This is something that crossed my mind, but I wasn't quite sure how to put it into words.



Even on Earth, there's numerous branches of life that diverged very early in their evolutionary histories, leading to a wide variety of creatures here. Having each planet have a specific value for how much variance can occur in living creatures would allow you to reflect planets that had cataclysms in their evolutionary history. For instance, life on Earth would be even more eclectic than it is today if the Permian-Triassic extinction event hadn't wiped out an average of 83% of all animal species on Earth at the time.



The more cataclysmic extinction events in a planet's history, the lower the variance in traits on the creatures that can appear on that planet. This is because more species are descended from common ancestors (the survivors of those cataclysmic extinctions) and thus have more similarity. A planet with very few cataclysmic extinctions will have a wider and more varied abundance of life on its surface.

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