When you ask Darwin’s question – why are phenotypes often similar? – you do indeed get Darwin’s answer. But if you ask instead why it is that some phenotypes don’t occur, an adaptationist explanation often sounds somewhere between implausible and preposterous. For example, nobody, not even the most ravening of adaptationists, would seek to explain the absence of winged pigs by claiming that, though there used to be some, the wings proved to be a liability so nature selected against them. Nobody expects to find fossils of a species of winged pig that has now gone extinct. Rather, pigs lack wings because there’s no place on pigs to put them. To add wings to a pig, you’d also have to tinker with lots of other things. In fact, you’d have to rebuild the pig whole hog: less weight, appropriate musculature, an appropriate metabolism, an apparatus for navigating in three dimensions, a streamlined silhouette and god only knows what else; not to mention feathers. The moral is that if you want them to have wings, you will have to redesign pigs radically. But natural selection, since it is incremental and cumulative, can’t do that sort of thing. Evolution by natural selection is inherently a conservative process, and once you’re well along the evolutionary route to being a pig, your further options are considerably constrained; you can’t, for example, go back and retrofit feathers.
tony curzon price on 16 Dec 07"When a sheep is ailing," said Socrates, "we generally blame the shepherd, and when a horse is vicious, we generally find fault with his rider. In the case of a wife, if she receives instruction in the right way from her husband and yet does badly, perhaps she should bear the blame; but if the husband does not instruct his wife in the right way of doing things, and so finds her ignorant, should he not bear the blame himself? [12] Anyhow, Critobulus, you should tell us the truth, for we are all friends here. Is there anyone to whom you commit more affairs of importance than you commit to your wife?" "There is not." "Is there anyone with whom you talk less?" "There are few or none, I confess." [13] "And you married her when she was a mere child and had seen and heard almost nothing?" "Certainly." "Then it would be far more surprising if she understood what she should say or do than if she made mistakes." [14] "But what of the husbands who, as you say, have good wives, Socrates? Did they train them themselves?" "There's nothing like investigation. I will introduce Aspasia to you, and she will explain the whole matter to you with more knowledge than I possess. [15] I think that the wife who is a good partner in the household contributes just as much as her husband to its good; because the incomings for the most part are the result of the husband's exertions, but the outgoings are controlled mostly by the wife's dispensation. If both do their part well, the estate is increased; if they act incompetently, it is diminished. [16] If you think you want to know about other branches of knowledge, I fancy I can show you people who acquit themselves creditably in any one of them."