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!

________

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.

14 Comments

  1. Luke
    Posted July 22, 2010 at 7:38 am | Permalink

    I was just reading Douglas Futuyma’s paper yesterday. It was a great overview of the issue today and its historical development. Unfortunately, it’s 15 pages long (minus citations) so I’m going to use NMNH’s paper to print it…

  2. Posted July 22, 2010 at 8:02 am | Permalink

    Nice post! I loved the paper by Deméré et al. when it came out, and have written about it a few times (i.e. – http://scienceblogs.com/laelaps/2009/09/what_is_your_favorite_transiti.php)

    Archaeocetes often get the most attention – it’s hard to beat walking whales – but, to me, the evolution of baleen whales from toothed, predatory ancestors is just as fascinating.

  3. Brian
    Posted July 22, 2010 at 8:38 am | Permalink

    I love the irony in calling the Blue Whale species musculus
    The little mouse indeed.

  4. americansun
    Posted July 22, 2010 at 8:45 am | Permalink

    Interesting stuff. I’m really pleased to see that this blog is becoming popular. It’s outrageous that the evolution argument is still going on in 2010. Anyway… here’s this thing.

    Things Republicans Hate: Evolution

    http://thethingsrepublicanshate.com/?p=11

  5. Delusional
    Posted July 22, 2010 at 9:11 am | Permalink

    Maybe the “sieve” also helped Aetiocetus better trap its large-ish (not microscopic) prey, like the hairs on a dragonfly’s legs?

    • Squiggy
      Posted April 2, 2012 at 7:38 pm | Permalink

      I’m bit late to the conversation but I agree. It seems likely to me that a rudimentary baleen would be very useful for whales transitioning their predatory habits from big fish to small schooling fish like sardines, possibly due to environmental pressures. The ability to expel massive amounts of water without losing a significant percentage of fish is definitely an advantage. Just a small step evolutionary-speaking to go to a true baleen capable of filtering plankton. It’s only a matter of time before all baleen whales adapt to FRAM filter technology. 😉

  6. blue
    Posted July 22, 2010 at 3:42 pm | Permalink

    “They include the largest animal that ever lived, the blue whale”

    I’ve never liked this ‘fact’. I think the best we can say is “the largest animal we know of”. Though we have a reasonable grasp of how many species of ginormous marine species are extant, we have no idea how many are extinct. I’m sure it’ highly likely, given what we DO know from paleontology, but 100% certainty on that one? I’m skeptical.

    • Microraptor
      Posted July 22, 2010 at 11:42 pm | Permalink

      Good point. Argentinosaurus and Sauroposeidon were both estimated to be comparable in mass to the blue whale, and it’s entirely plausible that there was some sort of extinct supersized fish, marine reptile, or prehistoric whale that reached similar sizes and didn’t live in an area we’ve recovered fossils from (say, the middle of the Pacific ocean).

  7. Sili
    Posted July 22, 2010 at 3:57 pm | Permalink

    Slaughtered whale showing baleen in the upper jaw.

    Looks like a giant yawning kitty.

  8. Evolution SWAT
    Posted July 22, 2010 at 5:17 pm | Permalink

    Awesome post! I linked to it on my Facebook page.Out of all the arguments in your book, I was most impressed by the evidence for whale evolution.

    I wish evidence like this had been presented to me when I studied biology in high school, at least in the biology textbooks. The teachers in my Christian school just said that they thought God used the same stuff, and that all the anatomical similarities pointed to a common creator.

    • llewelly
      Posted July 22, 2010 at 6:39 pm | Permalink

      The teachers in my Christian school just said that they thought God used the same stuff, and that all the anatomical similarities pointed to a common creator.

      Well then. Why did he make a foot out of fin, only to make a fin out of the foot millions of years later? Why not just pull and re-use the original fin? Or maybe the much better fins of the ray-finned fish?
      And we need to ask why he made feet back into fins so many times – ichthyosaurs, plesiosaurs, mosasaurs, pinnipeds … and don’t even get me started on penguins and great auks.

  9. Posted July 23, 2010 at 8:57 am | Permalink

    Awesome. Thanks for rescuing this from potential obscurity. I’m including it in a link roundup!

  10. Arek W.
    Posted May 17, 2012 at 6:20 am | Permalink

    Hello.
    Nice and clear post (although not the first one I’ve read about this paper).

    I’d like to ask a question about pictures. Do you need special permission for using them in a blog article?

    I’m asking because I’d like to write my own article about baleen evolution (not only about Demere’s paper), but in that paper (pdf) I found that

    “Any substantial or systematic reproduction,
    re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden.”

    On this page
    http://sysbio.oxfordjournals.org/content/57/1/15.full
    I found “Request Permission” but it seems, that you have to pay for permission (which I cannot afford).

    thanks for considering my question,
    Arek Wittbrodt.

  11. Posted February 27, 2013 at 7:38 pm | Permalink

    Hello there just stumbled upon your blog via Bing after I typed in, “Baleen whales: a lovely transitional form Why Evolution Is True” or perhaps something similar (can’t quite remember exactly). In any case, I’m
    grateful I found it simply because your content is exactly what I’m looking for (writing a college paper) and I hope you don’t mind if I gather
    some material from here and I will of course credit you as the
    source. Thank you very much.


8 Trackbacks/Pingbacks

  1. […] Why Evolution Is True Baleen whales: a lovely transitional form […]

  2. […] Jerry Coyne at Why Evolution Is True: Baleen whales: a lovely transitional form. […]

  3. […] need a whale fix?  I know I do!  Over at Why Evolution is True, we’ve got Baleen Whales: A Lovely Transitional Form.  Interesting factoid:  The earliest baleen whales actually had […]

  4. […] embryos also contains hints that their distant ancestors once bore teeth. They still grow tooth buds that disappear before the young whale is born. Charles Darwin must have had the toothed ancestors […]

  5. […] […]

  6. […] Baleen whales: a lovely transitional form (whyevolutionistrue.wordpress.com) 37.975336 23.736150 Share this:TwitterFacebookPrintEmailRedditLinkedInStumbleUponTumblrDiggPinterestLike this:LikeBe the first to like this post. […]

  7. […] המונעות מהם לפעול ולקודד יצירת חלבונים פעילים. גם ממצא זה, שפורסם במגזין Systematic Biology, וגם ממצאי המאובנים מאשרים כי […]

  8. […] Deméré’s article was brought to me by this reference. […]

%d bloggers like this: