Is Evolution Reversible

It depends what you mean by reversible, really. One could make an argument that snakes, sea cucumbers and dolphins represent evolutionary reversals, for example.

I had a thought about this. You seem to be thinking of forward, and backward as directions evolution can take. This is, of course, not true. If you want to consider an evolutionary change as having a direction, you'd need to consider it in a (very) multi-dimensional space. For example, if you had DNA coding for just this short section of protein LIAGMP, a change in a single one of those amino acids is effectively a direction evolution can take, so with just six amino acids you have six directions! But, wait, that's not right because there are 19 different possible other amino acids each one could change to.As you can imagine the number of "directions" rapidly becomes bewilderingly high. So, in fact, "backwards" is not a choice of two options; it's a choice of billions so the chance that a new mutation will directly reverse a prior mutation is tiny*.* - unless we're considering bacteria which have small genomes, huge population sizes and tiny generation times meaning the probability of any change occurring is very high indeed.

Hmm... not sure about that. Humans have 46 chromosomes, but we almost certainly evolved from animals with 48 (given that Gorillas, Chimps and Orangs all have 48).