For example: snakes have venom to kill their prey. Why don’t all carnivores have venom? Porcupines have quills to deter predators. Why don’t all mammal prey species have quills? Antelope and horses developed hooves to help run faster. Why don’t canines have hooves?
Because (take your pick):
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Everything has tradeoffs, which will vary by animal type. Otherwise, there would only be exactly one kind of animal, which had all the same characteristics. Perhaps the food type needed to generate venom, for example, isn’t in the creature’s environment, or is more dangerous to obtain.
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(Most likely) The random mutation that allowed feature X to appear in the population never occurred (or the creatures in which it occurred didn’t live long enough to pass it on for other reasons), so there was no chance to select for it, or
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It didn’t generate an advantage for the creature, with respect to either not having it or whatever it replaced. Since it didn’t increase survivability, it wasn’t selected for, or
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The mutation did occur, did increase survivability, but was “unlucky” for some reason (the population containing it was wiped out, an even better mutation occurred in the population, whatever).
Natural selection (better surviviability == more survival == increasing percentage of the popuylation) isn’t a particularly random process, but mutation, which generates the attributes that are selected for or against, IS mostly random. You can’t select for what doesn’t exist.
Generally three answers can apply:
(1) random nature of evolution. This one can apply anywhere. Even if something always provides substantial benefit at very low cost, some lifeforms will evolve without them, and some of these will be successful to survive even if they are competing against other species with those traits.
(2) extra ‘cost’ to grow these features
(3) the trait could serve as a liability in certain circumstances.
Why doesn’t a lion, (say) have venom?? All three would apply here. Lions are pretty successful predators already, and don’t need another attack trait. There are complicated venom glands that take a certain amount of energy to grow and keep working, and all poisonous species have to be careful that they won’t end up dying of their own venom. Most venomous species are relatively small, so I would think that the cost of protecting yourself goes up, somewhat, with mass, so it would be a very bad fit for a species like lions that is quite massive. (And that generally can let its mass work for it.)
Why doesn’t a rabbit have quills? Not so sure about this one… how much energy does it cost to grow quills? Can they be an inconvenience for a quick little rabbit trying to get something to eat??
Why doesn’t a wolf have hooves?? I kind of think the third one is the answer here. Wolves have evolved their paws to go fast in a different way than the hoof works… maybe not as efficient for sheer velocity, but perhaps they make up some in agility and quick turning. Also, front paws are useful for attacking prey and tearing out chunks to eat??
Just my $0.02 worth as usual.
on preview: hi timewinder.
Because they aren’t intelligently designed. If whatever characteristic that results from a chance DNA inheritance works to ensure survival that characteristic persists.
If a rabbit had quills, it wouldn’t be able to run so fast (due to lack of streamlining), and wouldn’t be able to hide so well (because of its bulk). A porcupine can afford to be slow and bulky because of its quills. Basically, having one method of defense - quills - precludes using other methods of defense - speed or hiding - very well. This is the “Jack-of-all-trades is master of none” principle. It’s simply impossible to be good at everything - if you try it you will end up being mediocre at everything. So many animals specialize for one “trade” (although there are some that are able to get by as generalists).
The OP is like asking why we don’t have one kind of vehicle - a car-boat-plane-helicopter-bicycle, for example. If you made one, it would be worse at any one particular mode than a more specialized craft - so it’s better to have separate vehicles for each mode of travel.
The question was adequately addressed in the early days of evolutionary theory.
‘Fitness’ is what wins an animal survival. Rather than competing against all the animals that already have quills or venom or wings, a very successfu strategy has been to develop a ‘niche’.
So snakes are the cold-blood, venom predator. Hedgehog, Porcupine and Echindna, the warm-blood, quill defence in their respective location. These are niches that are full.
Hence the variety of survival strategies present in the kingdom. Not that ‘strategy’ is a good word, implying as it does forethought.
[QUOTE=chrisk]
Why doesn’t a rabbit have quills? Not so sure about this one… how much energy does it cost to grow quills? Can they be an inconvenience for a quick little rabbit trying to get something to eat??
[QUOTE]
Rabbits don’t have quills because quills were the gift that Lord Frith gave to hedgehogs. The gift of El-ahrairah is in his strong hindquarters to outrun his thousand enemies.
“All the world will be your enemy, Prince With A Thousand Enemies, and whenever they catch you, they will kill you. But first they must catch you, digger, listener, runner, prince with the swift warning. Be cunning and full of tricks and your people shall never be destroyed.”
Heh…nice Watership Down reference.
Back to the OP, there is an underlying misconception in your basic question; although we casually classify animals often by function and behavior or diet–carnivores, omnivores, herbivores, insectivores, et cetera–these are ad hoc classifications that have little to do with their origin and development. That an octopus and a sea otter both eat clams isn’t evidence of common–or even parallel–evolutionary development, but rather that both species have found an available source of high energy protein.
