Evolution of poisonous vs. non-poisonous snakes

A long time ago, a member known as nemesis juggernaut pointed out while arguing against evolution that if natural selection was real then we would see species with a lot more beneficial characteristics. For instance, humans could always use wings to fly around when need be.I suspect that what you're thinking of is along the line of why don't non-venomous snakes retain their venom because it's always useful, oui? I don't blame you for thinking this way.I'm a backpacker, and sometimes I would spend days and weeks in the wild with my buddy. It's always a pleasant thought to have many things with me just in cases of emergency. I'd love to have at least a hand gun with me along with radios, meal packs, etc. There are at least a dozen things that I'd like to take with me. But you see, when you wander in the wild like I do, weight becomes an issue. Every little ounce counts. Why? Because every little extra ounce means I have to carry that much more weight on me while we climb the rockies or crossing the desert. Therefore, it is in every backpacker's interest to go through several times the stuff they will take with them to weed out all non-essential things. And even then, when I start my trek, I'd still have about 70-80 pounds of stuff on my back even after I've cut myself down to the bare minimum.Now, try to apply this concept to biological systems. Every bio-chemical process costs energy. Having an extra pair of wings just for the sake of convenience costs energy! Having a bio-chemical process to produce venomous saliva costs energy! That's this much more energy the snake has to spend. Now, if the snake doesn't really need the venom, then wouldn't you say that it's wasted energy to try to keep producing the venomous saliva?That's why we don't have a pair of wings or a tail. That's all just wasting energy.Added by edit.We see the same concern in engineering as well. Idealistically, we'd like a plane that could lift off vertically and fly horizontally once up at a certain altitude. When they first started drawing up engineering plans for such a plane, the biggest draw back to the engineers was the fact that the engine responsible for vertical ascend would be doing nothing but sitting there and waste precious fuel while the plane is flying horizontally. Why? Because it costs energy for the vertical ascend engine to be there even though it's not doing anything most of the time.Idealistically, I'd like to see a plane that could go straight up, go horizontally, and also be able to dive into water and manuver about like a submarine. And I'm sure with enough mental exercises engineers can design such a vehicle. Why don't they make these very convenient flying/diving vehicles? Because large parts of the vehicle would be doing absolutely nothing but wasting energy most of the time. It will be extremely fuel inefficient.Biological units go by the same general principles. Why have venom if it's just going to sit there most of the time wasting energy?Edited by Taz, : No reason given.Edited by Taz, : Changed day to they and larges to large in my sleep deprived state.

Speaking as an arachnid collector, I know what you mean. I frequently have to correct people on this as well. The most common question I always get from visitors is "are those tarantulas poisonous?" My answer's always "no, but they are venomous..."