Sticklebacks, Manatees, and Creationists
The other night our local PBS station re-aired a NOVA two-hour special, What Darwin Never Knew. It was pretty cool stuff, and incidentally featured Sean Carroll of UW Madison. I mention that because I want to digress for a moment. I live in Madison and since July of 2009 have been organizing Madison Science Pub. Every month I invite a different UW science professor to come to Brocach Irish Pub on the downtown square and talk about their field to a very interested, attentive, and inquisitive audience. I have an open invitation to Dr. Carroll to come talk, but he always seems to be too busy or something. Yes, yes, I know he runs a lab, and is Vice President for Science Education at the Howard Hughes Medical Institute, teaches, publishes, has a family, etc., etc., but come on, Sean… free beer! There. I’ve said my piece, back to the matter at hand.
The show was full of great stuff, but I had an authentic “oh wow!” moment about halfway through the program. The scene switched to a lake in British Columbia populated by fish called sticklebacks. Species of sticklebacks in the ocean have “a pair of fins on its belly that are like spikes. They are for defense. The spikes make the stickleback hard to eat,” but the lake sticklebacks have lost them.
Researchers David Kingsley and Dolph Schluter wanted to find out how the lake sticklebacks lost their spikes, and went digging through the fish’s DNA. “We know these genetic switches exist. But they’re still very hard to find,” Kingsley said. “We don’t have a genetic code that lets us read along the DNA sequence and say, ‘There’s a switch,’ to turn a gene on in a particular place.”
(In the show transcript Kingsley is identified with the HHMI. Hey, Kingsley, next time you see Carroll getting something out of the vending machine in the hall, remind him about Science Pub.)
Eventually Kingsley and colleagues found the switch, and sure enough it was mutated and no longer turns on the gene that makes spikes. They believe that this has implications for other more distantly related species and might even explain why manatees lost their legs when they left land for water. (I’ve developed this habit whenever I hear something like this to immediately wonder what Answers in Genesis has to say on the matter. Keep reading to find out.)
Additionally, the sticklebacks teased the researchers with a tantalizing clue. From the show, “the lake stickleback may have lost its spikes, but evolution has left behind some tiny remnants: the traces of bones. And they are lopsided, bigger on the left than on the right.”
“Wouldn’t it be amazing if, in fact,” Kingsley said, “this classic unevenness is the signature of using the same gene to control hind-limb-loss in incredibly different animals?” Well, the evidence continued to mount, because Kingsley and team then examined “boxes” of manatee bones and found the same left-right lopsided pattern. Manatees have left pelvic bones bigger than their right.
After the show I plugged “sticklebacks manatees limb loss” into Google and started clicking links. The most laydude friendly post I found, unsurprisingly, was on one of my favorite sites, sciencedaily.com, published on June 4, 2009. It’s an interesting piece and I highly recommend you read it, but I’ll touch on a few points here.
Mike Shapiro, first author on the paper cited in the Science Daily post, said, “We knew that in many cases of evolution, the same gene has been used over and over again - even in different species - to give the same anatomy.” But here the story gets more complicated, because, at least according to this post, different genes may be responsible for the spike loss in different species of stickleback. “This is very surprising because these species are fairly closely related,” said Shapiro.
What’s noted in this piece that was absent from the PBS show is the actual gene responsible for the sticklebacks spikes: Pitx1. Interestingly, in the last paragraph of the post we read, “While the new study shows different genes can control the same trait in two closely related species of sticklebacks, researchers already knew that in some cases, the same gene can control similar traits in distantly related species. Pitx1 controls loss of the pelvis in threespine sticklebacks and is tied to club foot in humans.”
In fact, it looks like Pitx1 was suspected even further back then the NOVA show, or the 2009 Science Daily post, because I also found this paper from the April 2004 issue of Nature. Shapiro and his co-authors closed the paper saying, “Many other populations show the same left-right asymmetry that is a characteristic feature of Pitx1-linked pelvic reduction in mice … Mutations in or closely linked to the Pitx1 locus may contribute to many other examples of evolutionary reduction of pelvic structures in natural populations, a possibility that can now be tested by further genetic studies and direct analysis of Pitx1 structure and regulation in multiple populations, species and genera.”
So it looks like a pretty compelling case for Pitx1, and regulatory switches related to its expression, being responsible for limb loss in many different species, including manatees. What’s interesting after reading up a bit on a topic like this is to search for it on the Answers in Genesis site. The sheer breadth of topics AiG manages to cover is pretty impressive, until you realize that the depth with which they cover them is, well, very unimpressive.
Plugging Pitx1 into AiG’s search engine finds an article called “How Manatees Lost Their Legs?” by Dr. Georgia Purdom. After recapping the basics of the research, she states that “[t]he changes in the regulatory region of the gene are thought to be quite ‘young’ (10,000-20,000 years ago).” Wait, what? I thought that the world was only about 6000 years old! But it’s okay; she’s just reporting what scientists think. She then adds, “[f]rom a creationist standpoint, this change may have happened as a post-Flood event [after about 2304 BC, that is] when rapid speciation occurred as a result of the drastic changes in environment and predator-prey interactions.”
Now prepare yourself for some weapons-grade irony, folks. In the very next section, under the heading “Where’s the Evidence,” Purdom faults the research for extrapolating the possibility that manatees lost their hind limbs through the same mutation and mechanisms that sticklebacks have in the wild, that have caused club feet in humans, that have caused reduced limbs in lab mice (more in the 2004 Nature paper), because the actual genes in manatees have not yet been sequenced to show the mutations.
There is more research to do, no doubt, but it’s more than reasonable after the work the researchers have done so far to think about Pitx1 in other species, including manatees. But for Purdom to make this the central point of her objection to the research after proposing spikeless sticklebacks the result of “a post-Flood event when rapid speciation occurred” is hilarious. If only AiG were held to the same standard for evidence as actual scientists!
If Purdom’s main problem with the research is that Shapiro “has not examined the Pitx1 gene in manatees yet,” wouldn’t this be a golden opportunity for Dr. Purdom, who has a PhD in molecular genetics, to ask Ken Ham for a budget to do the research and show that there is no correlation between Pitx1 and limb loss or reduce pelvis size in manatees? (This is yet another project for the list of unfunded creationist research that I started assembling in my previous post, of course.) I’m sure that Shapiro, Kingsley and a lot of other scientists, would welcome the opportunity to review her published results as she has done theirs.
(This piece is cross-posted to ScienceDenial.com.)