Phenotypes–the expression of genes in terms of physical features–are developed from previously appearing genes by amplification, elimination, mutation, drift, and so forth, as best suits the organism to survive and procreate. It might be handy for monkeys to have wings, for instance, but because their predecessors in order Primates do not, it is very, very unlikely that they will develop them without much time and a significant environmental impetus. Creatures who have spines, or eyes, or big nashing teeth have generally evolved from other species who have something like these, or some similar, more primative feature which became these accoutrements. To misquote our current Secretary of Defense, a species evolves with the genes it has, not the genes it wishes it had.
Some features–like eyes–are so valuable that many orders develop them independently in what is termed parallel (or sometimes convergent) evolution. But all features require energy (to grow and maintain) and have tradeoffs. You ask, for instance, why canines don’t have hooves; the answer to this (aside from that the predicessors in order Carnivora from which they descended don’t have hooves) is that flexile paws offer the greater dexterity to not only run but weave, dodge, climb, claw, and scrape; in other words, they offer greater mobility in chasing down prey. For the unglates (hooved animals) who, with the exception of hyraxes, aardvarks, and ocean-going formerly hooved creatures, are herbivorous herd animals, hooves make a lot of sense; they stand up well to the long ranging travel over rough terrain that a grazing diet requires. And a herd animal need not evolve to be faster than the animals that prey upon its species; like the hiker and the bear, it need only be more agile than its fellow herdmates to avoid being eaten. For the predator, however, raw speed and dexterity, as well as an ability to use paws for leverage to tear away its chunk of dinner (or in the case of the cats, to use claws to grapple and slash) outweighs the need for long-range travel.
I beg of you not to think of animals as being “perfectly evolved” (or in the minds of some, Intelligently Designed), but rather they are the minimum of required survival capabilities to maximize reproductive fitness. Species don’t thrive because they’re well suited to their environment; they’re well-suited to their environment because they thrive in it (and those that don’t become extinct). Nature is a cruel bitch that only rewards those who happen (not choose or are endowed by El-ahrairah or anyone else) to survive and propagate on a least-cost (or in game theory terms, minmax) basis.
In other words, in evolution, cheapskates and tightwads win. Bear that in mind next time you’re trying to win the favor of a girl sitting next to you at the bar by buying her a drink.
Stranger
Except that the mammals in the order *Carnivora *are united by common decent.
Yes, but not all carnivores are in order Carnivora, nor are most members of Carnivora, save for the cats, strictly carnivores.
Stranger
And of course some of them are almost exclusively grass eaters.
There’s another problem; sometimes you can have to much of a good thing. Scientists have run simulations of ecological systems for years and found some interseting things. In one such experment involving evolving “agents” that roamed a simple simulated world, the tried an experiment. Instead of letting them evolve, they wrote a “perfect” agent, one characteristics were optimized. Result : “They were a plague upon the land”. They ate all the resources and overbred.
To use a more physical example; suppose you asked a human to create the perfect predator. You wouldn’t get a wolf or bear; you’d get a horror-movie nightmare that included all the nastiness he could dream up. If such creature were ever created and released, they’d do quite well for a while - but in a few generations they’d eat and breed themselves into a famine.
Animals are as efficient as they need to be to survive and breed. Too little and they die out; too much and they overuse their resources. A population of wolves can sustain themselves indefinately; some sort of sci-fi killing machine would eat it’s environment bare.
Not really. What you are discussing isn’t purely hypothetical, it is simply an example of the introduction of a novel predator to a naive ecosystem. We’ve seen exactly this happen numerous times, often with the introduction of that most advanced of predators Homos sapiens.
And what invariably happens is an initial glut and massive population growth followed by famine and civil war, very rapidly settling down into a stable relationship with the new and much impoverished ecosystem. In very few cases is there evidence for the predators breeding themselves out of existence, and in no case has this been established with any degree of certainty. There is no real reason to assume that any other predator would be any different. The Earth is simply to diverse and to resilient to allow that to occur.
Organisms don’t die out because they become too efficient. Indeed if that was the case then all organism would be extinct. That’s because evolution acts on the individual, and in the vast majority of cases the biggest competitor for an individual is its closest relative. As a result every individual ‘strives’ to be more efficient than the current most efficient organism. They have to survive. So in that respect all organisms are already maximally efficient.
Wolves aren’t sub-efficient to avoid starvation, they are sub-efficient because they haven’t yet developed the mutations to allow them to be more efficient. But rest assured that if a mutation developed tomorrow to allow a wolf to potentially run at 1000km/hr with no other detriment whatsoever it would rapidly spread to the entire population, and many prey species would become extinct as a result.
Which more or less brings us to the answer to the OP. These changes don’t occur overnight. A wolf can only be born with a mutation to allow it to run a bit faster. If that helps it catch more deer then the deer themselves will evolve. Either they will evolve to run faster themselves, or they will evolve to stand and fight because running is futile.
What that shows is that there is no one ideal design for even a behaviourally simple organism like our hypothetical deer. There are at least two ways to evolutionarily evade faster predators, one by getting lighter and faster and one by getting heavier and more aggressive. In reality there will are several others including camouflage, herding behaviour, the development of an unpleasant taste or behavioural changes such as moving to areas where fast wolves can’t hunt like dense forest or treetops.
