A critical appreciation of a poem is a critical (close) reading of a poem. It does not mean to criticize the poem. Rather, a critical appreciation will examine a poem's meaning, themes, mood, tone, rhyme scheme, meter, figures of speech, and other poetic elements. Your critical appreciation may certainly explore each of those things in turn, or your critical appreciation can focus on a few of those items.
A critical appreciation can seem overwhelming, so my recommendation to students is to start with the easier, concrete parts of the poem. Sherman Alexie's "Evolution" is made up of five different stanzas. The stanzas are each three lines long, so there is some uniformity to the poem in that respect. All of the lines are of similar length as well; however, the poem is written in free verse. This means that the lines of the poem do not rhyme, nor is there a regular meter. Feel free to explore why you think the poet chose free verse. Perhaps the chaotic nature of free verse helps to mirror the chaotic deception and abuse faced by Native Americans in the poem.
That basic interpretation of the relationship between the poem's verse and meaning can lead you into further discussion about the poem's meaning. You could explore who "Buffalo Bill" is, and you could definitely explore how he is symbolic of the White ruling class or the United States government. There is clearly a negative tone in the poem, as the speaker finds fault with Bill placing his pawn shop right next to the liquor store and systematically taking all belongings and culture from the Native Americans. However, the final lines of the poem do seem somewhat caustic against Native Americans themselves for letting it happen and being willing to return to a museum that has false or warped references to Native American culture.
If you want to examine specific poetic devices that are used in the poem, I would examine the poem's use of enjambment. This is the continuation of a line without a pause created by a punctuation at the end of the line. Essentially, the line spills over into the next line or stanza. This poem does contain commas and periods that force a reader to pause, but those never occur at the end of a line.