What type of diagram do scientists use to show how species are related

Cladograms

Explanation:

A cladogram is a diagrammatic representation prepared by the evolutionary biologists to represent the relationship of the group of organism on Earth.

Nowadays The cladograms are prepared by comparing the molecular homology between the species called phylogeny. The cladogram is used by the scientists to study the relationship between the species and their origin as the cladogram is represented in the form of a tree with branches.

Thus, a cladogram is the correct answer.