Personally, I don't view it as so much "evolution", but rather as simple similarity...
An AT-RT has 2 legs as does an AT-ST, but to me I look at that as being no different than my car having 4 wheels the same as most other vehicles on the road...
The ARC-170, while having some similarity to an X-Wing, doesn't jump out at me as being an evolutionary step... Backstory's laying claim that it's an Incom ship, and that's fine, but the S-Foils strike me as something more along the lines of a feature seen on many types of ships by many types of manufacturers...
EU shows many ships with a general "shape" like the X-Wing that really have no tie to the X-Wing... The supposed role of the XW and ARC are completely different, so I see them on separate evolutionary paths... Honestly, I see the ARC as a closer kin to the Y-Wing (though if you go by EU, it too was in service during the waning days of the Republic... Perhaps the Y-Wing lead to the ARC instead?).
For my money's worth, these ships and vehicles more stand alone rather than "lead into" each other... The AT-ST is in the walker class, but shares no heritage from the AT-RT. To me, the AT-RT maybe evolved into the AT-PT instead which pops up in the Empire in EU materials...
The ARC, to me, doesn't lead into the X-Wing so much as the Z-95 does as EU's established, and the ARC simply fades from existance or goes into public service or whatnot.
I agree wholeheartedly that there was too much "effort" from Lucasfilm to tie looks of things between trilogies... EU's fleshing it out some so that it makes a little more sense and isn't just the straight "Hey, the ARC has the same shape as an X-Wing, sorta, so it must be what they design X-Wing's off of!" kind of mentality. Too simplified for my liking, personally, and treads too much on things I enjoy that were already established (The Z-95, which is a ship still in service under the Alliance, will forever be the true forebearer to the X-Wing, and the ARC will have had no influence aside from perhaps the S-Foils being a "good" idea for venting increased heat output during combat).
The Corellian Corvette(-ish) in ROTS, which I believe was wholely intended to be the ship in ANH from Lucasfilm's POV but which has become obscured, I'm torn on as to how to explain... I agree with some guys at a site I know that debate ships a lot, that the large section missing in ROTS would be awkward as an "upgrade" to the ship that happens between 3 and 4... On the other hand, as Nick noted, the ship's only referred to as Tantive in ROTS I guess, so that's possibly a whole other class of Corellian ship... The basic design, in EU, is varied... The Mysteries of the SIth game had a larger version of the Corvette in service in the New Republic Era that had a large/spacious hangar capable of holding a defensive force of Rebel fighters, for instance, but had the basic shape of the Tantive IV down exactly (only larger). The Corellian Gunship from EU also sports an incredibly similar look to the movie Corellian ships. So for what that's worth, it could very well be a sorta smaller version of the Tantive IV we see in ANH, but not the same ship...
Every source out there's continually referring to it as the Tantive IV though for some reason.
The ship/vehicle issue is really interesting... What I find interesting is how EU's sorta blending things, or blurring lines all together. For instance, AT-AT's were basically placed in the Clone Wars era EU now... They look almost identical to the ESB AT-AT's, based on the late conceptual art for AT-AT's (basically, a slightly boxier looking AT-AT then). AT-TE's have been spotted in the Imperial-Era EU now too, which I like the thought of them still seeing service... The AT-AP now may show up as well, and all the other walker variants mixed in from all the EU from all the eras... Lucasfilm intended visual tie-ins, but EU's debunking them as "evolutionary steps" somewhat. I like that, personally.