Baleen whales: a lovely transitional form

The paper I want to write about today, by Thomas Deméré and his colleagues, is two years old, but deserves wider circulation than it’s gotten.  I came across it as a citation in a new Evolution paper by Douglas Futuyma on evolutionary constraints (reference below; do read Doug’s paper if you’re in the field, as it’s one of the few sensible articles on the topic).  Deméré et al. is one of the best papers I’ve seen about a transitional form.

The transition is between modern baleen whales and their toothed ancestors.  Baleen whales are in the suborder Mysticeti of cetaceans; all of the species are toothless. (The other order, the toothed whales, is Odontoceti.)  Instead of teeth, mysticetes have baleen, a basketlike substance in the upper jaw which is used in filter feeding.  Baleen is not bone: it’s made from keratin that’s secreted from the whale’s palate.  The secreted substance is abraded by the whale’s tongue, producing a filamentous, fringe-like structure :

Fig. 1.  Slaughtered whale showing baleen in the upper jaw.

The baleen basket is used in feeding, and acts like a sieve.  The whale gapes its mouth, gulping in a huge volume of water containing small fish, zooplankton, and invertebrates. It then closes its jaws a bit and, using its tongue, squeezes out the water, trapping the prey against the baleen. They then go down the hatch.  Deméré et al. describe a baleen whale’s feeding gulp as “the world’s largest biomechanical event,” since it can take in, and expel, some 70 tons of water at one time.

Although their prey are small, baleen whales can get very big. They include the largest animal that ever lived, the blue whale (Balaenoptera musculus), which eats as much as 1640 kg (1.8 tons) of food per day.

Baleen had a lot of uses during the days of whaling; one was to make the supporting struts in “whalebone” corsets.  It doesn’t fossilize well, so the ancestry of baleen whales is deduced from other skeletal features and from DNA. From these we know that the earliest baleen whales actually had teeth. The paper by Deméré is about this transition.

Before I mention their results, I want to show one photo from their paper that supports this evolutionary scenario.  Baleen whales, though toothless, develop tooth buds when they’re embryos.  In toothed whales these buds go on to become the adult teeth, but in baleen whales they degrade and disappear.   I mention this in WEIT as evidence for evolution—creationists can’t explain why teeth begin to form and then disappear—but Deméré show a nice photo of these tooth buds that I hadn’t previously seen:

Fig. 2 (from Démeré et al.).  An embryonic fin whale (Balaenoptera physalus), a mysticete, dissected to show tooth buds in upper jaw.

Deméré et al.’s main result is this: they document the existence of a transitional form between baleen whales and their toothed ancestors.  This form apparently had both teeth and a small fringe of baleen from the upper jaw.  It could thus not only chomp its prey, but strain it.  This was a step on the way to the fully-sieving lifestyle of modern baleen whales.

The paper includes two kinds of evidence for a tooth-baleen transition.  The first is paleontological, based on a fossil whale, Aetiocetus weltoni, found in Oregon.  It dates from 24-28 mya, close to the time when we see the first truly toothless mysticetes.  The whale is clearly on the mysticete side of the toothed/toothless branching based on several skeletal features, but has teeth; it’s an early toothed mysticete.  But when the authors reexamined they jaw they found previously overlooked grooves (“nutrient foramina”) in the lateral parts of the palate.  In modern baleen whales, these slits allow passage of both nerves and blood vessels that feed the baleen-forming parts of the epithelium. (Baleen grows continuously throughout the whale’s life.)

These slits aren’t found in modern toothed whales. Here they are in the mysticete fossil:

Fig. 3.  (From Démeré et al.)  Upper jaw of the Aetiocetus weltoni. fossil. Left: Teeth.  Right: Enlargement of section in the rectangle.  Arrows show the depressions (sulci) and nutrient holes (foramina) that, in modern baleen whales, allow passage of nerves and arteries to baleen-forming epithelium.

They also found these slits in two other species of fossil mysticetes.  It seems likely, then, that these fossils, which occur at just the right time in the fossil record, had both teeth and some rudimentary baleen.  They are transitional forms because of both their morphology and the time when we see them.

The second line of evidence is genetic—DNA sequences.  The temporary “teeth” in fetal mysticetes have dentin (the calcified tissue that makes up much of the teeth), but lack enamel.  Working under the hypothesis that baleen whales evolved from toothed whales that had both dentin and enamel, the authors made the following prediction:

Given that edentulous [toothless] mysticetes recently descended from ancestors with fully mineralized dentitions, we predicted that enamel-specific SCPP [secretory calcium-binding phosprotein] genes would be present, but not functional, in modern baleen whales.

That is, the baleen whale genome should contain “dead genes” that made tooth enamel in their ancestors. (As I explain in WEIT, when evolution eliminates a structure, it does so not by excising the DNA for the structure from the genome, but by silencing the genes.)

And, by doing DNA sequencing of three enamel-specific genes from 12 species of mysticete, 4 species of odontocete, and some related terrestrial mammals, their prediction was confirmed.  Two of these genes were completely dead: they had experienced “frame-shift” mutations that throw the DNA code completely out of whack, as well as mutations to “stop codons” that prematurely terminate the production of a protein.  The other gene had a large section missing.

What better evidence for a toothed-whale ancestry for mysticetes? Together with the existence of transitory teeth in embryonic baleen whales, and now the fossil evidence, it’s dead certain that modern baleen whales weren’t created, but evolved from toothed whales about 30 million years ago.

Why did it happen?  The best guess is that some individuals among toothed whales had genes for secreting a substance from their palate that could act as a sieve, allowing them to eat not only large prey, but smaller ones at the same time.  By being better fed, the genes of these individuals would stand a better chance of being passed on to future generations.  Presumably there was a reproductive advantage to further specialization, so that this lineage completely lost its teeth and relied entirely on sieving.  Here’s what Aetiocetus weltoni probably looked like:

Fig. 4 (from Démeré et al.).  Artist Carl Buell’s reconstruction of Aetiocetus weltoni showing the simultaneous presence of teeth and baleen.

The authors say their results are important because they overturn an earlier view of a near-instantaneous origin of baleen whale feeding:

However, most recent phylogenetic analyses of Mysticeti instead imply a direct saltatory transition from an ancestral form with tooth-lined jaws to the modern condition where the jaws are toothless with right and left racks of baleen suspended from the palate.

But that is weird. No evolutionist would say that baleen-feeding arose from tooth feeding in a single sudden leap. There are many modifications of the skull and palate required to go from one lifestyle to the other, and that would take time.  And it’s highly unlikely that a single mutation got rid of teeth and produced the full baleen basket in a single step.  The “sudden origin theory” is a straw man. But that doesn’t matter. The authors have shown the one thing that counts: that there was a transitional form—one that was a plausible improvement over the ancestral condition—between toothed and toothless whales.  Further, the genetic evidence shows that evolutionary biology is not just in the business of concocting stories: it can make predictions, test them, and verify them. Yay us!

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Deméré, T. A., M. R. McGowen, A. Berta, and J. Gatesy. 2008. Morphological and molecular evidence for a stepwise evolutionary transition from teeth to baleen in mysticete whales. Systematic Biology 57:15-37.

Futuyma, D. J.  2010.  Evolutionary constraint and ecological consequences.  Evolution 64:1865-1884.