Creationists are fond of the “it can’t happen” argument: they like to point to things like the complexity of the eye or intricate cell lineages and invent bogus rules like “irreducible complexity” so they can claim evolution is impossible. In particular, it’s easy for them to take any single organism in isolation and go oooh, aaah over its elaborate detail, and then segue into the argument from personal incredulity.
Since Evolgen recognizes the importance of evo-devo, I’ll return the favor: bioinformatics is going to be critical to the evo-devo research program, which to date has emphasized the “devo” part with much work on model systems, but is going to put increasing demands on comparative molecular information from genomics and bioinformatics to fulfill the promise of the “evo” part. I’m sitting on a plane flying east, and to pass the time I’ve been reading a very nice review of the concept of modularity in evo-devo by Paula Mabee (also a fish developmental biologist, and also working in a small college in a small town in the midwest…but rather deservedly better known than yours truly). In addition to summarizing the importance of the concept of modularity to evolution and development, the paper also does something I always appreciate: it summarizes the key questions that the modern evo-devo research program is working to answer.
Fellow scienceblogger Evolgen has seen the light—evo-devo is wonderful. He’s attending a meeting and listening to some of the bigwigs in the field talk about their work, in particular some research on the evolution of gene regulation. While noting that this is clearly important stuff, he also mentions some of the bickering going on about the relative importance of changes in cis regulatory elements (CREs) vs. trans acting elements, transcription factors. I’ve got a longer write-up of the subject, but if you don’t want to read all of that, the issue is about where the cool stuff in the evolution of morphology is going on. Transcription factors are gene products that bind to regulatory regions of other genes, and change their pattern of expression. The things they bind to are the CREs, which are non-coding regions of DNA associated with particular genes.
One of the most evocative creatures of the Cambrian is Anomalocaris, an arthropod with a pair of prominent, articulated appendages at the front of its head. Those things are called great appendages, and they were thought to be unique to certain groups of arthropods that are now extinct. A while back, I reported on a study of pycnogonids, the sea spiders, that appeared to show that that might not be the case: on the basis of neural organization and innervation, that study showed that the way pycnogonid chelifores (a pair of large, fang-like structures at the front of the head) were innervated suggested that they were homologous to great appendages. I thought that was pretty darned cool; a relic of a grand Cambrian clade was swimming around in our modern oceans.
However, a new report by Jager et al. suggests that that interpretation may be flawed, and that sea spider chelifores are actually homologous to the chelicerae of spiders.
I’m going to introduce you to either a fascinating question or a throbbing headache in evolution, depending on how interested you are in peculiar details of arthropod anatomy (Mrs Tilton may have just perked up, but the rest of you may resume napping). The issue is tagmosis.
I just learned (via John Lynch) about a paper on cetacean limbs that combines developmental biology and paleontology, and makes a lovely argument about the mechanisms behind the evolution of whale morphology. It is an analysis of the molecular determinants of limb formation in modern dolphins, coupled to a comparison of fossil whale limbs, and a reasonable inference about the pattern of change that was responsible for their evolution.
One important point I’d like to make is that even though what we see in the morphology is a pattern of loss—whale hindlimbs show a historical progression over tens of millions of years of steady loss, followed by a near-complete disappearance—the molecular story is very different. The main players in limb formation, the genes Sonic hedgehog (Shh), the Fgfs, and the transcription factor Hand2, are all still present and fully functional in these animals. What has happened, though, is that there have been novel changes to their regulation. Even loss of structures is a consequence of changes and additions to regulatory pathways.
I’m going to briefly summarize an interesting new article on cnidarian Hox genes…unfortunately, it requires a bit of background to put it in context, so bear with me for a moment.
First you need to understand what Hox genes are. They are transcription factors that use a particular DNA binding motif (called a homeobox), and they are found in clusters and expressed colinearly. What that means is that you find the Hox genes that are essential for specifying positional information along the length of the body in a group on a chromosome, and they are organized in order on the chromosome in the same order that they are turned on from front to back along the body axis. Hox genes are not the only genes that are important in this process, of course; animals also use another class of regulatory genes, the Wnt genes, to regulate development, for instance.
A gene can only be called a Hox gene sensu stricto if it has a homeobox sequence, is homologous to other known Hox genes, and is organized in a colinear cluster. If such a gene is not in a cluster, it is demoted and called simply a Hox-like gene.
Hox genes originated early in animal evolution. Genes containing a homeobox are older still, and are found in plants and animals, but the particular genes of the Hox system are unique to multicellular animals, and that key organization arrangement of the set of Hox genes in a cluster is more unique still. The question is exactly when the clusters arose, shortly after or sometime before the diversification of animals.
If you take a look at animal phylogeny, an important group are the diploblastic phyla, the cnidarians and ctenophores. They branched off early from the metazoan lineage, and they possess some sophisticated patterns of differentiation along the body axis. We know they have homeobox containing genes that are related to the ones used in patterning the bodies of us vertebrates, but are they organized in the same way? Did the cnidaria have Hox clusters, suggesting that the clustered Hox genes were a very early event in evolution, or do they lack them and therefore evolved an independent set of mechanisms for specifying positional information along the body axis?
Four of my favorite things are development, evolution, and breasts, and now I have an article that ties them all together in one pretty package. It’s a speculative story at this point, but the weight of the evidence marshaled in support of the premise is impressive: the mammalian breast first evolved as an immunoprotective gland that produced bacteriocidal secretions to protect the skin and secondarily eggs and infants, and that lactation is a highly derived kind of inflammation response. That mammary glands may have had their origin as inflamed glands suppurating mucus may not be the most romantic image to arise in a scientific study, but really—they got better and better over the years.
Zimmer has a summary of the new analysis of the human and chimpanzee genomes that suggests that human speciation wasn’t sudden (no surprise there, I don’t think), and that our ancestors dallied with chimpanzee ancestors over a fairly prolonged period. I can’t wait to read the creationist response to that!
I haven’t read the paper yet—I’m still waist-deep in grading hell—but the deadline for grade submission is midnight tonight, and then at last I will be a free man again!
Crap. Coturnix tagged me with this beautiful bird meme, and I am the wrong person to ask. I don’t get out much, preferring to sit in the lab or the library, so my favorite birds are all in pieces and dead. But OK, since he asked…
Bird digits
Bird teeth
Bird brains
Jurassic bird brains
Bird lungs
Oviraptor pelves (does that count?)
Cretaceous bird embryos
Four-winged birds
Waimanu
And Archaeopteryx, of course.
