Thought I should point out that Rock has no resistance to Electricity. That would be the Ground type.SpcyhknBC said:Well, if we assume pokemon types are equivalent to taxonomy, all rock type pokemon are from the same family, grass type the same and so on, each type of pokemon could have a different specific biochemistry. Now, let's say that rock/ground types contain a specific gene that is unique to them. Whatever protein it expresses, it allows for the resilience against electricity, but also negatively interacts with high quantities of water. Maybe something akin to lactic acid buildup for humans, but more drastic in its immediate effects. You can train to become more tolerant of lactic acid buildup, but never completely get rid of it because it's part of your genetics. So essentially, rock/ground are fine with water intake so far is required for their biochemical pathways, but excessive amounts cause formation of some compound that harms them.
As for the psychic weakness against bug types, that can actually be explained very simply. Many have already suggested the hive mind mentality and/or simple brain which could be considered partially correct. If we assume that we can break pokemon kingdom into two main branches, vertebrates and invertebrates just like it is in modern taxonomy, bug type are clearly invertebrates. The insect brain is significantly different from a vertebrate brain. It is this difference which the immunity to psychic can be attributed. This doesn't denote a difference in intelligence necessarily, but does indicate a different neurological control scheme. It's not like Independence Day where a Mac can infect alien hardware with a computer virus.
Please forgive any inaccuracies in this statement, it has been a long time since I took a course in evolutionary biology.
Also, now I've completely overdone this.