Is evolution always unidirectional?
It's possible, but very unlikely.  Usually it takes a substantial number of genetic differences to qualify two organisms as different species.  One might think that reversing the direction of selection pressure could reverse a genetic change, but genetic change begins with random mutations which are not determined by selection pressure.  It's very unlikely for one random process to undo another random process (imagine mixing black and white sand in a jar by shaking it, then trying to unmix by unshaking!).