Buck is a dog who is the protagonist of The Call of the Wild by Jack London, and this novel tells his story from benevolent captivity to wild dog. While Buck doe snot become the leader of the sled dog team right away, we see some leadership qualities in him from the beginning as well as throughout the rest of the story.
Buck is the protector of the estate he lives on in California, which is a precursor to his later role as leader. Of course, there is admittedly not much to guard in his idyllic life, but he nevertheless seems to be a born leader.
Once he is stolen and shipped to Alaska, Buck has to learn how to adapt. This is a quality of leadership, and Buck does it. Although he is abused, has to fight for his position with the other dogs, and nearly starves to death, Buck adapts. That is what effective leaders do when they have no control over their circumstances.
Buck also perseveres. Everything in his life changes--his environment, his owners, his food, his treatment, even his name--yet Buck perseveres. When he is beaten with a club, it takes Buck several times to figure out that he will not be successful. He learns the hard way, it is true, but he learns and he will not forget. Once Buck finally learns this lesson, he again adapts to his new surroundings, his new owners, and his new method of discipline.
Buck is also physically imposing, making him fit to be the new leader of the pack.
His development (or retrogression) was rapid. His muscles became hard as iron, and he grew callous to all ordinary pain.... He could eat anything, no matter how loathsome or indigestible.... Sight and scent became remarkably keen, while his hearing developed such acuteness that in his sleep he heard the faintest sound and knew whether it heralded peace or peril. He learned to bite the ice out with his teeth when it collected between his toes; and when he was thirsty and there was a thick scum of ice over the water hole, he would break it by rearing and striking it with stiff fore legs. His most conspicuous trait was an ability to scent the wind and forecast it a night in advance. No matter how breathless the air when he dug his nest by tree or bank, the wind that later blew inevitably found him to leeward, sheltered and snug.
These are the physical attributes which Buck has developed through his deprivation and the harshness of his new life, and they are the qualities of an effective leader. Without these skills, of course, Buck would never survive his life in the wilds of Alaska. He knows how to take care of himself, and although it may not be his personal responsibility to make sure the other dogs take care of themselves, he does set a good example for the others to follow.
Buck's inherent nature has also been somehow activated, and this is another reason Buck makes a good leader. He trusts his instincts and uses them to keep the others in line. Buck learns that his latent (previously hidden) instincts are still strong and he revels in them throughout most of this story.
Finally, Buck is loyal. We do not see much of that until after he takes over the leadership, but we do see it.
Buck protects what he has been given, he adapts when necessary, he perseveres despite harsh and cruel treatment, he is physically strong and imposing, he has grown closer to the native instincts of his species, and he is lyal. These are the qualities which contribute to making him effective as the new leader of the team.