One of the things that Pat, Melissa and I were interested in, and which we kept coming back to all the time in the discussions, is if you see an animal behaving in a particular sort of way, or its behavior being modified depending on the environment in which it is in, how do we understand this, whether we understand it as learning, or the expression of innate developmental rules, and also how much is adaptive.

You see a tremendous variety of colors in tropical fish, for example. Do we have to assume that each of those has actually been selected, or not? Let me give you the classical example — Pat uses it in his book and also in his chapter for the book Hilary and I are editing: on some of the islands there are flamingos which are a beautiful pink color. Back even before the present fights, back in the beginning of this century, Thayer, an American naturalist and illustrator suggested that the pink color of the flamingo was an adaptation so they would be less visible to predators against the setting sun.

But in fact the pink color depends on their diet. If they eat a lot of shrimp they go brighter pink, if they don't eat a shrimp diet they go a paler pink. It would be very hard to argue that the pink color was a Darwinian adaptation to protect against predators rather than an epiphenomenon, a consequence of the diet. That's one of the things that is actually an extremely important issue within biology: what's adaptive, what's not adaptive, what's accidental in some sort of ways. And the flamingos make a very good example.

Then Pat and I were talking about another one — I keep cats, he breeds cats, and apart from birds he knows a great deal about cat genetics and cat breeding. We all know that a cat sits on your lap and purrs. Is that purring adaptation? Why do cats purr?

The answer is, we haven't the slightest idea though one can think of it as some type of social signalling mechanism. But how could one study that experimentally? You might exploit natural variation in the amount of purring and look for correlations in the response of social companions or to try rather brutal surgical interventions to de-purr a cat, and you'd have to deafen its litter-mates and its mother as it was being reared; you'd have to look at what effect that had on its behavior and its social organization. No one wants to do an experiment like that.

But if you ask why cats purr, and you asked ultra-Darwinians, they would probably say there has to be an evolutionarily adaptive explanation for it — an alternative could be what Steve Gould calls an exaptation, something which arose during evolution for other reasons or by accident and then got coopted for current purposes, the point is that we need to be much more eclectic and much more open to there being multiple explanations of why anything happens in nature. That there are explanations is clear, but they cannot simply be reduced to the working out of the imperative of the selfish genes.