Final answer:
The theory of evolution, which explains the diversity and similarity of life through the process of natural selection, is the central scientific theory addressing variation among organisms.
Step-by-step explanation:
An important scientific theory that explains why organisms are at once so similar and also so diverse is the theory of evolution. This theory explains life's diversity as having originated primarily through the process of natural selection. The theory of evolution is central to understanding the biological sciences, as it provides an explanation for the diversity of life that we see around us. It is supported by scientific evidence from a variety of disciplines and explains that the genetic makeup of populations changes over time, often driven by the mechanism of natural selection where organisms with beneficial traits are more likely to reproduce and pass on those traits to successive generations.