Which is the real answer to the OP. There simply is no one ideal solution, Every organism is in an arms race, and every path taken in one direction excludes a dozen other options that may have been equally or more successful. So in a very real sense every solution that works is equally valid.
Poison for example doesn’t come free of charge. It requires the expenditure of protein and energy, and also requires extra glands in the mouth, and glands are prone to infection. Moreover in the early stages poison may not offer much of an advantage at all, it’s probably only the difference between an organism struggling for 3 days before dyinig of septicaemia form the bite, and struggling for 2.5 days. Not much of a difference practically speaking, and obviously only applicable for predators that hunt primarily by bite and release. For large predators like wolves or bears it simply could never evolve because prey never lasts more than an hours or so if it is going to be taken at all.
Spines are also inconvenient. They are almost impossible to groom and so make the animal the target of parasites. They also make mating very difficult, while at the same time the animal has to have limited mobility to avoid puncturing itself. They aren’t in any way free, and simply wouldn’t be practical for a cat or a deer which rely on mobility for survival.
To ad one micro point to Blake’s superb piece, don’t think of evolution as a process that ended yesterday or 500 years ago, with today’s animals being the end-point of a long journey.
In reality, every species is evolving every day. Today they have the characteristics they have today. 50 years from now some animals will be different. On a 50 year scale, nothing much more complex than unicellular animals will have changed detectably.
But in 500 or 5000 years, you’d be able to see distinct differences in complex species vs today. And those changes would make those animals better suited to their environment than their present-day counterparts would be.
I didn’t say they would destroy themselves, unless they are on an island. They will never prosper the way a more moderate species will; compare the common cold or even AIDS to Ebola. Ebola never spreads for long, because it kills so fast; it’s the disease version of that sci-fi monster I imagined. It tends to kill an entire village, then it stops because it destroyed it’s own resources.
Strip this question down to the bare essentials and it becomes simply Why is there diversity at all - why isn’t everything the same? The answers (above) are much easier to grasp in this context.
Very mild hijack possibly, but still, I think, relevant to the question. There is a slight problem with this type of question and these types of answers. The problem is that the popular media often present an account of evolutionary theory which might be described as relatively superficial, science-lite and easy-to-understand, and that also borders on being non-falsifiable, and hence non-scientific.
What I mean by this is that any trait which seems to confer a survival advantage is heralded as (clear and helpfully self-evident) evidence supporting evolutionary theory. However if:
(a) a trait seems to confer no survival advantage, or
(b) a trait seems to actually constitute a disadvantage in survival terms, or
© the absence of a trait seems disadvantageous,
a glib and easy reason is proferred as to how this also supports the evolution hypothesis. In other words, you can never falsify the theory.
The danger with this approach is that it unnecessarily gives ammunition to the creationist and ID advocates.
I believe that evolutionary theory is correct, although we may, over time, find that our understanding of its operative mechanisms is currently rather crude and needs to be refined. I also believe that the truth and correctness of evolutionary theory can be shown and substantiated via empirical data satisfying the highest rigours of scientific and rational enquiry.
What I do not believe is that to say, “look, this trait confers a survival advantage, which therefore backs up evolutionary theory” is good thinking or good science. It’s a lazy kind of correlation, and a dangerous one, because it means you also have to accept the equivalent correlation of the form ‘this trait confers a survival disadvantage, and thereby falsifies evolutionary theory’.
You cannot accurately say having venom confers a survival advantage. The species might well have survived even without the venom trait (as others have) and for all we know closely related species with very similar venom or even better have died out and are now extinct. All you can say is:
(a) random genetic mutation occurs, and is part of the mechanism by which species diversity comes about
(b) in any given species we can study, we notice traits which seem to have helped the species to survive but this is not a perfect and failsafe correlation. It is not a perfect correlation because we have no control group. We cannot assess the relative survivability of the exact same species plus or minus one specific, named trait. If we could, we might find some surprising results. We might find that while we imagine trait X confers a survival advantage, the same species lacking trait X does even better.
So do spiders, which are only distantly related to snakes. With (mostly) random mutation and natural selection, you get what you get. The offspring of similar species can diverge, losing traits or gaining new traits due to mutation and selection. Divergent species can converge somewhat phenotypically if they have the appropriate genetic background and environmental forces favor venomous substances, for instance. “Venom” is a rather vague catagory anyway. It could be anything poisonous introduced by bite or sting that is a secretory product of an animal. All animals have secretions. That some here and there wound up adventitiously poisonous isn’t much of a stretch, conceptually, given the enormous diversity of animal species. That some didn’t is also not so mysterious, in turn.
Sorry Loopy…how the heck are snakes ‘only distantly’ related to spiders? If you mean distantly as in ‘primordial soup’ distantly, I understand what you mean…I think. 
I’m afraid I don’t really understand your question. They’re certainly more distantly related than snakes are to other vertebrates.