Biology Reference
In-Depth Information
C
B
D
A
E
Fig. 11.
An example of a CMP phylogram.
rates. Two lineages [
AD
] and [
AE
] could have evolved at precisely the
same rate since they diverged from their cenancestor A, but if one line-
age [
AD
] has more internal nodes along the path from node
A
to the leaf
D
, its total length will inevitably tend to be larger than the total length
of lineage [
AE
]. The phylogram (Fig. 11) correctly tells us that the rate
of transformation is higher in branch [
BD
] than it is in branch [
BC
], but
the length of branches [
AC
] and [
AD
] is not directly comparable to that
of branch [
AE
].
This size illusion effect is the same as in a small-scale (or low-
resolution) map displaying roads. When switching to a larger-scale map
(one with a higher resolution), more and more curves in the road
become apparent. Measuring the length of a road on both maps will give
different results and the one with more detail will invariably give a larger
result. A road appears to become “straighter” when the resolution is
decreased. The same holds true for CMP analysis: since the states of all
characters at the internal nodes are reconstructed, the more nodes there
are on a path, the more precise the measured evolutionary distance will
become. As precision grows, the measured dissimilarity can only become
larger. Thus, comparing two branches with a different number of
nodes along each one is like measuring two roads on maps of different
resolutions.
Another, more subtle effect is long-branch attraction.
5,6
When two or
more lineages evolve rapidly, the probability of convergence arising by
pure chance increases. This effect is particularly pronounced because of
the small alphabet size of nucleotide sequences. The overall result is that
rapidly evolving lineages — or long branches in the tree — tend to be
placed phyletically close to each other. Long-branch attraction may be
avoided by adding taxons that are related to those which have evolved
rapidly in order to break up the longer branches.
