

  5. Just a quibble: Falco sparverius is more properly known as the American Kestrel. ‘Sparrow Hawk’ is a long outmoded name that more correctly applies to hawks of the genus Accipiter and allied genera.

    1. Yes, I know that American kestrel is used these days, but I deliberately used sparrow hawk because I object to the notion of ‘official’ common names. Common or vernacular names are just that– the names that people call them. Falco sparverius is called ‘sparrow hawk’, ‘killy-killy’ (which is actually my favorite) and ‘kestrel’, the latter used mostly by bird watching enthusiasts. Cervus canadensis was called ‘elk’ by early English settlers, despite the fact that in Europe elk referred to Alces alces (what we call moose in North America), and so some people prefer to call it ‘wapiti’, but most people still call them elk. Falco sparverius got its common name by a similar switch in usage, from the European Accipiter to the American Falco. I also prefer sparrow hawk because it gives a nice set of vernacular names for the three common American Falco: duck hawk for the large Falco peregrinus, pigeon hawk for the medium Falco columbarius, and sparrow hawk for the small Falco sparverius.


      1. I object to the notion of ‘official’ common names

        Even for birds?! But the ornithologists and twitchers have worked so hard to standardize!

        You’re hardcore.

        1. Strictly speaking, you’re right, of course, Greg. The whole concept of ‘official common name’ is an oxymoron.

          Still, there’s something to be said for the standardization of common names, if only in the interest of reducing ambiguity in day-to-day conversations. Most of the birders I know, for example, are completely ignorant of the official binomials of the birds they see, but they could easily list off ten points of distinction between the Sparrow Hawk (Accipiter nisus) and the American Kestrel (Falco sparverius).

          1. Pete–you’re right that there is something to be said for standardization. I don’t mind nearly so much the idea of ‘official English names’, which are recognized to be a somewhat artificial construct (like scientific names), and not a representation of the vernacular (i.e. what people call them in everyday usage). Many field guide writers (e.g. the late James Bond) and ornithological organizations (e.g. the American Ornithologist’s Union) make this distinction, and refer to “English names”, which may or may not be common names.


      2. Good rule of thumb: If you object to common names, please don’t use them. If you must, use the least confusing of all possible names, and/or list more than one.

        The consensus English name for these birds is Kestrel, and F. s. sparvarioides is the Cuban Kestrel, and it’s also worth mentioning what the locals call the birds: Cernícalo.

        This also needs correction:

        Falco sparverius got its common name by a similar switch in usage, from the European Accipiter to the American Falco.

        Not true. These vernacular names are based on confusion between birds from two entirely different families — the Falconidae and the Accipitridae — and are primarily based on their related sizes, not ecology (otherwise, F. columbarius would be the sparrowhawks and F. sparvarious would be mousehawks or something).

        Both occur in Europe as well as N. America, so it isn’t a Europe vs. America thing… it’s an ignorance thing. Currently, in both North America and Europe, many of the smaller Falco sp. are “Kestrels” and the smaller Accipiter sp. are “Sparrowhawks.”

        1. I don’t object to common names, just official common names. In fact, I’m rather interested in common names, and make a point of learning common names of animals from people who know them when traveling or conducting field work.

          As regards the switch of the name ‘sparrow hawk’ from a European Accipiter to an American Falco, I’m simply correct, and I can’t figure out from what you’ve written what the nature of your objection is. I never said anything about what families the birds were in, nor anything about their ecology (although the size differences among Falco do correspond to a difference in average prey size). And you’re wrong about the sparrow hawk of Europe (Accipiter nisus) occurring in America– it doesn’t (except as a rare vagrant); and none of the American Accipiters are called ‘sparrow hawk’ as an ‘official’ common name. Accipiter nisus has been called ‘sparrow hawk’ in England for centuries. There are Accipiters in America that resemble it, but the settlers, for whatever reason, did not apply that name to them, but rather to Falco sparverius. The early settlers also called Turdus migratorius the ‘robin’, even though they are quite distinct from the English ‘robin’ (Erithacus rubecula; both English and American robins do have red breasts though).

          The best rule of thumb is that when there is possible ambiguity, alleviate the ambiguity (e.g., by using the scientific name; I note that no one had any trouble knowing what bird I illustrated in the first picture above).


          1. By “both occur in N. America and Europe” I didn’t mean A. nisus and F. sparvarius. I meant small Falco sp. (for example, F. sparvarious) and small Accipiter sp. (for example, A. striatus). There are many species of kestrel and sparrowhawk, under the “official” names.

            To clarify my point: when discussing relatively common organisms like kestrels in the Americas or sparrowhawks in Europe, using old naming conventions throws your readers and makes them wonder whether or not you know what you’re talking about. I get your point about official common names, but perhaps next time you can be more clear about making such non-standard name choices for such commonly discussed organisms 😉

  6. I hit a true albino squirrel with my car once (nothing to be done; one of those quick darts out of the woods into the direct path of my tire). It was in Bucks Co. PA, a single local mutant (which I had spotted a few times before killing it).

  7. Not to suirrel away the topic, but the local cases I know of are mostly plants, where I take it it is more common? (For example varieties of spruce such as the local “snake spruce”. So named for a (tentative, I believe) mutation that inhibits side branches while still apparently being survivable.)

    One exception that I have only read about is IIRC polymorphism among perch. Two morphs can be found in some lakes, one short stout hugging vegetation and one sleeker in the open. IIRC it is believed to be the result of intense predation and on the path to speciation (due to different breeding grounds). Also, I seem to remember this happen elsewhere as well, but I can easily be mistaken.

  8. Surprised to hear that albino squirrels are the rarer variant. since they’re not uncommon down here in SE England, where melanistic ones are, at least in my experience, unknown. I have however only seen albinos in urban areas, not out in the woods.

    1. NeilF–

      Are the squirrels you’ve been observing the introduced American Sciurus carolinensis, or the native English Sciurus vulgaris?


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