February 9, 2012 | David F. Coppedge

How the Zebra Got Its Just-So Story

Evolutionists have updated Kipling’s fanciful story, “How the Leopard Got Its Spots,” with a new, improved, scientific tale, “How the Zebra Got Its Stripes.”  They actually gathered empirical data to show for it.  Not all evolutionists are convinced, however, that it changes the genre from just-so story to scientific explanation.

So how did the zebra get its stripes?  Previous theories explained the stripes as camouflage to confuse predators, as body temperature regulators, or patterns for recognizing mates, but none have proven satisfactory alone.  The new story is that they evolved the elaborate coat patterns to repel horseflies.  PhysOrg reported how Gabor Horvath, Susanne Akesson and colleagues from Hungary and Sweden performed field experiments to show that striped patterns attracted fewer bloodsucking insects.  BBC News shows the experimental setup.  The team erected four fake horses in a horsefly-infested field.  They were dressed in black, white, brown and striped hides coated with sticky material to catch bugs.  By surprise, the striped pattern attracted fewer flies than the light and dark hides.

The scientists even came up with a physical explanation.  The stripes defeat the polarized light the bugs use to hone in on their targets.  “”We conclude that zebras have evolved a coat pattern in which the stripes are narrow enough to ensure minimum attractiveness to tabanid flies,” the team said.  “The selection pressure for striped coat patterns as a response to blood-sucking dipteran parasites is probably high in this region,” meaning Africa, even though the experiment was conducted in a field near Budapest.

Rachel Kaufman at National Geographic News had some words of caution about this explanation:

1.  The study was not conducted on live zebras, but on models.

2.  it was not conducted in Africa, where the zebras live.

3.  There may well be other factors the flies sense, like the zebra’s breath or heat, that are the primary attractors.

4.  Human breeding may have altered factors in horses that changed their attractiveness to flies.

5.  The BBC News article added that the stripes may have multiple explanations, not just one.

6.  The BBC News article also questioned why, if stripes are so effective at repelling flies, other mammals did not follow suit.

Other challenges can be lobbed against the conclusion.  For instance, did they test whether the stripes simultaneously deter mosquitoes, houseflies, lice, fleas, or bees?  Stripes that repel horseflies might backfire and attract other pests.  Did they test for pleiotropic effects – i.e., if natural selection favored stripes, did the genetic mutations cause deleterious effects elsewhere?  Given the ambiguity of the results, the explanation found on Uncommon Descent might work as well as any other:  “Don’t bite!! I’m just the test pattern. World of Blood comes on in a minute.”

We do not fault the team for trying.  Getting out in the field and performing a clever experiment is certainly better than armchair theorizing. Putting forward a physical theory like polarized light is also classical scientific method that can be tested.   The team has added some information to our knowledge – just not necessary and sufficient knowledge to explain the origin of the stripes.  (Another data point in the articles is worth noting: zebra embryos start out black; the stripes materialize before birth. )

The story illustrates a difficulty biologists have with scientific explanations: for every rule, there are exceptions.  If stripes-as-bug-repellants were a law of nature, all animals would have them, and the flies would go extinct or learn to eat plants.  Do zebra finches have stripes to deter houseflies?  Do zebra fish have stripes to deter water fleas?  When science has to keep changing the law of nature to account for different instances of the same pattern, or keep multiplying the laws of nature to account for different patterns, it’s hard to boast of a unified theory.

The main fault in the study is the use of evolutionary theory as an assumption.  The stripes “evolved to” do something: e.g., “The team wondered whether the zebra’s stripy hide might have evolved to disrupt their attractive dark skins and make them less appealing to voracious bloodsuckers,” PhysOrg stated.   This is the just-so element of the story.  It is fallacious on two accounts: (1) Evolution, being an unguided, purposeless, moment-by-moment mechanism, cannot “evolve to” do anything.  (2) Making the “zebra’s stripy hide” the subject of “to disrupt” is a personification fallacy.  Hide cannot do anything (except maybe hide).

A third problem with the evolutionary assumption is it rules out from the outset any non-evolutionary explanation, like design.  A zebra suit has fashion written all over it.  That’s why intelligent clothiers design gowns on the pattern (example).  It should be obvious that fashion models wear such gowns to attract mates (not deter flies) by design.  If the Designer of animals is intelligent, he might have his own purposes for outfitting each creature differently, including artistic as well as functional reasons.

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