Life is reproduction, mutation, and selection.
Although mutation and selection are quantified almost on a regular basis nowadays, accurate practical estimates of reproduction are more elusive, for reasons given below. Quantifying reproduction would seem to be central to several biological subspecialties. In microbiology, for example, it could help assess antiviral or antibacterial therapies, and dissect biological correlates of microbial virulence. In zoology, it could help determine reproductive success during accidental pest introductions or deliberate wildlife releases. For many populations of interest, of course, the reproductive cycle can vary from individual to individual, and even a single individual can give rise to progeny at different times.