Over the next few days I’ll post a few “holiday snaps” from my week-long trip to Texas: beasts, boots, and BBQ. Today we have a very rare animal I was privileged to see in central Texas, the Texas blind salamander (Eurycea rathbuni). It’s a cave-dwelling animal, of course, and so its eyes are either absent or vestigial and, like many cave beasts, it’s lost its pigment. Highly endangered, it’s endemic to the underground caves found in the Edwards Aquifer around San Marcos, which is itself about 40 miles south of Austin. It’s also completely aquatic, and so breathes through external gills, which it can regenerate if damaged (a Federal wildlife guy told me that he once had a salamander whose gills were completely ablated, and it managed to breathe through its skin until the gills grew back).
They can grow up to about six inches, and eat invertebrates. You can see a nice film of them here (note that they give the species name of Typhlomolge rathbuni; the genus name has been updated).
Here’s a picture of one I took at the San Marcos National Fish Hatchery and Technology Center, where I was kindly given a tour. You can see the vestigial eyes, nearly completely covered with skin. Only the left eye is visible: variable and asymmetric expression of vestigial traits is common. The eyes are much more visible in young salamanders, and then gradually disappear as the skin grows over them.
I need hardly point out that the presence of nonfunctional, vestigial eyes in an animal descended from ancestors that had functioning eyes is pretty good evidence for evolution (see chapter 3 of Why Evolution is True).
Here’s a close-up of the head and gills. This individual has no visible eyes (variable expression among individuals!). I was told that the large swellings at the base of the head contain neurons for processing olfactory information: this beast lives by smelling rather than seeing. This guy also has a bit more pigment than the one above. (I say “guy,” but I don’t know the sex. The only way to sex these things is to hold them up against a strong light and look for either eggs or sperm ducts.)
Phylogenies show that, like all cave animals, the blind salamander descended from a sighted species that lived aboveground. Why do cave animals lose their eyes? There are several explanations. One is that, since they’re not useful in the dark, mutations that make them degenerate are not selected against. The other two explanations involve positive selection. Eyes are easily damaged and infected, and if you don’t need them, individuals with mutations reducing the eyes are less likely to risk injury/infection.
Alternatively, eyes take metabolic energy and bodily substance to build; if you don’t need them, you may have higher fitness if you divert that energy away from eyes and toward other structures or functions that enhance reproduction. And of course all three factors could operate together. I don’t think we know in any case the precise evolutionary forces that led loss of eyes (or pigmentation) in a cave species. The hypotheses are reasonable, though I don’t know of any experimental test (one would be to look at the genes for eye or pigment loss and see if the DNA sequence shows a signature of positive selection, though that test can be misleading).
Here are the eggs photographed in water (and slightly out of focus). You can see the tiny newts developing.
And finally, my friend Jim Bull, a UT Austin professor, built a birdhouse in his backyard. It was taken over by a screech owl, who, at twilight, would poke his head out of the hole for an hour or so. Really cute!
This is probably an eastern screech owl (Megascops asio), but I can’t tell from the photo. Both eastern and western screech owls are sympatric in some parts of Texas. This one, like most owls, hunts at night, and probably poked his head out each evening to scope out the surroundings.
Thanks for sharing. Very cool.
It could also be pleiotropy. Studies have indicated that developmental genes, I think sonic hedgehog and tiggy winkle hedgehog, if memory serves, influence eye development. Of course, these genes also influence other structures of the jaw and face. So it could be that in the absence of selection for vision, that positive selection for a different developmental pattern may indirectly result in eyelessness. A wrinkle on the above-mentioned options. Eyelessness could be a byproduct of positive selection for something else.
Correct me if I’m wrong, but the hypothesis “if it’s fitness neutral then mutations accumulate”, is the null hypothesis. Positive selection for NOT having eyes requires an extra step – parsimony argues against those hypotheses. Perhaps I have some bias I’m not recognizing, but to me this is one of those “stands to reason” self evident things – of COURSE it’s just accumulating mutations, why would anyone even consider an alternate hypothesis? What data has indicated that there is ‘something else’ going on? I’d love to be illuminated on this one…
I am not sure how important pleiotropy is with regard to eye degeneration. Indeed, mutation accumulation is an excellent hypothesis. However, there are researches examining pleiotropy. Here is an example.
