No Such Thing As A Zebra?

No such thing as a zebra?  Sure there is, it's an African animal, black with white stripes.  Or maybe, a white animal with black stripes?  But, who's asking damn fool questions, when there's a group of them right there!

Well, who's asking the question is Stephen jay Gould in his essay What, If Anything, Is a Zebra?  I wrote about Gould recently in The Science of Writing.  It's a simple question which Gould writes about to illustrate a wider point: that there is no perfect way of classifying organisms, even though there must be a "right answer" if only we knew enough about evolutionary descent.  In his words,

"The potential dilemma for zebras is simply stated: they exist as three species, all with black-and-white stripes to be sure, but differing notably in both numbers of stripes and their patterns. ... Do these three species form a single evolutionary unit? Do they share a common ancestor that gave rise to them alone and to no other species of horse? Or are some zebras more closely related by descent to true horses or to asses than they are to other zebras? If this second possibility is an actuality, ... there is, in an important evolutionary sense, no such thing as a zebra."

To give an extreme example from human life, all redheads are not more closely related to each other than to their parents and siblings!  In fact, Gould concludes that the issue is not proven, that zebras may be a complete group, that the word "zebra" may really mean something precise and biological, but that it may not.

Just to doubly confound us, the same is apparently true of two other common terms - there is certainly no such thing as a fish!  Or a tree!

To quote from Ennos' Trees,

"..trees are not what biologists call a natural group. Whereas other successful groups of plants, such as grasses, all descend from a single common ancestor, the tree form has evolved many times in the course of history; their ancestors were quite unrelated plants."

In fact, there are trees which are grasses, roses, mosses, and clovers, and many, many more.

And to go back to zebras, in How the Zebra Gets Its Stripes, Gould concludes - on evidence from embroyology and pigmentation - that the zebra is a black animal with white stripes, not the other way round.

Comments

[Peter the flautist]

Clustering correctness (re coment by Jno): that was my point. In mutivariate analysis there are a number of techniques for clustering. In different applications different approaches have different utility, I was making a more general point than cladistics, which is a technique whch is indeed appropriate for understanding evolution. However other techniques may be more useful if indeed you wish to select on observable morphology (as most gardeners do). I think we are essentially in agreement.

DP

[Jno]

DP wrote:
> Now there is no "correct" method of clustering

I think the point is that there *is* a correct method of clustering for the purposes of evolutionary biology, viz. cladistics, but the tools and information to cluster organisms in this way are only recently available. The good old Gilbert White approach to natural history, of classifying organisms on the basis of their observable features, results in groups like fish (and possibly zebras) which are not natural groups; that is they do not map to clades.

[Peter the flautist]

Thanks for the comment and the Oyster Band track! Dancing Cat

[lindsay]

Fascinating response from Dark Puss - and a very pleasing one from Rhys. There's another, entirely frivolous, side to this - the fun you have on safari telling the earnest guide that there's no such thing as a zebra!!

[Peter the flautist]

A number of interesting issues are raised by this post. Disregarding evolution (I.e. ignoring any underlying mechanism) I suppose we could regard classifcation of animals/plants/galaxies/music etc. as an exercise in clustering in a multidimensional parameter space. Now there is no "correct" method of clustering, and indeed there are a whole variety of techniques available to the analyst. The question often boils down to "is it useful"? That is if there is a traning set that we can develop the technique with will the technique have a good efficency at separating out the test data with reasonable accuracy. Imperfect sampling, particularly when there is some desirable characteristic, can often generate articicial splits in otherwise conrinuous distributions. For example short dark-skinned, black-haired women look very different from pale-skinned, tall red-headed women, but they are not distinct in terms of the underlying genetic information. Because of our exceptional human ability to recognise patterns, we are very good at making such distinctions and the problem then comes when we try to associate morphology with the evolutionary sequence.

Returning the the Zebra theme, the now extinct Quagga was one of the first (possibly the first) animal to be subject to ancient DNA analysis. The results suggest strongly that the Quagga evolved from the Plains Zebra somewhere between about 150k and 250k years ago, and illustrates how rapidly the coat colour could evolve in an isolated population, possibly under external climatic influences. Another example, from horticulture, the whole classifaction of the genus Rhododendron was revised significantly by Cullen and Chamberlain (Notes from the Royal Botanic Garden Edinburgh 39:1 and 39:2 1980) in a move away from the horticulturally-based approach of Balfour to a more herbarium (i.e. anatomical/cytological/phytochemical) based approach. This caused some uproar in hoticultural circles at the time when beloved distinct species were "shown" to be just variants selected by the great plant hunters who were often particularly on the look out for horticulturally intersting variations. Indeed many were funded by nurseries and private collectors.

To conclude, is there a zebra? I think yes in the sense that there is a usefully distinctive group of animals looked at from an external perspective. Are they uniquely distinct, in the same sense that boron is not beryllium, no they are not? DNA based classifcations help us greatly to unravel evolutionary trajectories and relationships, but they won't tell you which "species" to grow in your flower beds if you seek variety.

Dark Puss

[Rhys]

This is all very interesring and thought provoking for a Saturday morning blog walk.