Darwinian Just-So Story Criticized
When Young and Brodie & son published their article “How the Horned Lizard Got its Horns,” (see 04/01/2004 headline), they apparently meant it as a bit of April-fool joke, not a real Kipling-style just-so story. Several respondents in the Sept 24 issue of Science,1 however, either didn’t think it was funny or concluded the story was just-so after all:
- William R. Fouts (Nevada State) was not amused by the Kipling reference, because he viewed their paper as “an important example of natural selection in the wild” and thought the title was a poor choice of words. He thought they should have examined the possibility of preadaptation: i.e., maybe the horns grew out of a nub that once upon a time appeared on the back of the lizard’s neck.
- John H. Christy (Smithsonian) thinks the authors did not prove that the longer horns function in defense against shrike predation. In his opinion, therefore, the authors’ explanation for the adaptive function of the horns is still a just-so story.
- R. Yosef described how shrikes actually kill their prey, and then whimsically remarked, “I suggest that the parietal horns developed as a defense against shrike attacks to the nape region and not against their being impaled after they are dead,” because clearly, “it does not make evolutionary sense for a trait to be incorporated into a prey species, as a result of a predator’s behavior, that results in all cases in its death (i.e., the impaling stage).”
The authors thanked the respondents for their insights on issues they claim were not discussed in the original paper due to space limitations. But then he chided them for not getting the joke: “The title of our paper was meant as an allusion to the Just So Stories of Kipling, which are often used as a shorthand criticism for unsubstantiated adaptive arguments. It is a bold statement, and we thought it so clearly over the top that it would not be taken as a literal explanatory title.” (Emphasis added in all quotes.)
1Letters to the editor, Science, Vol 305, Issue 5692, 1909-1910, 24 September 2004, [DOI: 10.1126/science.305.5692.1909b].
Here was a rare, valiant attempt at providing just one clear, unambiguous association between a trait and a survival advantage, and even their fellow evolutionists were not convinced. So what are the rest of us supposed to think about the validity of adaptive stories in the Darwinian tradition?
The critics’ points were pointless as far as helping Darwin. A nub turns into a horn, right. Silly. For support of the “preadaption” or “exaptation” hypothesis, Fouts refers to the panda’s thumb and tetrapod limbs (see 04/05/2004 headline). How does said nub get into the genome and developmental pathways, and become established in the population before it functions in defense? Darwinism allows no foresight, yet Fouts argues:
Perhaps the role of preadaptation in evolution is of great importance and is deserving of more widespread appreciation. Given the possibility of a preadaptation scenario in the evolution of crown horns in horned lizards, I find it ironic that Young et al. commented on the weakness of “just-so stories” and also chose a title that reads remarkably like the titles of Kipling’s stories. Until presented with evidence suggesting that the horns were mere nubs until the onset of shrike predation, I will remain convinced that “How the horned lizard got its horns” is a poor choice for what is presumably meant to be an informative title.
Sheesh, think the authors; can’t a guy take a joke? Their response undermines the hope of proving a trait arose by evolution:
The question of whether any horns on the head of horned lizards existed before shrike predation drove them to elongated states (i.e., were “preadapted”) is an interesting one, but one that is only answerable through comparative analyses with full phylogenetic information and ancestral environmental conditions. Although we have not performed such an analysis and could probably never reconstruct the ancestral predation conditions, it is worth noting that of the 13 species of horned lizards currently extant, P. mcalli has the longest relative horn lengths and belong to the most derived species group, while some other species in the genus (e.g., P. douglassi) have virtually no parietal or squamosal horns (i.e., the nubs mentioned by Fouts).
So how did the nub-challenged lizards get along? If nubs are cool, every lizard would want some, especially when the shrikes are dive-bombing down on their necks. Yikes! Shrikes! Man your nub stations! (Or do they say, “lizard your nub stations”?) It’s survival of the nubbiest. May the best nubs win!
The authors agreed with Christy’s comments, but in so doing, again undermined any hope of providing a Darwinian explanation for anything:
Christy correctly points out the two primary shortcomings of any covariance analysis of selection: It is impossible to rule out every unknown unmeasured character that could drive the observed selection, and covariance analyses usually cannot assign a mechanism of selection because they are not manipulative studies.
Yosef didn’t get it, either. Of course they didn’t mean that selection acted after the lizard was impaled on the tree; they merely assumed that longer horns prevented attacks in the first place. Obviously, they couldn’t ask the shrikes how they feel about the effectiveness of the horns, so they relied on personal experience. Visualize the scientist at work: “When attacked or grasped, flat-tailed horned lizards stab their spines into the offending object. In the case of human fingers, this behavior often results in bleeding and immediate release of the lizard.” Ha! This obviously means they evolved to ward off junior-age kids.
So yes, as entertainment, the original article and the criticisms are “clearly over the top.” Why do you think the Darwin Party is so sensitive to the charge of storytelling? Guilty conscience? We feel honored to be included among those who, in the spirit of promoting good science, often use the phrase “just-so stories” as a “shorthand criticism for unsubstantiated adaptive arguments.” Grab your baloney detector and join the fun.