Populus is the genus or family name for cottonwood, poplar, and aspen. There are probably a half dozen different types of poplar listed under this name and the quality of the poplar may depend upon the area and the type of poplar that is being harvested and made into boards.
If you look at the chart that Bob Henry supplied, please recognize that there are actually two types of poplar listed but they are listed under their more common name, cottonwood, with only a secondary listing for one as poplar. Notice the differences listed between black cottonwood and the balsam cottonwood.
In several places, I have read that the reason for using a framework is primarily so that you will have something to anchor your screws into. After all, using 1/8â€