The classification is not important as some species are mis-named or just changed by science for some reason some genuses are split but it's a long job.
The scientists studied the endlers for over 30 years (1975-2006) before finally describing the as a new species. Many people before this classed them as guppies as they are easily crossed with all lines of guppies, but until they started using genetic make up of the fish they discovered the genetic differences to classify them as a new species.
The primary way to differentiate species are.
Lateral line count
Lateral line to highest scale count
Lateral line to lowest scale count
Dorsal fin ray count
Caudal fin ray count
Gill raker count
Extra with livebearers
Gonopodium structure.
With livebearers the primary way to place fish into a genus is by the gonopodium. The structures is firstly grouped together to form the genus this is why platies and swordtail's are in the same genus and not two different genuses.
This is a typical shape of a gonopodium of a Xiphophours
And now two different species of Gambusia. (primary differece is the gonopodium is pointed in gambusia, classing them closer the the Brachyraphis than than Poecilia)
Gonopodia structure of Poecilia reticulata in Fig a. Poecilia winge, in Fig b and c
As u can see the structure with in the same species can be slightly different but on the basics it's the same design.
Anyway I've rambled on here but think I've explained the basics now and crossing between different species is difficult as the structures are different and makes fertilisation very difficult with out artificial insemination being used.
thanks for the greeeeeeeeaaaat info~~~ could u tell where i could get info kind like this? do u get anything about molly's Gonopodium?
