The Emergence of Whales, Chp. 4
Synopsis of Chapter 4: Cetaceans are Highly Derived Artiodactyls
Michel C. Milinkovitch Evolutionary Genetics, Free University of Brussels, Belgium
Martine Berube Department of Population Biology, Copenhagen University, Denmark
Per J. Palsboll Department of Ecology and Evolutionary Biology University of California-Irvine
Fortunately a short chapter, with much that I can’t reproduce (complex phylogenetic trees and the like). Which is good, as they are some long chapters ahead.
Good first paragraph:
“Cetaceans (whales, dolphins, and porpoises) form one of the most dramatically derived group of mammals and modern representatives are easily recognized by the telescoping of the skull, posterior movement of the narial openings, isolation of the earbones, shortening of the neck, loss of external hind limbs, reduction of the pelvic girdle, and addition of vertebrae (ref). These skeletal character states are the most conspicuous features within a suite of transformations that cetaceans experienced in basically all of their biological system during their adaptation to the aquatic environment.”Let’s reemphasize that last part: “during their adaptation to the aquatic environment.”
Previous genetics work (including Gatesy’s previous chapter) showed close relationship between cetaceans and ungulates, particularly artiodactyls. A surprising result has been that “the suggestion that cetaceans are nested within the artiodactyl phylogenetic tree, making artiodactyls paraphyletic”. Subsequent genetic work supports this (with some brief critical comments on earlier studies). Gatesy is cited in particular. Shimamura et al. looked at “retropositional events that lead to the insertion of short interspersed elements (SINEs) at particular loci in the nuclear genome of various artiodactyl and cetacean ancestors.” Shimamura et al. agrees with Gatesy and this study, and that’s good, because “the likelihood of SINEs being independently inserted at the same locus in different lineages, or precisely excised in different lineages, seems virtually nil.”
So, it seems that the results from molecular data pretty much indicate artiodactyl paraphyly. But as good scientists should, these guys are going to test that with alpha-lactalbumin sequences.
Materials and Methods
Be serious. This is a synopsis. These guys provide DETAIL. A mild example:
“The sequencing products were separated by vertical electrophoresis on a 5% denaturing polyacrylamide gel (Long Ranger^TM) and visualized by autoradiography.”In case you want to know what goes into a DNA amplification solution, I can now tell you. Let’s move on.
Here’s the species contributing DNA for the test:
Next section describes how the phylogenetic analyses were performed. Some of the same software programs that Gatesy used were also used here. (Does that induce a bias in the results?)
Relative apparent synapomorphy analysis (RASA) and likelihood mapping were also performed.
Guess what? Baleen and toothed whales group together, whales and
dolphins group together, then with hippopotamus, then with the
goat and the bovine. Cool.
The final section is somewhat easier to get into, with a bold statement at the end.
Morphology vs. Molecules
First they justify doing this kind of analysis, pointing out that they’d like to have DNA from extinct taxa, but that’s kind of hard to get. They point out that some molecular data is not very noisy (as in the Introduction), as apparently some criticisms of the methods have pointed out that molecular data is noisy.
They justify the contrast between the molecular data and the morphological data by noting that one of the reasons the morphological result (artiodactyl monophyly) is different could be “missing data” (!!). Meaning: because no complete hind limbs for fossil whales have been found and because modern whales don’t have hind limbs, the diagnostic synapomorphy for artiodactyls — the famous trochleated astragalus — hasn’t been found (see end note). Mesonychians had a nontrochleated astragalus. They also discuss divergence of dental characters.
(I did a little searching: the trochleated astragalus is described as a “pulley” on the heel bone. Aids in locomotion. I’d still like to see an anatomical diagram. The problem is, when you Web search, astragalus is also a plant.)
So they defend their results, stating “We do not consider that molecular data are necessarily superior to morphological data for phylogeny estimation. We are well aware of the problems inherent in molecular sequence data such as [4 problems]. We consider that these potential problems are unlikely to have played a significant role in the molecular analysis of artiodactyl and cetacean phylogeny because of the diversity of the genes analyzed (e.g. alignment was trivial for some of these loci). On the other hand, we consider that it has not been convincingly demonstrated that well-known problems inherent to morphological data (such as the difficulties in objectively defining morphological characters and character states) have not played a significant role in the support of the artiodactyl monophyly hypothesis.”
The bold result (!!)
“When all of the morphological and molecular data available to date are considered, the suggestion that cetaceans are nested within the artiodactyl phylogenetic tree (making artiodactyls paraphyletic) is the best-supported hypothesis for the origin of whales. To acknowledge that whales, dolphins and porpoises are, most likely, highly derived artiodactyls, we consider that the taxonomic rank of cetaceans should be lowered from an ordinal level and included in the order Artiodactyla.”That’d be fun.
Get ready to sink your teeth into Chapter 5, “Phylogenetic and Morphometric Reassessment of the Dental Evidence for a Mesonychian and Cetacean Clade”
(!!) This just in (as of October 1998): Thewissen jacked up the stakes on this 1998 book with a quick paper in Nature (10/98), in which he and co-authors described fragmentary archaeocete ankle bones, which didn’t have a trochleated astragalus, making things a bit more interesting. Still, the ankle bones did have a feature found only in artiodactyls and Eocene cetaceans, so the taken all together, the artiodactyls seem favored over the mesonychians.
I’ll go into more detail when I get to Thewissen’s own chapter, Chapter 16. The January 1999 issue of Scientific American had a short write-up about the whole state of affairs on page 26 (in case anyone might claim there was a cover-up).
This page was last updated September 1, 2001.