Nature's Microevolutionary Gems Part 1: Lizards, Fish, Snakes, and Clams
Size Matters in Stickleback fish and Anolis lizards
One of Nature's "gems" was said to show "Natural selection in speciation." The study cited found that reproductive isolation between different populations of stickleback fish was established based upon the trait of body size: "Levels of reproductive isolation are well accounted for by differences in a single trait, body size." Since body size can be determined by genes or the environment, it wasn't entirely clear whether the selected traits were heritable. In fact, when individuals from different stickleback populations were manipulated in the lab to the "preferred" body size of a different "ecotype" (e.g. a member of the same species from a different habitat), mating occurred even though the sticklebacks had previously been from different reproductively isolated populations.
While this study is a nice demonstration of how assortative mating can lead to sympatric speciation (so long as we define "speciation" as mere "reproductive isolation" and don't expect significant morphological change), what this shows is that even after untold generations of reproductive isolation, these fish are still reproductively compatible so long as they like the "size" of their partner. And what sort of morphological divergence is observed between the different stickleback populations? A difference of 2-3 centimeters in length. It goes without saying that small changes in the size of stickleback fish are not going to explain the evolution of sticklebacks in the first place. Have we really witnessed differences that show large-scale evolutionary change is possible, or even "speciation"?
(See Jeffrey S. McKinnon, Seiichi Mori, Benjamin K. Blackman, Lior David, David M. Kingsley, Leia Jamieson, Jennifer Chou & Dolph Schluter, "Evidence for ecology's role in speciation," Nature, 429:294-298 (May 20, 2004).)
Another "gem" claimed to find "Natural selection in lizards." Well, it wasn't exactly "natural" selection. Somewhat like the way researchers once glued peppered moth on trees to see if they'd be eaten by birds, evolutionary researchers artificially introduced a predatory lizard to small islands in the Caribbean to see if there was any impact upon populations of smaller Anolis lizards native to the islands. And they didn't just introduce the lizards to the island. They also artificially released "curly-tailed" predatory lizards right in front of their would-be prey, the lizard species Anolis sagrei, to see how Anolis lizards would respond. Here's what your taxpayer-funded NSF grant dollars supported:
On four of the experimental islands, we conducted focal animal observations on individual A. sagrei to investigate their immediate reaction to the introduction of curly-tailed lizards. Lizards were approached and an experimental object -- either a live curly-tailed lizard (n ¼ 24) or, as a control, an inanimate object of approximately the same size (n ¼ 23) -- was placed 0.5-1.0m from the lizard on the ground and clearly in its visual field.
(Jonathan B. Losos, Thomas W. Schoener & David A. Spiller, "Predator-induced behaviour shifts and natural selection in field experimental lizard populations," Nature 432:505-508 (November 25, 2004).)
That the experiment was not entirely "natural" is no great reason to criticize it and in fact it does serve as a nice illustration of what natural selection might be able to do. Confirming prior studies, the Anolis lizards were found to undergo selection for both larger body sizes (in females) and longer limb (in males) because this allowed them to better escape the predatory "curly-tailed" lizards. And Anolis lizards may be small but they aren't stupid: they also started spending less time on the ground and perched higher up in trees to escape their newfound predators. I'm sure that the slower, smaller Anolis lizards didn't appreciate falling prey (literally) to this experiment -- in Darwin's words, such experiments show "Nature red in tooth and claw" at its finest. But we've still seen nothing beyond extremely small-scale changes in lizard sizes. Much like the peppered moth story said nothing about the origin of moths, what does this study tell us about the origin of lizards? Not much.
Toxic Examples of Evolution
It's long been discussed by critics of neo-Darwinian that the evolution of antibiotic resistance entails the evolution of essentially no new functional biological information in the genome. Nature calls "Toxin resistance in snakes and clams" an "evolutionary gem," but what's really going on in the studies cited?
In the case of snakes, a species of garter snakes predate upon certain newts which produce the toxin tetrodotoxin (TTX). The toxin "causes paralysis and death by binding to the outer pore of voltage-gated sodium channels and blocking nerve and muscle fiber activity." It turns out that by substituting valine for isoleucine in a gene for a particular protein involved in the sodium ion channel, a small amount of resistance to TTX is gained. A couple other amino acid substitutions in certain snake species also seem to confer additional resistance. Meanwhile, the sodium ion channels continue to perform their functions. So we see that toxin-resistance requires small-scale genetic changes that entail the origin of no new genes.
(See Shana L. Geffeney, Esther Fujimoto, Edmund D. Brodie III, Edmund D. Brodie Jr, & Peter C. Ruben, "Evolutionary diversification of TTX-resistant sodium channels in a predator-prey interaction," Nature 434:759-763 (April 7, 2005).)
As for the clams, the packet reports that "Resistance to the toxin in the exposed populations is correlated with a single mutation in the gene that encodes a sodium channel, at a site already implicated in the binding of saxitoxin." (See V. Monica Bricelj, Laurie Connell, Keiichi Konoki, Scott P. MacQuarrie, Todd Scheuer, William A. Catterall & Vera L. Trainer, "Sodium channel mutation leading to saxitoxin resistance in clams increases risk of PSP," Nature 434:736-767 (April 7, 2005).)
In both cases, we're talking about strong selection pressure causing a couple changes (or even just one change) in the amino acid sequence of structural proteins. No new functions or structures are evolving and all we've seen is the loss of the ability of a toxin to bind to its target -- a protein involved in sodium channels. This is similar to the breaking down of a function -- losing the ability to bind through a mutation. Interesting and important research for sure, but if we're trying to showcase "just what is the evidence" for the grander claims of Darwinian evolution, this will not suffice.