Juv, evolution is not different from other scientific theories, you're testing it to a higher standard than other theories.
The evolution of life is extremely complicated and not easy to model, however it is still based on simple principles that are proven to be correct.
Let me give you an analogy, i'll even provide a link:
http://blueballfixed.ytmnd.com/
Now, when you look at this link, think of physics, think of gravity. What causes the balls to fall? What causes all of the machinery to move?
You'd say, "Newtonian physics, and gravity".
Well, that's great, now prove how it actually happened, to someone who doesn't believe in newtonian physics and gravity, and prove it one blue ball at a time!!!
I mean, a lot of the questions you ask are hard. It's not really obvious, to anyone, why bacteria never evolved into anything multicellular. I'm really just giving you my best guess with my predator eukaryote hypothesis. It's like looking a trainwreck from earlier in the day and trying to calculate using newtonian physics why this particular log or cart ended up here, all while not really knowing, but having a good idea, of where the log and cart started in the first place.
I mean, take a train wreck, not knowing for sure what the train was like before it was wrecked, and then try computer modeling the whole process to the utmost detail. It's a really daunting task.
Now, you can model newtonian physics by reproducing simple physical processes in the laboratory, calculate it all using simple formulas, and completely ignoring the train wreck outside, you can prove physics works just like you think it does.
Same goes with evolution... people breed e-coli, fruit flies, etc, in the laboratory, and get all kinds of interesting results, and they also find evidence for macro evolution in the massive selection of dead animal fossils scattered around everywhere, as well as tell-tale signs in DNA of the nested heirarchy which is charicteristic of evolutionary processes.
Evolution predicts some complicated results, same as physics does, and both theories have very simple principles:
Evolution basically works on the following principles:
1) Between generations, animals mutate (change) in small ways which are more or less random. The mechanism for this is transcription errors in DNA.
2) Natural selection: The rare mutations which benefit the individual will cause the individual to be more likely to survive and reproduce, thus making the beneficially mutated variation of the animal more common over time, as it outcompetes it's neighbors.
3) Over long periods of time, the above mechanisms can lead to entirely different animals than what you started with.
...
let me go into a little more detail:
The average human genome undergoes around 100 random mutations per generation per animal. That means the average baby has 100 mutations which are transcriptions in the genome.
of these 100 mutations, 99% are neither negative or positive, some have no discernable effect, and the other 1% are mostly negative. If it's negative to a significant enough extent, the animal dies.
However, occasionally a mutation that is beneficial will turn up, making that organism more likely to survive. But let's do some math here:
let's say you have a population of 10,000,000 animals, over a course of 10,000,000 years, that reproduce once per year, with 100 mutations per reproduction.
How many mutations do you have? well, 100 * 10,000,000 * 10,000,000 = 10,000,000,000,000,000. That's a lot of mutations. How many are beneficial? Probably a good number of them.
You've got a lot of information to work with, as for what survives and what doesn't, it's all about whether the mutated creature can find it's own niche or else be better at exploiting it's current niche. That said, the utilitary reason for why X trait exists or never appeared is not always entirely obvious.
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