Did you predict that in each interaction there would be an evolutionary pressure to reduce negative effects or increase positive effects? Those predictions make sense, but why, after 3.5 billion years of evolution, do we still have interactions with negative effects?

Yes, in each interaction, there would be an evolutionary pressure with negative and positive effects

Explanation:

Parasitism and mutualism are two fundamental relationships in evolutionary studies with negative and positive effects respectively. Even after 3.5 billion years of evolution, we still have some form of parasitism among animal species that result in negative effects. This may be due to the necessity of evolutionary interactions and the balance maintained by ecological nature for both fauna and flora. Individual species compete for the limited resources available for ultimate survival. In mutualistic units with two species, there must be a parasitic character.