Or all designs evolve towards a distinctive shape which is similar. Assuming (I know that's generally where the problems start, "Assuming" anything, but for the sake of arguement...) speed is the greatest asset of piston engined fighter from the Second World War, then Radial engine fighters all head towards the same shape (with alterations based on design parameters).
It will have the largest engine possible up front to give the absolute maximum in horsepower, thinnest wings (no Hurricane Wings, closer to Laminar Flow, aka P51A wings) to reduce drag but give the lift needed, bubble canopy (see them before they see you), heavy armaments (higher the speed, less time you have to get a shot in hence the more heavy slugs in the air at one time increasing the chances of critical damage), armor and survivability equipment (inert gasses pumped into the fuel tanks, CO2 or other fire extinguishers, emergency release equipment- sometimes even a hachet!) and wide track gear to make landings easier on low time pilots (and the high time too!).
What you end up with is a fast, manuverable (high roll rate), heavily armed and relatively heavy aircraft. The Tempest, Fury, Bearcat, Ki-100 and Lagg-9 are all similar in appearance.
I think the early war aircraft all had a distictiveness that was based on differing design philosophies (for instance radial engined aircraft such as the F4F, the Skua, Zero and P36 all showed a design philosophy reflecting biases or requirements of the 30s where aircraft were more elegant than in the middle 40s-imho). By the time the end of the war had come around, radial engined aircraft were built much more functionally than their predecessors. They were, for the most part, disposable. As such, form followed function.
Same thing held true with the inlines but not to the degree of development. Seems inlines (in fighter aircraft) pretty much stopped in developement at the end of the war. Whereast the radials continued to be developed (mostly for COIN or Ground Attack) the inlines were replaced by turboprops. Probably the ability of a radial to absorb damage gave it another 20 years of design life.
Again, IMHO.