Thanks! I hadn't realized but Tierra and Avida are quite similar to SproutLife. I found this video that explains a bit about it https://www.youtube.com/watch?v=ouF8wKxXWFQ&t=1050s

I wonder if they also encountered the issue where the genomes/programs get simpler over time. In some ways one would expect evolution to increase complexity, but what I found happening is that prioritizing reproduction means creating pressure to decrease complexity, thereby increasing the reproduction rate and decreasing the footprint for negative mutations. The way SproutLife counteracted this was to add a "competitive" metric where bigger organism win over smaller ones, kind of like big trees casting shade to block smaller shrubs.

As it seems like you’re seeing, what evolves depends a lot on what you’re selecting for and the environment they’re ‘growing’ in. This is another paper with more details. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7123229/pdf/978... My favorite stories are the ones where the organisms evolved to be able to detect when they were in the test environment and only performed the high-cost, selected-for behavior there.

If you haven’t run across it yet, Karl Sims’ virtual creatures are also really fascinating. https://www.karlsims.com/evolved-virtual-creatures.html And anything by Melanie Mitchell. Her book “An Introduction to Genetic Algorithms” is great.