There is no evolutionary ladder. I think that when people use that phrase they typically mean that a species is evolving and therefore acquiring traits that are "better" than the traits that they had generations ago. In science lingo, we would call that improved "fitness" for the environment. When a species evolves greater speed, intelligence, camouflage, or vision a "climb up the ladder" could be inferred. The problem with that description is that it assumes that better fit means greater success. This is not always the case. Environments change and generalist often fare better during periods of change than species with greater fitness. In addition, species co-evolve, so that faster predators lead to faster prey. Success is temporary.
· 7 years ago