Darwinian puzzle  A Darwinian puzzle is a trait that appears to reduce the fitness of individuals that possess it. Such traits attract the attention of evolutionary biologists. Several human traits pose challenges to evolutionary thinking, as they are relatively prevalent but are associated with lower reproductive success through reduced fertility and/or longevity. Some of the classic examples include: left handedness, menopause, and mental disorders. These traits are also found in animals, a peacock shows an example of a trait that may reduce its fitness. The bigger the tail, the easier it is seen by predators and it also may hinder the movement of the peacock. Darwin, in fact, solved this "puzzle" by explaining the peacock's tail as evidence of sexual selection; a bigger tail confers evolutionary fitness on the male by allowing it to attract more females than other males with shorter tails. The phrase "Darwinian puzzle" itself is rare and of unclear origin; it's typically talked about in the context of animal behavior. Darwinian puzzles are evident in nature, even though it appears to reduce the fitness of the individual that possesses it. Different individuals use the odd phenomenon in particular ways such as toxins, fitness demonstration, and mimicry. There are a few contributing factors in biology which may affect Darwinian puzzles. This may be based on common descent. Animals may share 1/4, 1/2, or even in some cases all of their genes with others. Identical twins have a coefficient of relatedness of r=1. Full siblings have a coefficient of relatedness of r=.50, and half siblings and first cousins have a coefficient of relatedness of r=.25. Depending on how related two animals are, they may be more likely to act altruistically to one another. Even if it is of no benefit to themselves, it helps to promote survival of at least some of their genes as they are shared with others closely related to them. The formula to calculate coefficient of relatedness is (RXY) = Σ (1/2)n. Hamilton's rule is often used to explain altruism in populations based upon relatedness. It can be represented by the formula (rB > C) where r represents relatedness, B represents benefit to the recipient, and C represents cost to the altruist. Animals may use Hamilton's rule in many instances where it will not promote their own fitness, but will have an evolutionary effect on the overall fitness of the species in general. The following phenomena are sometimes called Darwinian puzzles: