There are Pokemon in the world, running around, that we can interact with to get seen-data or just for kicks.  
Now, I suggest:    
[glow=red]Only have overworld Pokemon on a map, that actually spawn on that map.[/glow]   
Maybe as a hint to what can be caught there, or just for making sense. 
For example, there's a Corsola here, while none can be caught on the map, which made me think to post this.