To settle the debate about Digimon evolutionary lines, this is how I understand it, as someone who's primary monster game has always and forever will be Digimon.
Digimon are compiled of data. They start as very simple programs with only very basic functionality but with limitless potential to be improved upon with only very slight limits. Digimon grow more powerful by absorbing the data of their defeated opponents [as seen in Tamers]. Depending on what types of data they accumulate depends on what they become.
Example: A Koromon is a basic Digimon which many know evolves into Agumon under normal circumstances. But say the Koromon were to absorb the data of an Aquatic-type Digimon? This particular Koromon now has a predisposition to that type, and evolved instead into Betamon, which evolves into the Seadramon line normally, or Dark Tyranomon with Dark-Type data. Koromon is essentialy a basic program for ANYTHING that eventually becomes a -dramon.
In games like World Championship, killing and absorbing foes is replaced by training under certain parameters which increase the program's data in certain attributes attributed to a certain type.
Example: Putting a Koromon in a volcano training area increases its 'Dragon' attribute, enabling it to digivolve into more dragon types. Increasing its Insect, Machine, Aquatic, etc type will yeild different evolutionary paths, which can be varies even more by changing attributes as you go.
All Digimon have a 'time elapsed' pathline too, which is generally considered the default. This is the one you see in the show, and the one wild peaceful Digimon tend towards, because it involves no extra data consumption.
