Phylogenetic Tree:
Look at the tree below
and describe what sister relationships it depicts:
Phylogenetic Tree:
Look at the tree below and describe what
sister relationships it depicts:
Tree 1:
Species 5 is the sister group to
species 6
Species 3 is the sister group to
species 4
The clade of species 3+4 is the
sister group to the clade of species 5+6
The clade of species (3/4)+(5/6) is
sister to species 2
Species 1 is sister to the clade
containing all the other species
Remember, trees can be rotated at the nodes
without changing the topology (i.e. without changing the
relationships represented in the tree).
For example, look at the tree below.
Tree 2:
Although tree 2 looks superficially quite different from
tree1, examine the relationships we deduced from tree 1 and see
if they still apply.
Species 5 is the sister group to
species 6. . . YES
Species 3 is the sister group to
species 4. . . YES
The clade of species 3+4 is the
sister group to the clade of species 5+6. . . YES
The clade of species (3/4)+(5/6) is
sister to species 2. . . YES
Species 1 is sister to the clade
containing all the other species. . . YES
THUS, Tree 1 and Tree 2 are identical trees
that are simply drawn differently (rotated at the nodes).
Now examine tree #3 below in the same way
Tree 3:
Species 5 is the sister group to
species 6. . .NO
Species 3 is the sister group to
species 4. . .YES
The clade of species 3+4 is the
sister group to the clade of species 5+6. . . NO
The clade of species (3/4)+(5/6) is
sister to species 2. . . NO
Species 1 is sister to the clade
containing all the other species. . . YES
Thus, although some branches in the tree are
similar to tree 1 and tree 2, tree 3 is NOT the same tree!
Inferring Ancestral Characteristics on a
Tree:
Look at the tree below and try to infer the
character state (Blue or Red) of the ancestral node
Let's begin by guessing that the mystery
ancestor was red
In this case, we would have to have the
evolution of blue flowers twice in order to best explain the
tree (i.e. 2 evolutionary steps).
Now let's see what happens if we guess blue
to be the ancestral character.
In this case, the simplest explanation is
that red flowers evolved once (one evolutionary step). Since it
only calls for one change rather than two, assuming blue to be
character at the ancestral node is the best and most likely
explanation.
Monophyly vs. Paraphyly:
The tree below first shows an outgroup
(family Outgroupaceae) to the left, and some other species
labeled 2 - 6.
If we want to name these ingroup
species as families, we want to be sure to only name
Monophyletic groups. At first, a logical way to group these
ingroup plants as families could be to segregate them by color
into two families, Blueaceae (containing species 2, 5, and 6)
and Redaceae (species 3 and 4).
First, we'll examine if Redaceae is a good
monophyletic group. Follow the branches down to the node that
would represent their common ancestor of species 3 and 4.
We can easily see that Redaceae would include
that ancestor and all of its descendants (only species 3 and 4).
Red color is a derived trait that is shared between those two
species(a synapomorphy), and thus groups them together as
a monophyletic group.
Now find the common ancestor of all of the
blue species
Follow all the descendant branches from that
common ancestor. The red species 3 and 4 are descendents of that
ancestor too!
Grouping the blue species together based on
the retention of an ancestral characer state makes it a
paraphyletic group. Thus, naming species 2, 5, and 6 as the
family Blueaceae is not acceptable.
So what are some monophyletic groups we could
name as families? If we include species 3 and 4 in the Blueaceae
(even though they're red!), it makes it a monophyletic family.
Taxonomists who combine formerly recognized
families in order to make a larger monophyletic family are often
called lumpers. Another way to create monophyletics is to be
what is called a splitter. Splitters divide the larger
monophyletic group into smaller monophyletic families, often
with some families containing only one genus or species.
A splitter might divide the ingroup into
three families in order to retain Redaceae as its own family. To
do this, you must separate out species 2 as its own family. In
this case, you could still call species 5 and 6 'Blueaceae'
since they form a monophyletic clade. Species 2 would have its
own family name (Neoblueaceae, perhaps?). And Redaceae would be
a monophyletic family that is no longer included in a larger
paraphyletic group.
Both splitting and lumping are equally
correct and viable solutions, as long as the resulting groups
are monophyletic. Sometimes lumping results in excessively huge
groups or causes a popular, easily recognized taxon to be
eliminated at the family level. Splitting can lead to a
ridiculous number of families to be recognized and
disintegration of natural groups at the family level. Both
philosophies are widely applied. And both provide headaches to
botanists who are constantly having to relearn plants under a
new naming system.
Polytomy:
When a phylogenetic tree has more than two
branches radiating from a node, it is called a polytomy.
Polytomies arise when the relationships between a group of taxa
are unresolved. For example, look at the polytomy in the tree
below
The relationships between species 3 , 4, and
5 are unresolved. Species 4 and 5 could be a sister group, with
species 3 as the sister group to their clade.
Alternatively, species 3 and 5 could be a
sister group, or species 3 and 4 could be a sister group.
The polytomy shown in the original tree means
that any one of the three trees that followed might be correct,
but it is unresolved, and we just don't know the true
relationships at that node..
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