The self-portrait and the selfie are two separate, though at times overlapping, efforts at establishing and embellishing a definition of one’s self.
Qualities like medium specificity, deeply rooted histories, and traditions (or lack thereof) that define these efforts only superficially differentiate the two. What has greater weight is the selfie’s inherently replaceable and even disposable quality. If after taking a picture of oneself the results are unsatisfactory, it is easily forgotten and replaced by a new picture.
The self-portrait, whether it is a carefully composed study or created in haste, often contains more decisions than could be easily erased. Calling a self-portrait by Rembrandt a selfie is not only anachronistic, it also negates the carefully calculated set of decisions that created the rendering.
This does not mean that selfies cannot be self-portraits, or that selfies by nature require the opposite of calculated intent. An artist could choose to represent him or herself through selfies; however, self-portraits don’t immediately signify selfies.