The word is derived from clanna , the Gaelic for children, and is notionally applied to all who descend from a tribe's eponymous progenitor. But although all who bear the name Macdonald, or one of its similar spellings, may claim to be of the clan, a descent from Donald of Islay is not an essential qualification. So clansmen need not be related.
Some clans have several different origins located in different parts of Scotland.
Anderson, for example, is the Son of Andrew, as is also MacAndrew, and groups of families deriving those two surnames from the popular christian name of Andrew have effectively merged into a single clan with no pretence of common ancestry. If someone wishes to be a member of a clan, acceptance of the Chief is in practice sufficient. Your Tartan Despite the ubiquitous nature of the deplorably ignorant charge that tartan was invented in the 19th century, its long history is irrefutable. However, the clear relationship between specific names and specific patterns that we know today did not always exist. In early times patterns were associated with districts, not names, and the colours of the designs were influenced by the availability of the required vegetable dies in those districts.