Bipedalism has many selective advantages. However, there is one particular disadvantage of walking on two feet: giving birth to offspring with a large head through the smaller pelvic opening that is necessitated by upright posture is very difficult. This situation results in a high percentage of deaths (of both mother and child) during birth compared to other primates. How do you explain the selection of a trait that is both positive and negative?
The molecular clock is an important tool in determining the number of years that have passed since two modern groups have diverged from a common ancestor. The only way that this method of determining relationships can be accurate is if you know exactly the rate at which DNA changes in the absence of selective pressure. How is the molecular clock calibrated?