There are various deviations of most surnames. I see Calvert deviations such as Colbert, Colvert. There is a surname study for Calvert at the Guild of One-Name Studies. There are also a couple of DNA projects for the Calvert surname which may be the best bet for determining which branch some may be connected with. With DNA branches may start to differ with a few markers but have a common source. The more markers tested and compared the better. FamilyTreeDNA has the biggest Calvert database with about 105 names. There are also other DNA projects with variations on spellings.
Colleen Fitzpaterick created a program to enter Cladograms where DNA results can be added to show how various results are spread out. Colleen wrote the book "DNA & Genealogy" and has some of this information free online.