Tag: Stephen Jay Gould

The wonders of camouflage

April 6, 2009 • 9:55 am

by Matthew Cobb

Hello everyone, Jerry has kindly (or foolishly) handed over the reins of the WEIT blog to me for the next 10 days or so. See if you can spot the difference!

Some of the most spectacular signs of evolutionary  adaptation are the many examples of camouflage shown by animals. Although many examples of camouflage are shown by prey animals seeking to avoid being eaten, predators also use camouflage to avoid detection. My good friend Professor Innes Cuthill of Bristol University, UK, studies animal camouflage, and has just posted this excellent audio slideshow on the BBC website.

Innes describes various cases of how camouflage works in different animal species, and there are some great pictures to go with it. Sometimes, changing color is not actually to do with camouflage – this is the case in chameleons, and  also in octopuses and squid, which can use rapidly changing patterns of skin color to communicate in ways we do not fully understand. And by ‘ways’ I mean both what they are communicating and how they change their color so quickly.

One of the most spectacular examples of cryptic camouflage can be seen in the octopus, in this video:

This is taken (without credit!) from a fantastic five-minute talk on underwater animals by David Gallo at Ted.com, which you can find here and which includes some great interactions between squid at around 2m 40s and some cuttlefish showing fantastic rapidly changing color patterns.

One example of camouflage given by Innes Cuthill is the zebra, which he suggests may have stripes because it disrupts their outline, making it more difficult for predators to decide where the zebra begins and ends. This may be true – but in reality we simply do not know what the adaptive advantage is. Indeed, it is possible that the stripes have nothing to do with what is really going on (they may be simply a side-effect of the true advantage), although that seems unlikely. A non-camouflage explanation is that zebra foals are born into a world of stripes, and that the stripes on their parents help to enable them to identify their fellow-zebras, and reinforce their herd identity.

The problem with all these explanations is that they are what the late Stephen Jay Gould called ‘Just So Stories’, after the children’s fables written by the British author Rudyard Kipling (‘How the elephant got his trunk’, and so on). They fit the facts, and they may be true, but they lack the decisive support that science alone can provide: experimental evidence.

In discussing this with Jerry last week, he pointed out that a simple test of the ‘disruption’ hypothesis to explain the zebra’s stripes would be to paint some all black or all white, and see what happens to the predation rate. I suspect that would not be possible, either ethically or practically, but some kind of experiments on zebras, lions, or both, will be required before we can be really sure why zebras have stripes. Post your explanations – and above all, think of a doable experiment that could test your hypothesis!

Stephen Jay Gould and the origin of jaws

March 4, 2009 • 5:34 pm

by Greg Mayer

There’s been some interesting discussion in the comments on the post on Change we can believe in concerning gradual and punctuational evolution, including the question of whether Stephen Jay Gould ever advocated macromutation. (Among his many accomplishments, Gould joined Niles Eldredge in the explication and elaboration of Eldredge’s initial suggestion of the idea of punctuated equilibria.) Over a very productive 30+ year career, Gould’s views were of course not static, but in a 1980 paper (Is a new and general theory of evolution emerging? Paleobiology 6:119-130) he does advocate macromutation:

Instead, I envisage a potential saltational origin for the essential features of key adaptations. Why may we not imagine that gill arch bones of an ancestral agnathan moved forward in one step to surround the mouth and form proto-jaws?

Gould refers here to the serial homology of vertebrate jaws to the gill skeleton, the discovery of which is one of the triumphs of classic comparative anatomy. He proposes jaws to have arisen from the gill skeleton in a single mutation (i.e. a macromutation). There is always the problem that what’s a big mutation to one person is not big to another, but I think most or all vertebrate morphologists would consider the conversion of a gill arch into a jaw in one step a macromutation. In context, Gould is not arguing that all the features of jawed vertebrates would have arisen at once, but that a very major feature would have. Two arches are involved in the jaws: the mandibular arch, forming the jaws themselves, and the hyoid arch, which supports and suspends the mandibular.  Since there are extinct fishes (the acanthodians) whose hyoid arch is little modified from a gill arch, both of the arches involved in jaws did not change in one step. The exact way in which jaws arose is not known, and is the subject of continuing  anatomical, developmental, molecular genetic, and paleontological research.  New fossils from the Chengjiang Lagerstatte in China are beginning to throw more light on early vertebrate evolution, but are a bit early for the origin of jaws; we may hope, and predict, that further discoveries will shed more light on the origin of jaws.