Final answer:
Unilinear evolution in biology is based on the theory of evolution, which explains how life has evolved and diversified from a common ancestor. Favorable traits passed on through generations lead to the unity and diversity of life.
Step-by-step explanation:
The basis of unilinear evolution in biology is the theory of evolution, which states that all life has evolved and diversified from a common ancestor. This theory, proposed by Charles Darwin, explains how variations within a population occur through mutations, and how individuals with favorable traits are more likely to survive and reproduce. Over time, these favorable traits can accumulate in succeeding generations, leading to the unity and diversity of life.