How can we tell that two species shared a common ancestor, and are therefore related?

Homologous structures provide evidence for common ancestry, while analogous structures show that similar selective pressures can produce similar adaptations (beneficial features). Similarities and differences among biological molecules

Explanation:

There are multiple ways you can tell

you can look at the bone structure of an organism and see if it's homolougous

Or use a cladogram to tell what species share a common ancestor ( hope this helps)

the correct answer is - ability to find a mate