And the Gurus were chosen by the last Guru. So a Guru in his lifetime would annoint a successor. But in case when they couldn't, like when they were martyred or died young; the other senior men/council selected a Guru. Though many of the Gurus were related, there was only once that a son succeeded a father. That was when the last Guru - Gobind Singh succeeded his father Tegh Bahadur when he was martyred by the Mughals. The 10th Guru was also the one who militarized the Sikh society and gave them the tenets that now give them their appearance i.e. Long hair.