(I commented this underneath the video already but I'm reposting it here so other people can read it as well. The comment might have also landed in your YouTube spam filter.)
Thanks a lot for the video! There is certainly a bug in here, but it's not really what seems to be the 'obvious' reason. In fact, I tried to solve this bug alone for a couple of days when I had time to work on the 2.0 update but I didn't manage to figure out what's going on.
That being said, I did a ton of testing and know certain things that are definitely not causing the bug. In short the problem is this: Sometimes the creatures in the "Best of" scene don't behave the same way they did during the actual simulation, even though they have the exact same brain as the best creature of that previous simulated generation.
I understand that when you (or anybody else) looks at the behaviour the first thing to think is: „The simulation clearly doesn’t pick the best one / It picks one of the bad ones.“ A lot of people say that. The real problem is that this is not the case (It would be really nice if it was because that would have been easy to fix).
This is definitely not the case though. The simulation consistently picks the creature with the highest fitness score of the generation as the best creature and stores that brain (in chromosome form, so as an encoded string). The fitness scores are also assigned properly. This is why 1) the creatures evolve at all and 2) the reason why you were seeing relatively high fitness scores (~17%) being displayed in the best creatures scene, while the „best creature“ was just falling over. It’s because the same creature (with the same brain) performed well and actually got 17% fitness during the simulation. It just for some reason doesn’t want to move when you put it in the „best creatures scene“. <- This is the really annoying bug that I’ve already tried to fix for days, but the problem is that I have no idea why this might be happening at all, so I can’t even consistently reproduce it.
As you have probably also noticed this doesn’t happen all the time. Most of the time the best creatures perform exactly the same as they did during the simulation and everything is fine. But that just makes it harder for me to find out the reason behind the bug. So if anybody can find out how to consistently reproduce this issue (instead of just randomly witnessing it) that would be of great help!
Another way to tell that the best creature is actually behaving differently (compared to when it was simulating) is that sometimes when this bug happens (not in this video) you can see that all of the creatures during the simulation actually walk - some better, some worse, but all of them walk - whereas the bugged out best creature just wiggles or falls over. This shows that this isn’t a matter of the wrong creature being picked but it’s actually a change in behaviour of the best creature (without any apparent reason).
That’s as much as I can say about this bug. The thing to take away from this is: While this bug is highly annoying and definitely something I’d love to be able to finally fix one day, it doesn’t affect the outcome of the simulation at all. It’s something that purely happens on the „best of“ scene.
Now you might ask yourself, well then why did the fitness score randomly drop from 17 to 5%?
This could have a lot of different reasons so I can’t say for sure why that is, but I’m pretty sure it’s unrelated to the above bug. Not only does the fitness drop „randomly“, it also goes back up as you can even see in this video. Additionally, the fitness neither drops exactly when the best creature bug happens for the first time, nor does it drop only when the bug occurs. Sometimes you can see these fitness fluctuations between generations without the bug even happening at all.
My current guess is that the creature brains have reached a point where you have (sort of) two groups of creatures. The ones that are doing pretty well at walking and the ones that can barely walk at all. Now when the next generation is created you can end up mating good and bad creatures which could cause the new creatures to receive one part of the bad creature’s brain and another part of the good creature’s brain. This could result in a bunch of creature that can’t walk at all (just like we’re seeing it in the video). However, the brain information of the previously good creatures is not fully gone; it’s still in the brains of the current generation. Therefore, during the next mating process you can still end up with creatures that receive both good brain parts and then „know“ how to walk again.
This is just my guess as to what might be happening here - especially since it seems to be happening in cycles and the good behaviour always keeps coming back after a certain number of generation after the fitness drops. The important thing is that this is unrelated to the bug described above and most likely not a bug itself.
That being said this bipedal creature is pretty amazing when it walks. Would you be okay with it if I record a little gif of one of them and add it to the collection at keiwando.com/evolution-gifs ? I would link back to this video of yours or another video/gif you might want to make yourself with this creature design.