Improving ArcPy script for ArcMap to only return rows of selected polygons instead of all polygons in layer

I am debugging a code provided to me that merges two polygons and alters the fields of “FID1′ and “CALC_AREA.”

I have realized that the current code searches every layer in the map document for fields containing “FID1” & “CALC_AREA”, sorts them ascendingly, chooses the ID matching the largest “CALC_AREA” selected, and assigns the value to all IDs of merged polygons.

def populateMergeID():

mxd = arcpy.mapping.MapDocument("Current")
df = arcpy.mapping.ListDataFrames(mxd)[0]
for lyr in arcpy.mapping.ListLayers(mxd):

    # Describe cursor object for selected polygons sorted by area
    search = arcpy.SearchCursor(lyr, fields="FID1; CALC_AREA", sort_fields="CALC_AREA A")
    FID1 = ""
    area = 0


    for row in search:
        print "Current FID1!: "
        print row.FID1 
    # Just want the first row FID1 value
        if row.CALC_AREA > area:
            area = row.CALC_AREA
            FID1 = row.FID1
# Corresponds to largest polygon in selection
    print "The container polygon has FID1 value!!: " + FID1

    del search

    with arcpy.da.UpdateCursor(lyr,"mergeid") as cursor:
        for row in cursor:
            row[0] = FID1
            cursor.updateRow(row)
            print row
    # del cursor

My question is that how would I write the script so that the search only looks for selected polygons instead of every polygon in the the map document.