What is tanistry?

TANISTRY-
A custom among various Celtic tribes- notably Scotland and Ireland- by which the king or chief of the clan was elected by family heads in full assembly. He held office for life and was required by custom to be of full age, in possession of all his faculties, and without any remarkable blemish of mind or body. At the same time and subject to the same conditions, a tanist, or next heir to chiefancy, was elected, who, if the king died or became disqualified, at once became king. Sometimes the king's son became king, but not because the system of primogeniture was in anyway recognized; indeed, the only principal adopted was that the dignity of chieftainship should descend to the eldest and most worthy of the same blood, who well could be a brother, nephew, or cousin. This system of succession left the headship open to the ambitious and was a frequent source of strife in families and between clans. Tanistry in Scotland was abolished by a legal system in the reign of James 1 (1406-37) and the English system of primogeniture substituted.
Source cited: Encyclopedia Britannica

It has been said that a female relative could not be a tanist, only a male could. I imagine it wouldn't have been uncommon for older men to marry younger woman or girls in hopes of a male heir. Could/would a chief have more than one wife or was divorce possible if a current wife didn't produce a male heir?

We know that the people of a clan revered their Chief. They looked up to him for protection and guidance. A chief could call an army together amongst his people very quickly and at little or no cost. A chief was judge and jury. He would rule between disputes and decided punishment.

Though the sentence up above says that tanistry was abolished many years ago, it is eveident among the clans today that this practice still lives on in some aspects.