Darwinism Cannot Explain The Peacock

As far as I know, all animals need vitamin C, but most animals synthesize it while some don't. So how then do those who don't synthesize it get the vitamin C that they need? Through their diet. Not just with vitamin supplements (very rarely, actually! -- only domesticated animals could get those!), but rather through their diet. For example, guinea pigs can get it through fresh, raw fruits and vegetables (such as broccoli, apple, cabbage, carrot, celery, and spinach) and not just through dietary supplements, so your statement, "they must receive vitamin C supplementation" is factually incorrect.I don't know whether their most probable wild ancestor (Cavia aperea, C. fulgida, or C. tschudii), is able to synthesize vitamin C or whether guinea pigs had lost that trait through the process of domestication as you seem to suggest, but those questions are moot. Whether loss of that ability is deleterious depends on whether they can still get that vitamin through their diet. If they cannot, then it is deleterious; if they can, then losing the ability to synthesize vitamin C is neutral, it doesn't matter.Why stop at guinea pigs? What about humans? We also have lost the ability to synthesize vitamin C. But since we can get it from our diet, that doesn't matter. Until we change our diet to one that doesn't include vitamin C, in which case we develop scurvy.Now consider cats. To my knowledge, their diet doesn't supply vitamin C, so if a cat were to lose the ability to synthesize vitamin C, then that would indeed be deleterious. But cats, unlike humans and dogs, have lost the ability to synthesize vitamin A. Is that deleterious? No, since their carnivore diet amply supplies them with that vitamin. The only time their inability to synthesize vitamin A would be deleterious would be when their environment changes; ie, when they are owned by an extreme vegetarian who imposes a vegetarian diet on his cat, in which case the cat will suffer severe problems unless also given vitamin A supplements. Feed your cat a proper diet and there's no need for supplements.Go back to the basic question of what makes a trait deleterious and you will find that the primary answer to that question is the environment. And if a population is in an environment in which a given trait is deleterious, then that trait will be selected against.