(back from nursing a cold and shoveling MORE snow)

"Can a mutation be evidence of potential evolution?"

Yes.. it can.

"mu·ta·tion
/myo͞oˈtāSHən/
noun
noun: mutation; plural noun: mutations
1.
the action or process of mutating.
"the mutation of ethnic politics into nationalist politics"synonyms:        alteration, change, variation, modification, transformation, metamorphosis, transmutation; More
humoroustransmogrification
"cells that have undergone mutation"

2.
the changing of the structure of a gene, resulting in a variant form that may be transmitted to subsequent generations, caused by the alteration of single base units in DNA, or the deletion, insertion, or rearrangement of larger sections of genes or chromosomes.synonyms:        alteration, change, variation, modification, transformation, metamorphosis, transmutation;
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Now a "variant form" may be tiny, such as small coloration differences which allow some individuals better blending to escape predation.. (there's an entire study of some moths)... or it may be large, such as extra fingers & toes, which may or MAY NOT be useful.

We tend to use 'mutation' to refer to the larger, more visible changes that happen suddenly, but the whole problem of the debate between Snail & Troubador about "show me some evolution" is that most of it is in small increments and is hard to pin down in its detailed steps. Those moths, and fruit flies...and the famous finches are examples which are **evidence** of an evolutionary process, because significant changes can be observed in a short period of time. In paleontology we have only a minuscule number of most lines of research, with a lot of guesswork & prediction involved as to where the missing pieces fit.

Now... one can debate precise linguistic use of the word 'evolution', but it does little to establish anything 'scientific' about the process itself. Sometimes it is useful to stop and examine whether much of the debate IS only about how a word is used.