Evolution__3rd_Edition

424 PART 4 / Evolution and Diversity
Species
Time
(a) (b) (c) (d)
A B C D C A B D A C B D A B C D A B C D
Figure 15.1
A phylogeny shows, for a group of species, the order in which
they share common ancestors with one another. (a) Species A
and B share a more recent common ancestor with each other
than either does with C; the group of species A, B, and C share a
more recent common ancestor with one another than any of
them do with species D. (b) In a phylogeny, any of the nodes
can be rotated without altering the relation shown: (a) and
(b) are identical, but (c) and (d) differ from (a) and (b) because
the order or the pattern of branching is altered. (e) Phylogenies
may be drawn with either right-angled or diagonal lines; the
information is identical: (d) and (e) are the same phylogeny.
What a phylogeny is
The only information in the phylogeny is the order of
branching: the x-axis does not necessarily represent phenetic
similarity. In particular, (d) does not imply that species B and
C show convergent evolution. Sometimes a phylogenetic
diagram does also display phenetic similarity (e.g., Figure 15.6),
but then it is explicitly drawn on. The vertical axis expresses
the direction of time, which goes up the page. However,
the axis is not usually exactly proportional to time: (a) does
not imply that the time between the successive branching
was constant. Some phylogenetic diagrams do display
absolute time, and then it is again made explicit (e.g.,
Figure 15.12).
15.1 Phylogenies express the ancestral relations
between species
A phylogenetic tree, or phylogeny, or tree, for a group of species is a branching diagram
that shows the relationships between species, according to the recency of their common
ancestors. For each species, or group of species, a phylogeny shows which other species
(or group of species) it shares its most recent common ancestor with. A phylogeny
implicitly has a time axis, and time usually goes up the page. In Figure 15.1a, for
instance, species A and B share a more recent common ancestor with each other than
either shares with any other species (or group of species). There are many possible phylogenies
for the four species, A, B, C, and D, in Figure 15.1. Maybe A shares its most
recent common ancestor with B, as shown in Figure 15.1a. Or maybe A shares its most
recent common ancestor with C, as shown in Figure 15.1c. Figure 15.1d is another possibility.
In all, any set of four species have 15 possible phylogenies. 1 The problem of
phylogenetic inference is to work out which of those 15 is correct, or most likely to be
correct. The answer has to be found by inference, rather than by direct observation or
experiment. The splitting events occured, and the common ancestors lived, in the past.
They cannot be directly observed.
1 We are assuming that the phylogenies only contain two-way splits, or bifurcations, and no higher order
splits, such as three-way splits, or trifurcations. The assumption is likely to be valid, given the number of
species that exist and the amount of time they evolved in.
(e)
..