If evolution is effective, why do men still have nipples? In extreme situations, can fathers lactate and breastfeed their children if the mother is not present to do so?
Males secrete factors that block the development of female ducts and structures. Once the male embryo produces testosterone, the hormone can influence other sex-specific traits around the body. Men having nipples doesn't really have any evolutionary advantage, but it usually doesn't hurt anything either.
Barring specific medical conditions—like a tumor on the pituitary gland—men generally lack the necessary levels of prolactin to stimulate lactation and cannot produce milk.
Despite having a limited amount of underdeveloped breast tissue, men are still capable of getting breast cancer. It is extremely rare for a man to develop breast cancer, and men account for less than 1% of all breast cancer cases, but it can happen. Risk factors include estrogen levels, obesity, alcohol consumption, and liver disease.
Evolution tends to select out traits that are harmful (in quite simple terms) AND expressed soon enough to negatively affect reproductive success. Nipples (and breast tissue) on males are extremely unlikely to have a negative effect on reproduction, which means there's no negative selective pressure on them. Since all embryos have the tissue required to develop breast, and since there's nothing to stop that developmental pathway, they go ahead and develop them. And since doing so does not negatively affect their reproductive success later in life, they are at no disadvantage to any potential male who might express a mutation that would stop the development of breast tissue, so they go right ahead and reproduce with their wildtype, which then allows this trait to continue in spite of its lack of utility.