In my previous comments about maternal effect genes, I was talking specifically about one Drosophila gene, bicoid, which we happen to understand fairly well. We know its sequence, we know how it is controlled, and we know what it does; we know where it falls in the upstream and downstream flow of developmental information in the cell. So today I’m going to babble a bit more about what bicoid is and does, and how it works.
In developmental biology, and increasingly in evolutionary biology, one of the most important fields of study is deciphering the nature of regulatory networks of genes. Most people are familiar with the idea of a gene as stretch of DNA that encodes a protein in a sequence of As, Ts, Gs, and Cs, and that’s still an important part of the story. Most people may also be comfortable with the idea that mutations are events that change the sequence of As, Ts, Gs, and Cs, which can lead to changes in the encoded protein, which then causes changes in the function of the protein. These are essential pieces in the story of evolution; we do accumulate variations in genes and gene products over time.
Last week, I received some delusional e-mail from Phil Skell, who claims that modern biology has no use for evolutionary theory.
This will raise hysterical screeches from its true-believers. But, instead they should take a deep breath and tell us how the theory is relevant to the modern biology. For examples let them tell the relevance of the theory to learning…the discovery and function of hormones…[long list of scientific disciplines truncated]
Dr Skell is a sad case. He apparently repeats his mantra that biology has no need of evolution everywhere he goes, and has never bothered to actually crack a biology journal open to see if biologists actually do use the theory. In my reply to him, I did briefly list how evolution is used in every single one of his numerous examples, but today I’m going to focus on just the one I quoted above: hormones.
Once upon a time, as a young undergraduate, I took a course in neurobiology (which turned out to be rather influential in my life, but that’s another story). The professor, Johnny Palka, took pains at the beginning to explain to his class full of pre-meds and other such riff-raff that the course was going to study how the brain works, and that we were going to be looking at invertebrates almost exclusively—and he had to carefully reassure them that flies and squid actually did have brains, very good brains, and that he almost took it as a personal offense when his students implied that they didn’t. The lesson was that if you wanted to learn how your brain worked, often the most fruitful approach was an indirect one, using comparative studies to work out the commonalities and differences in organization, and try to correlate those with differences and similarities in function.
At about that time, I also discovered the work of the great physiologist, JZ Young, who had done a great deal of influential work on the octopus as a preparation for studying brain and behavior. (Young, by the way, went by the informal name “Jay-Zed”, and there you have another clue to my affectation of using my first and middle initial as if it were a proper name.) It was around then that I was developing that peculiar coleoideal fascination a few of the readers here might have noticed—it was born out of an appreciation of comparative biology and the recognition that cephalopods represented a lineage that independently acquired a large brain and complex behavior from the vertebrates. To understand ourselves, we must embrace the alien.
Young’s attempts to understand mechanisms of learning in memory in the octopus were premature, unfortunately—they have very complex brains, and we made much faster progress using simple invertebrates, like Aplysia, to work out the basics first—but it’s still the subject of ongoing research. I was very pleased to run across a general overview of the octopus brain in The Biological Bulletin.
Maybe half of my audience here will be familiar with this problem. You’re a man, and you’re hauling this massive, ummm, package around in your pants everywhere you go. Other men fear you, while the women worship you…yet at the same time, your e-mail is stuffed to bursting with strange people making friendly offers to help you make it even bigger. It’s a dilemma; you think you would be even more godlike if only it were larger, but could there possibly be any downside to it? (There is a bit of folk wisdom that inflating it drains all the blood from the brain, but this is clearly false. Men who are stupid when erect are also just as stupid when limp.)
A couple of recent studies in fish and spiders have shown that penis size is a matter of competing tradeoffs, and that these compromises have evolutionary consequences. Guys, trash that e-mail for penis enlargement services—they can make you less nimble in pursuit of the ladies, or worse, can get you killed.
One of many open questions in evolution is the nature of bilaterian origins—when the first bilaterally symmetrical common ancestor (the Last Common Bilaterian, or LCB) to all of us mammals and insects and molluscs and polychaetes and so forth arose, and what it looked like. We know it had to have been small, soft, and wormlike, and that it lived over 600 million years ago, but unfortunately, it wasn’t the kind of beast likely to be preserved in fossil deposits.
We do have a tool to help us get a glimpse of it, though: the analysis of extant organisms, searching for those common features that are likely to have been present in that first bilaterian; we’re looking for the Last Common Bilaterian by finding the Least Common Denominators among living species. And one place to look is among the flatworms.
Two short articles in this week’s Science link the orb-weaving spiders back to a common ancestor in the Early Cretaceous, with both physical and molecular evidence. What we have is a 110-million-year-old piece of amber that preserves a piece of an orb web and some captured prey, and a new comparative study of spider silk proteins that ties together the two orb-weaving lineages, the Araneoidea and the Deinopoidea, and dates their last common ancestor to 136 million years ago.
Araneoids and Deinopoids build similar looking webs—a radial frame supporting a sticky spiral—but they differ in how they trap prey. Deinopoids spin dry fibers that they fluff into threads that adhere electrostatically to small insects; Araneoids secrete glue onto the the strand, which takes less work (no fluffing), and is much more strongly adhesive. The differences are enough to make one question whether there was a single origin of orb weavers, or whether the two groups independently stumbled on the same efficient form of architecture.
PvM at the Panda’s Thumb has already written a bit about this issue in the article “Human Gland Probably Evolved From Gills”, but I’m not going to let the fact that I’m late to the party stop me from having fun with it. This is just such a darned pretty story that reveals how deeply vertebrate similarities run, using multiple lines of evidence.
The diagram above shows the early cleavages of the embryo of the scaphopod mollusc, Dentalium. You may notice a few peculiarities: the first cleavage is asymmetric, producing a cell called AB and a larger sister cell, CD. Before the second division, CD makes a large bulge, called a polar lobe, and it almost looks like it’s a three-cell stage—this is called a trefoil embryo, and can look a bit like Mickey Mouse. The second division produces an A, a B, a C, and a D cell, and there’s that polar lobe, about as large as the regular cells, so that it now resembles a 5-cell embryo. What’s going on in these animals?
That Stephen Hawking guy is saying that we need to get colonies out there in space to preserve the human race. I’m a space opera fan, I think space exploration is a worthy endeavor, but I have to admit that watching Chris Clarke whomp on Hawking is very entertaining, and I agree. Hawking has it all wrong.
