I strongly recommend Larry Moran’s analysis of the paper on mammalian macroevolution that I briefly described earlier today.
Pharyngula
Evolution, development, and random biological ejaculations from a godless liberal
I strongly recommend Larry Moran’s analysis of the paper on mammalian macroevolution that I briefly described earlier today.
The mammalian tree is rooted deeply and branched early!
That’s the message of a new paper in Nature that compiled sequence data from 4,510 mammalian species (out of 4,554) to assembly that lovely diagram above. Challenging the ‘conventional wisdom’ that mammalian diversity is the product of an opportunistic radiation of species after the dinosaurs were wiped out at the end of the Cretaceous 65 million years ago, the authors instead identified two broad periods of evolutionary expansion among the mammals: an early event 100-85 million years ago when the extant orders first appeared, and a radiation of modern families in the late Eocene/Miocene. A key point is that there is no change in rates of taxon formation across the Cretaceous/Tertiary (K/T) boundary—mammalian diversity was rich before the dinosaurs disappeared.
The latest Nature reveals a new primitive mammal fossil collected in the Mesozoic strata of the Yan mountains of China. It’s small and unprepossessing, but it has at least two noteworthy novelties, and first among them is that it represents another step in the transition from the reptilian to the mammalian jaw and ear.
Here is a spectacularly pretty and weird animal: stalk-eyed flies of the family Diopsidae. There are about 160 species in this group that exhibit this extreme morphology, with the eyes and the antennae displaced laterally on stalks. They often (but not always) are sexually dimorphic, with males having more exaggerated stalks—the longer stalks also make them clumsy in flight, so this is a pattern with considerable cost, and is thought to be the product of sexual selection. The Sphyrocephala to the right is not even an extreme example. Read on to see some genuinely bizarre flies and a little bit about the development of this structure.
What do you know…just last week, I posted an article dismissing a creationist’s misconceptions about pharyngeal organization and development, in which he asks about the evidence for similarities between agnathan and gnathostome jaws, and what comes along but a new paper on the molecular evidence for the origin of the jaw, which describes gene expression in the lamprey pharynx. How timely! And as a plus, it contains several very clear summary diagrams to show how all the bits and pieces and molecules relate to one another.
The short summary is that there is a suite of genes (the Hox and Dlx genes, which define a cartesian coordinate system for the branchial arch elements, Fgf8/Dlx1 genes that establish proximal jaw elements, and Bmp4/Msx1 genes that demarcate more distal elements) that are found in both lampreys and vertebrates in similar patterns and roles, and that vertebrate upper and lower jaws are homologous to the upper and lower “lips” of the lamprey oral supporting apparatus.
I was reminded of one of the more comical, but persistent misconceptions by creationists in a thread on Internet Infidels, on The Coelacanth. Try doing a google search for “coelacanth creation” and be amazed at the volume of ignorance pumped out on this subject. I’ve also run across a more recent example of the misrepresentation of the coelacanth that I’ll mention later … this poor fish has a long history of abuse by creationists, though, so here’s a brief rundown of wacky creationist interpretations.
Here’s a nicely focused blog: R. Ford Denison, of the UM Department of Ecology, Evolution, and Behavior, has a new blog titled This Week in Evolution, and he’s planning to put up one post each week summarizing a recently published paper in evolutionary biology. He has specific criteria:
Each week, I plan to discuss a scientific paper that meets the following criteria:
- published during the previous month;
- about some aspect of evolution;
- published after peer review in a journal with a citation impact of at least 1.0 (i.e., no third-tier journals);
- containing significant amounts of data, not just mathematical modeling or discussion.
It’s an excellent plan—check in each week!
Halkieriids are Cambrian animals that looked like slugs in scale mail; often when they died their scales, called sclerites, dissociated and scattered, and their sclerites represent a significant component of the small shelly fauna of the early Cambrian. They typically had their front and back ends capped with shells that resembled those we see in bivalve brachiopods. Wiwaxiids were also sluglike, but sported very prominent, long sclerites, and lacked the anterior and posterior shells; their exact position in the evolutionary tree has bounced about quite a bit, but some argument has made that they belong in the annelid ancestry, and that their sclerites are homologous to the bristly setae of worms. One simplistic picture of their relationship to modern forms was that the halkieriids expanded their shells and shed their scales to become molluscs, while the wiwaxiids minimized their armor to emphasize flexibility and became more wormlike. (Note that that is a very crude summary; relationships of these Cambrian groups to modern clades are extremely contentious. There’s a more accurate description of the relationships below.)
Now a new fossil has been found, Orthozanclus reburrus that unites the two into a larger clade, the halwaxiids. Like the halkieriids, it has an anterior shell (but not a posterior one), and like the wiwaxiids, it has long spiky sclerites. In some ways, this simplifies the relationships; it unites some problematic organisms into a single branch on the tree. The question now becomes where that branch is located—whether the halwaxiids belong in a separate phylum that split off from the lophophorate family tree after the molluscs, or whether the halwaxiids are a sister group to the molluscs.
A reader pointed me to this German documentary (with English subtitles) on evolution and creationism—it has a nice 10 minute primer on mechanisms and evidence for evolution (with evo-devo, especially of fruit flies and zebrafish, prominently mentioned, appropriately enough for the country of Christiane Nusslein-Volhard).
There’s also a segment on creationism that is a bit lacking in nuance—they are all lumped together as young earth creationists—which is the kind of opening creationists use to disavow association with those other kooks, while glossing over the foolishness they do believe. Never mind the theological hairsplitting, though, YECs and IDist are fundamentally identical in their rejection of science for dogma.
Aside from that, it’s a simple introduction to evolution that emphasizes the molecular evidence (yay!), has eye-catching graphics and animations, and scathingly dismisses creationism and the general descent into mystical thinking. Do any of my German readers know of this fellow? Was this broadcast on German television?
The article about gastrulation from the other day was dreadfully vertebrate-centric, so let me correct that with a little addendum that mentions a few invertebrate patterns of gastrulation—and you’ll see that the story hasn’t changed.
Remember, this is the definition of gastrulation that I explained with some vertebrate examples:
The process in animal embryos in which endoderm and mesoderm move from the outer surface of the embryo to the inside, where they give rise to internal organs.
I described frogs and birds and mammals the other day, so lets take a look at sea urchins and fruit flies.
