Several sources I read described black squirrels as a “melanistic phase” of the Eastern gray squirrel. “Phase,” in this instance, confuses me. Common usage of “phase” is something that one passes through. (“He’s just going through a phase.”) The black squirrel doesn’t change into a gray squirrel (like a stoat changes into an ermine in the winter). What, in this context, does “phase” mean?
Biologists sometimes use the term simply to refer to a characteristic that distinguishes some individuals of a group. It doesn’t have to be a stage the organism passes through, although the word is used in this sense as well.
The dictionary is, as always, your friend.
This bit of jargon is admittedly confusing.
“Phase” can apply to the different permanent color varieties found in a species, such as gray and rufous phases in screech-owls or melanistic phases in hawks. These are also known as “morphs.”
It also is applied to seasonal color variations found in some species, such as the white (or winter) phase of snowshoe hares or stoat/ermine.