W. R. Jeffery. Adaptive Evolution of Eye Degeneration in the Mexican Blind Cavefish. Journal of Heredity 2005 96(3):185-196
From the abstract:
“A key discovery is that Hedgehog midline signaling is expanded and inhibits eye formation by inducing lens apoptosis in cavefish embryos. Accordingly, eyes could have been lost by default as a consequence of natural selection for constructive traits, such as feeding structures, which are positively regulated by Hh signaling. We conclude from these studies that eye degeneration in cavefish may be caused by adaptive evolution and pleiotropy.”
The paper is online at this site:
http://jhered.oxfordjournals.org/cgi/content/full/96/3/185
I, having no dog in this fight, am perfectly happy to consider all of the hypotheses. As so often is the case, they could all have an impact. It may be difficult to tease apart the causal contribution of any one of them. Nonetheless, I think pleiotropy is a very interesting possibility.
Ooh! This is exactly what I was looking for, thank you very much!
I always get suspicious of myself when something seems self evident – sometimes it’s a good sign that some strong bias is going on.
Thanks.
Sheees, salamanders and owls. Skip right to the good stuff. Did you find any chupacabras?
Texas is a good place for owl watching. One evening near Houston we managed to see great horned, eastern screech, barred and barn owls — with the help of a birdwatching friend who knew where to look (and had recordings for calling the barred and screech owls).
There is also Ray Lankester’s migrational selection:
http://pandasthumb.org/archives/2008/07/lankesters-migr.html
Like nipples on men and vestigial tailbones on every hominid, fish w/ vestigial eyes have always held a special fascination for me. I believe that vestigial anything is perhaps the simplest and most obvious morsel of evidence that can be offered to refute arguments for a designer god.
More than 200 years ago William Paley speculated that the fine inner workings of a watch necessitated the existence of a “designer.” No one – not even Darwin – ever thought to ask Paley how he ever came to the conclusion that only one god was responsible for this perceived “design.” After all, it took more than one designer to make the first timepiece. Someone had to make the springs, another to make the links for the metal chainwork holding the balance weights and yet another to make the cogs. If one looks back a little further in human history, it becomes obvious that didn’t take too much brainwork to place a verticle rock on a flat, circular one and then sit back, fire up a pipe full and watch the sun cast shadows around it at different points during the day. The Mayans learned this watchmaking trick about 1000 years before Paley and Aquinas were even conceived. Neither of these brainiacs ever contemplated that Mayan rock-timekeeping devices were divinely inspired.
Coyne, in his book, “Why Evolution is True,” makes another interesting postulation that easily refutes “creation” and affirms evolution when he writes: “… the nested arrangement of life was recognized long before Darwin … as biologists began classifying animals and plants, discovering that they consistently fell into what was called a “natural” classification. Strikingly, they came up with nearly identical groupings. This means that these groupings are not subjective artifacts of a human need to classify, but tell us something real and fundamental about nature. The natural classification of nature is itself strong evidence for evolution.” To make his point even clearer he adds: Take cardboard books of matches, which I used to collect. They don’t fall into a natural classification in the same way as living species. You could, for example, sort matchbooks hierarchically beginning with size, and then by country within size, color within country, and so on. There are many ways to order them and everyone will do it differently. There is no sorting system that all collectors agree on. This is because, rather than evolving, so that each matchbook gives rise to another that is only slightly different, each design was created from scratch by human whim.” “Matchbooks,” he wrote, “resemble the kind of creatures expected under a creationist explanation of life.”
My buddy had a screech owl take up residence in a birdhouse just like this one. Same thing, every evening the owl would look out the hole for a while. For his amusement(?), he put a platform on his laundry pole where he would, at dusk, set out a live mouse for the owl. Of course, said owl caught on to the routine in a big hurry. The mouse was snatched and flown away with in short order. The owl never took it back to the box.