Galaxies are sufficiently distant that only a few can be resolved into stars to test the SSP results. The closest galaxy is, of course, our own. Thus, we can look at the stars in the solar neighborhood to see what the stellar population from our Galaxy consists of. The recent Hipparcos mission to determine parallax's (i.e. distance) to tens of thousands of nearby stars results in the following HR diagram:

Note that the HR diagram from the solar neighborhood is unlike a globular cluster, mostly because of recent star formation that produces the hot massive stars in the upper left quadrant. However, the existence of a red giant branch indicates that there are some old stars in the Galactic disk.

In fact, all the features of the Galactic HR diagram can be explained by summing a series of SSP models under the assumption of constant star formation for the last 10 Gyrs.

One of the few nearby galaxies that can be examined by HST is the Phoenix dwarf galaxy. Its HR diagram is shown below.