Interesting topic and a difficult one.
First of all, let's remind that the "Supspecies" concept (as the concept of race) is always tricky. Indeed, subspecies should be geographically isolated and morphologically different. A cline refers to a gradual change in a specific trait (often colors or patterns in butterfly species) across a geographical gradient within a species.
If the hybridization is very low (because the species doesn't move a lot), you can have 2 distinct populations one near another one. It often happens in Amazon between North and South Amazon. Amazon is very large (from 2 to 16 km !) and very few specimens cross it. So you have 2 subspecies but may find some "hybrids".
But how is a ssp described ? You know one species pattern in a place. You find a new population in a distant place which is really different. You describe a new ssp. Years later, other populations are found between the 2 localities and you can see a clear cline pattern... the 2 ssp are in reality a cline. This has to be proven and the 2 ssp have to by synonymised. It requires time, money (especially for Agrias) and will.
A classic example in Agrias :
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Agrias pericles pericles described in 1860 from Itaituba (Rio Tapajos) by the great HW Bates. This ssp has a lot of green on its typical form. Agrias are rare and it was described from few specimens.
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Agrias pericles mauensis described in 1921 by Fassl from Maues has no green on its typical form and was thus described as a new ssp. Agrias are rare and it was described from few specimens.
100 years later, we have caught much more specimens and we now know that these "green form" of pericles pericles can be found in ssp. mauensis too (and the contrary). The more you go towards Itaituba, the higher the % of green form you will find. It is a cline. If you compare 10 specimens of Itaituba and 10 specimens from Maues, you see 2 ssp. If you compare 1000 specimens and try to find specimens from localities between Maues and Itaituba, you discover a cline.
The taxonomy of Agrias is even more difficult than other genera because :
- there are a lot of forms within a ssp (genetic forms). This led to new ssp described by error.
- a lot of people want "their name" on an Agrias and are naming a lot of forms / ssp. Sometimes rightfully, sometimes for nothing.
- hybridization does happen. Sometimes hard to understand the level of it. Some ssp seems "pure", some are doubtful as explained above with pericles mauensis.
- anti-science Brazilian laws doesn't help. We should collect much more specimens to better understand what is happening.
- some populations (forms or ssp) have disappeared due to deforestation or other factors.
All these forms, hybridizations, ssp are what makes Agrias interesting to study ! It is very complex, especially in Amazon. You need a good knowledge about all Rios of Amazonia to better understand what is happening.
Some "scientists" will elude the topic saying "forms are not recognized by the Code", don't bother with that.
Some "scientists" will try to understand the genetics and population dynamics behind all these forms and that is a very interesting topic. David Lohman (City College of New York) has made a tremendous work on the Delias genus (another difficult genus). We need someone to do the same on the Agrias (sub)genus.