But what does it mean to be “more
closely related”? Relatedness should be
understood in terms of common ancestry—
the more recently species share a common
ancestor, the more closely related they are.
This can be seen by reference to pedigrees:
You are more closely related to your first
cousin than to your second cousin because
your last common ancestor with your first
cousin lived two generations ago (grandparents),
whereas your last common ancestor
with your second cousin lived three
generations ago (great-grandparents).
Nonetheless, many introductory students
and even professionals do not find it easy to
read a tree diagram as a depiction of evolutionary
relationships. For example, when
presented with a particular phylogenetic
tree (see the figure, left), people often erroneously