T HE TREE of LifE 33
else we can learn from phylogenetic studies. This
chapter and Chapter 16 delve into these questions.Phylogenetic Trees
There are two major processes in the evolution of
a higher taxon, which is a named group of organ-
isms above the level of species. These are ana-
genesis, which is evolutionary change of features
within a single lineage (species), and cladogenesis,
or branching of a lineage into two or more descen-
dant lineages. Following cladogenesis, anagenesis
in each of the descendant lineages results in their
becoming more different from each other (diver-
gence, or divergent evolution). A phylogeny is the
history of the events by which species or other taxa
have successively arisen from common ancestors.
The branching diagram that portrays this history
is called a phylogenetic tree (FIGURE 2.6). Other
kinds of evolutionary events are sometimes also
represented in such diagrams, such as extinction
(e.g., taxon F in Figure 2.6) and reticulation, which
occurs when two lineages merge or form a hybrid
descendant, so that the tree has a netlike structure.
We will focus on branching trees here.
The phylogenetic tree in Figure 2.6 shows three
living species: human (A), chimpanzee (B), and bonobo (C). (A similar tree could also
represent three higher taxa, such as lizards [A], crocodiles [B], and birds [C].) Each
segment in the tree is a lineage, or branch, which may split at an internal branch point
or node (such as D), representing the formation of two descendant lineages (B and
C) by speciation from their common ancestor. All the descendants of any one ances-
tor form a clade (also called a monophyletic group); thus B and C form a clade that is
“nested” within the larger clade A + B + C. Two clades that originate from a common
ancestor are called sister groups. (If B and C are species, they are sister species.)
The tree in Figure 2.6 represents the genealogical relationships among the taxa,
meaning the temporal order of branching by which they have originated from the
common ancestor (E in this case). The lineage leading to the most recent com-
mon ancestor (MRCA) of all the species in the phylogeny is called the root of the
tree. Thus a tree has an implicit time scale from past (at the root) to more recent
time (e.g., the present). This time scale (which is often omitted from published
phylogenetic diagrams) is often relative, implying only the order of branching. In
some cases, however, an absolute time scale is used, and branch points are drawn
to match the dates at which the branching events are thought to have occurred.
As we will see, phylogenetic trees can convey information not only about the rela-
tionships among species and their time of divergence, but also about evolutionary
changes in phenotypic and genetic characteristics and geographic distributions.
The order of branching in a phylogenetic tree defines which species are more
closely and which are more distantly related. Two species are more closely related
to each other than to a third species if they are derived from a more recent com-
mon ancestor. By analogy, two siblings are more closely related to each other
than they are to a cousin because they share more recent common ancestors
(their parents) with each other than with their cousin (a grandparent).
Closeness of relationship is not the same as similarity. A person might more closely
resemble her cousin than her sister with respect to eye color or many other features,Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_02.06.ai Date 11-02-2016TimePresentPastA B CDEFTerminal nodeNames of terminal
taxa (e.g., species)Lineage, or branchExtinct
lineageInternal nodes
(common ancestors)Root of treeFIGURE 2.6 A phylogenetic tree of three taxa, human (A), chimpanzee (B),
and bonobo (C), illustrating major phylogenetic terms. The time scale in most
phylogenetic trees is a relative one, but the tree always implies the passage of
time from the root of the tree toward the tips of the branches.02_EVOL4E_CH02.indd 33 3/23/17 8:59 AM