Is evolution reversible?
Evolution is not a process that can be reversed. The core principle behind evolution is that traits with a positive impact on survivability or reproduction tend to passed on to subsequent generations. Evolution itself is merely the slight alteration of a species gene pool over many generations. Perhaps you can see that the question "Is evolution reversible is something of a misnomer. Is the process by which you change something reversible In certain processes the thing that is changed can be returned to its original state, but this process also requires change. If you intended to reverse an evolutionary process the only way you could do so would be through evolution. This type of process has no non-reflexive logical inverse.
Perhaps what the question intended to ask was whether a species could "revert" back to a "less evolved" state. An example to explain this would be the reversion of humans to a more apelike genetic structure. This is certainly possible. If traits that were more apelike positively impacted the survival rate or the reproduction rate of the humans that had them than humans would begin to evolve to become more and more like our simian ancestors.
Answer 2 : There are certain examples of reversal in evolution .Most familiar trend can be seen in evolution of Dinosaurs which were of small size in Triassic period , evolved to maximum size in Jurassic period ,and again decreased in size in Cretaceous period .
Answer 3: The process of Evolution doesn't have a "direction", it doesn't favor one type of organism or trait as being "higher" than another. So in a broad sense evolution simply goes where environmental conditions dictate, there's no backward (reverse) or forward.
Could traits that were once non-adaptive reappear in a population if environmental conditions changed to make them adaptive again? Yes
Can an individual organism reacquire older traits that were not part of it's original genetic makeup? No
Do some older traits occasionally reappear in vestigial form in some individual organisms. Yes (there are instances of humans being born with a short tail etc.)
Evolution is basically a stochastic phenomenon. In order for evolution to "back-track", every single change in the genome that had occurred would have to be reversed in the exact reversal of the order that they occurred in. This is so extremely unlikely to ever occur that, were it observed, we would have to conclude aliens were pulling our legs.
But although evolution is not reversible, it is possible for traits to evolve a similarity with an earlier form. Consider for instance the fish-like shapes of whales: they evolved from land-dwelling mammals, which in turn can trace their ancestry back to early fish. Same shape, completely different genetics.
If you cloned an extinct animal, it might count as evolution going backwards. Not sure about that though, cause it's a pretty philosophical question.