Would you immediately assume that it could be an evolutionary accident with no adaptive significance then start to wonder if you could rule out such an explanation? Can random genetic drift of neutral alleles explain the zebra's stripes?
Or would you immediately start thinking of adaptive explanations for why all three extant species of zebras have stripes but no other large mammals in the same environment are striped. Most other horses don't have prominent stripes but many have faint stripes on some parts of their bodies (Darwin, 1859).
I argue that you have to rule out the null hypothesis (drift) before invoking adaptationist explanations. In other words, the first question you need to ask is whether zebra stripes are adaptive. But that's not the adaptationist approach. Adaptationists begin with the assumption that stripes are adaptive, then they start looking for adaptive explanations.
What if the favorite adaptive explanation is refuted? What does an adaptationist do next? Gould and Lewontin (1978) provide the answer ...
We would not object so strenuously to the adaptationist programme if its invocation, in any particular case, could lead in principle to its rejection for want of evidence. We might still view it as restrictive and object to its status as an argument of first choice. But if it could be dismissed after failing some explicit test, then alternatives would get their chance. Unfortunately, a common procedure among evolutionists does not allow such definable rejection for two reasons. First, the rejection of one adaptive story usually leads to its replacement by another, rather than to a suspicion that a different kind of explanation might be required. Since the range of adaptive stories is as wide as our minds are fertile, new stones can always be postulated. And if a story is not immediately available, one can always plead temporary ignorance and trust that it will be forthcoming .... Secondly, the criteria for acceptance of a story are so loose that many pass without proper confirmation. Often, evolutionists use consistency with natural selection as the sole criterion and consider their work done when they concoct a plausible story. But plausible stories can always be told. The key to historical research lies in devising criteria to identify proper explanations among the substantial set of plausible pathways to any modern result.Let's look at the history of this question as summarized in a recent paper by Egri et al. (2012).
The most characteristic aspects of zebras are the bold black-and-white striped patterns on their body surface (Fig.1). Embryological evidence (Prothero and Schoch, 2003) has shown that the background colour of zebras is black, and the white stripes and bellies (where the production of dark pigmentation is inhibited) appear only in a later embryonic developmental stage. The reason for the striped coat pattern in zebras has long been debated, and Wallace suggested that zebras evolved striped coats as camouflage against carnivores in tall grass (Wallace, 1867; Wallace, 1879). Darwin, however, who had closely studied the inheritance of colours and stripes in horses and zebras, criticized this hypothesis as an explanation (Darwin, 1871), as zebras do not occur in areas with dense vegetation but rather prefer open savannah habitats with short grass.Isn't that interesting? The question has been around for 145 years and nobody has suggested the null hypothesis. It certainly hasn't been ruled out. Why do adaptationist explanations continue to be so popular if most of them have been discredited?
Since the 19th century, a number of alternative hypotheses (Waage, 1981; Ruxton, 2002; Lehane, 2005; Caro, 2009) have been proposed to explain the striped pattern of zebras, including predator defence, social interaction, indication of physical condition, thermoregulation, and protection from tsetse flies (a more detailed account is given in the Appendix). These and more explanations have been thoroughly discussed and criticized by Ruxton (Ruxton, 2002) and Caro (Caro, 2009), who concluded that the majority of these hypotheses are experimentally unconfirmed, and thus the exact cause of stripes in zebras remains unknown. Nevertheless, the explanation of Waage (Waage, 1981) for the benefit of zebra stripes (i.e. protection from tsetse flies) has been the only hypothesis to be partially supported experimentally (Turner and Invest, 1973; Brady and Shereni, 1988; Gibson, 1992; Ruxton, 2002; Lehane, 2005; Caro, 2009).
Egri et al. propose that the stripes on zebras serve to protect them from horseflies (tabanid flies). They carried out several experiments to measure whether horseflies preferred black, white, or striped backgrounds. The results indicate that horseflies tend to avoid landing sites that have lots of stripes. In one experiment the authors measured the number of horseflies that were attracted to full-sized models of zebras that were black, brown, white, or striped. (The photo shows a spotted model but a white one was used in the reported experiment.)
The result was remarkable. The black and brown models attracted 562 and 334 horseflies over 60 days while the white models only attracted 22 horseflies. Only 8 horseflies landed on the striped model in 60 days! The authors conclude ...
We have shown here that the evolution of stripes with brightness and/or polarization modulations disrupting the homogeneous pattern of reflected light might be a selective advantage in avoiding attacks from polarotactic tabanids. We conclude that zebras have evolved a coat pattern in which the stripes are narrow enough to ensure a minimum attractiveness to tabanid flies.There are a lot of steps between showing that flies avoid striped patterns in field experiments and concluding that stripes confer a selective advantage because there ain't so many flies on zebras. Egri et al. have constructed a plausible story that's consistent with adaptation but they have not demonstrated that stripes confer a selective advantage.
The author of the BBC news article (Victoria Gill) sought the opinion of Matthew Cobb who thinks that none of the adaptive explanations is satisfactory (but doesn't entertain any non-adaptive explanations).
Prof Matthew Cobb, an evolutionary biologist from the University of Manchester pointed out that the experiment was "rigorous and fascinating" but did not exclude the other hypotheses about the origin of zebras' stripes.
"Above all, for this explanation to be true, the authors would have to show that tabanid fly bites are a major selection pressure on zebras, but not on horses and donkeys found elsewhere in the world... none of which are stripy," he told BBC Nature.
"[They] recognise this in their study, and my hunch is that there is not a single explanation and that many factors are involved in the zebra's stripes.
[Hat Tip: RichardDawkins.net [Zebra stripes evolved to keep biting flies at bay] who copied a BBC news article.]
Darwin, C. (1859) On the origin of species (Chapter 5).
Egri, A., Blahó, M., Kriska, G., Farkas, R., Gyurkovszky, M., Åkesson, S., and Horváth, G. (2012) Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes. J. Exp. Biol. 215:736-745. [doi: 10.1242/jeb.065540]
Gould, S.J., and Lewontin, R.C. (1979) The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme. Proc. R. Soc. Lon. B Biol. Sci. 205: 581-598. [PubMed] [doi: 10.1098/rspb.1979.0086]