It is on the branch leading to C and D.
On the second and the third phylogeny or bifurcating tree,
it occurred once in each individual branch leading to C and D.
The same applies to the third possibility.
If you map all characters in this way,
onto the three different bifurcating trees or cladograms,
you end up with three different distributions of the black squares.
Which one should you then choose?
Number one?
Number two?
Or number three?
In fact, the right answer is number one.
And why should you choose it?
Well, you should choose it simply because, it's a simplest solution of all
hypotheses as all the other hypotheses, all the other, the two others
require one more character evolution than the one we have chosen.
This is in fact a scientific principle known as Occam's razor.
You should always choose a hypothesis that requires the least amount of
extra hypotheses,
which is called parsimony or you should choose the simplest possible
explanation explaining your character state distribution or your observation.
It is very important to stress that this is not a statement about evolution.
It does not indicate that evolution is parsimonious.
It is just a scientific principle.
This picture shows a standard published phylogeny.
I will admit, it's pretty boring, but
you do have to remember that almost anything we do when we reconstruct
the tree of life is hypothesis, from species circumscription to final trees.
This may be surprising, but
every detail is subject to scrutiny and can be changed accordingly.
So, there's a couple of important things you have to know about phylogenies.
The only real important thing about
phylogenies is that it is a matter or it shows a branching order.
It's almost like with the London tube map.
It can be drawn in an endless number of ways, but the order and stations and
the actual lines that stop at each station is the only real important information.
It does make sense if B and C are more closely
related to each other than they are to A, then this also applies to C and B.
Basically, everything can be broken into
statements involving three entities on a phylogeny, just as we have done.
If there are not three entities either implicitly or
explicitly, relatedness has no meaning.
Let's do a simple analogy.
No matter whether you like it or not, I'm related to you.
I am also related to Alice in the US who's also watching this program.