Question

____ happens when a lineage splits into two or more lines of descent.

Answer 1

Speciation happens .

Answer 2

When a lineage splits into multiple line of descents, then, it leads to speciation. It is a process by which a single population evolves into different species.  The process of speciation leads to the development of two or more species as  the lineages are splitted. These two evolved species are genetically independent.

Hence, the blank can be filled with 'speciation'.