This sounds like a discussion as a whole that concerns the concept of species developing/evolving in responce to galactic influence.
Saxton's got a thread on that:
http://www.theforce.net/swtc/humans.html
Basically, Ryloth's only major export besides Ryll are slaves, mostly female. Therefore, over the millenia the individuals and families with traits more resembling galactic (ie human) standards, the more prevalent and prosperous they are and provide for their descendants' success.
Same deal with Falleen and perhaps Togruta (of which Shak-Ti is the only example I know of, though WotC implies they only come in white & red due to camouflage with their planet's fauna), while to a lesser extent the facial features of the Cathar, probably the arms of the Saurin (subspecies of Trandoshan), as perhaps many other prevalent species in the galaxy with otherwise inexplicably identical anthropormorphic features. You're less likely to buy Corellian starships, SoroSuub blasters, and spread accross the galaxy if you're stuck with lobster claws afterall.