Well, I guess the point is I don't know! :-D
I'm only thinking and commenting from the standpoint of efficiently explaining evolution, because parent's video disappointed me. Maybe "in depth" is indeed not an option in a short video.
I like @lotsofmangos 's explanation, it strongly shows that we're bound to have some evolution.
But ideally we'd also want a simple conjecture that would predict what kind of evolution at what velocity within what range we're bound to have. And then present how what we know about the past fits into that.
To answer that question you'd basically need a model of the galaxy at the atomic level. The fitness of genes is not determined by the genes themselves, but by the environment where the organism lives. But "environment" means everything: all the other organisms it comes into contact with (of the same species and every other species), the food that's available, whether organisms it could mate with are awake at the same time, the climate, forest fires, climate change, gamma ray bursts, and so on.
It's an enormous question and an inherently chaotic process. I'm not sure how you could model all that accurately. Also, there's more than one solution to a given selective pressure, and at the end of the day, DNA mutations are truly random.
Let's consider a simple system: a cave full of insects and some crabs that can eat them. Initially the crabs don't live in the cave, but given that there are no other animals there, when they do randomly come upon the cave it's an all-you-can-eat buffet. The cave is pitch black, which isn't a problem for the crabs, but their eyes are useless and actually a liability, since they can be pierced more easily than skin, they can get infected, and they take protein and nutrients that, if not used, could allow the crab to live on less food, or be more vivacious and able to produce more offspring.
So there's a selective pressure on the crabs to get rid of the eyes. But this can be achieved in many ways: the genes that cause the eyelids to separate during development could be damaged, causing a layer of skin to permanently seal the eyes. Other genes being mutated may cause the eyes to never form in the first place. Another may prevent blood vessels from growing where the eyes need them to thrive, resulting in weird, misshapen but much smaller eyes.
So what solution will occur? It turns out: it's mostly random. If one adaptation can occur with only a single mutation, it's obviously more likely to occur before another that takes ten mutations becomes prevalent. But even in this tiny, simple system it's still difficult to make a prediction as to the specific timing and mechanism that would result.
But ideally we'd also want a simple conjecture that would predict what kind of evolution at what velocity within what range we're bound to have.
To drill down to trying to even roughly predict rates and kinds of evolution, you are no longer at a simple conjecture as you have to include all the detail of the interplay of biochemical reactions, which is insanely complicated.
