Someone who studies evolution might discuss the "fitness landscape", a mathematical landscape which represents the reproductive success of an organism. The higher the elevation of an organism on the landscape, the more "fit" it is; the lower the elevation, the less likely the organism can survive long enough to reproduce at all.
The process of evolution yields species which, over time, discover higher and higher points on the topology of animal-space. Eventually, the successful ones end up at the top of the hill in their ecological niches, at which point they stop changing, since there is no higher point which is accessible by the short distance traveled by random mutations.
Jellyfish and horses are both at the top of the landscape, yet they reside in very different locations. There is a great valley between them. If a jellyfish slightly closer to horse-space is hatched, it will be a worse-than-average jellyfish and produce few offspring if it even manages to surv