Whale relations

I've been trying to make some sense of how the baleen whales are related. The baleen whales (also called mysticetes) are divided into four groups: the rorquals, which are fast, sleek whales that engulf their prey in big mouthfuls; the rights, which are slow, bulky surface-skimmers; the grays, which are a bit like rorquals that suck prey out of mud; and pygmy rights, which are small and fast, and probably skimmers, but not well known at all.

How these groups are related is not something that's 100% understood yet, but the past few years have seen some real progress using genetic techniques. I found this paper today: Baleen Whale Phylogeny and a Past Extensive Radiation Event Revealed by SINE Insertion Analysis. Things make a lot more sense now. I'm going to gloss over the genetics, because I don't personally understand the methods well enough to describe them, but basically the researchers use it to create a phylogenetic tree of baleen whale species.

Phylogenetic trees are a tool of what's called cladistics; in a sense, cladistics is Linnean taxonomy 2.0. Linnean taxa (the groups like genus, order, kingdom, and so on -- I mentioned this stuff here) are ultimately arbitrary -- the only absolute unit is the species, and everything else is somewhat relative. It's useful to describe characteristics of certain groups, but it isn't really suited to precisely describing how species are related. The Linnean system is a bit like the Dewey Decimal System: it'll tell you that C.S. Lewis's books are on the same shelf as J.R.R. Tolkien's, but not that the two were close friends who influenced each other's writing.

Cladistics, on the other hand, is more like a genealogy of species. It maps organisms by their evolutionary history in groups called clades, as suggested by shared characteristics like genetics and body structures. All organisms in a given clade are descended from a common ancestor, just like all members of a given family are descended from the same individual -- but in cladistics, the common ancestor is a species, rather than a single organism.

Here's a diagram from the paper I mentioned, which lists all the mysticetes known when it was published:

Common Ancestor
|
+-- Hippopotamus
+-- Cetaceans (Clade A)
     |
     +-- Odontoceti (toothed whales and dolphins)
     +-- Mysticeti (baleen whales, clade B)
           |
           +-- Balaenidae (right whales, clade I)
           |     |
           |     +-- Eubalaena (right)
           |     +-- Balaena (bowhead)
           |
           +-- (Clade C)
                 |
                 +-- Neobalaenidae (pygmy right)
                 +-- (Clade D)
                       |
                       +-- Eschrichtus (gray)
                       +-- (Clade G)
                       |     |
                       |     +-- Balaenoptera musculus (blue)
                       |     +-- (Clade H)
                       |            |
                       |            +-- Balaenoptera borealis (sei)
                       |            +-- Balaenoptera edeni or brydei (see below)
                       |
                       +-- (Clade F)
                       |     |
                       |     +-- Megaptera (humpback)
                       |     +-- Balaenoptera physalus (fin)
                       |
                       +-- (Clade E)
                             |
                             +-- Balaenoptera acutorostrata (minke)
                             +-- Balaenoptera bonaerensis (Antarctic minke)

To be precise, where I wrote "Clade A", that actually refers to the common ancestor species of everything in Clade A. (I've named the clades here in the same way as the paper, since most of them don't have names in the Linnean system.) If you start at a given clade and follow the lines down or to the right, any species you reach belongs to that clade.

A set of data represents a single historical tree, but it's not always 100% which of several possible arrangements of the data is correct. Clade D, for example, actually contains some conflicting genetic data that can't be read as a neat binary tree. This suggests the clade diverged into new species very quickly from the ancestor species, which doesn't leave as clear a genetic trail to follow.

EDIT: What this cladogram says, then, is that of the whales that are still around, the first to diverge from their most recent common ancestor were the Balaenidae; the rest, in Clade C, are descended from an earlier sister species to the Balaenidae. Neobalaenidae diverged from Clade C's common ancestor, and then for some reason, the Eschrichtiidae and all the Balaenopteridae rapidly diverged from the Clade D common ancestor in an order that's not quite clear. The paper cites other research that suggests this happened about 10-20 million years ago.

Oh, a quick note about Bryde's whales: after this paper were published, it was shown that there are actually two, and possibly three, species of Bryde's. The original Bryde's, which was B. edensi, is now B. brydei; the Eden's or pygmy Bryde's is now B. edensi. I don't know which species the data actually refers to (possibly both), but in all likelihood, the two are so closely related that it doesn't change things.

So here's what I got from all this:

* Sperm whales are in the same clade as the rest of the toothed whales. There was some evidence presented a few years back that they might be more closely related to the mysticetes, but I don't think this is very likely.

* Right whales are the most basal mysticetes -- 'basal' means it diverged from the common ancestor earlier, as in 'more base'. I'll have to look into whether paleontologists have an idea what that ancestor looked like.

* Gray whales are very closely related to the rorquals, if not one of them.

* The genus names, or rather the morphology they're based on, don't reflect the actual relationships of these animals. Both papers mention this uncertainty. A second paper that I'm reading says that the cetacean families mostly correspond to distinct clades, with the exception of the rorquals and grays; I haven't looked at the toothed whales as much, but I do know their phylogeny has been studied more thoroughly.