So this leads to what's referred to as the parsimony method for tree-building.
Basically what we're gonna do is we're gonna compare the traits
to what we believe the ancestor would have had in terms of the trait.
Now typically speaking the ancestor not usually still around,
we don't usually get to look at them.
So what people do in practice is they look at what's referred to as an out group.
So, for example let's say you were looking at phylogeny of insects.
Let's say these were all different insects.
The out group you might use might be for example, a spider.
So something that's similar but clearly not an insect and so it's just outside.
Using the spider, and looking at all these insects, you would then try to identify
which forms for each trait are ancestral, or basically have been around
since that common ancestor and might, for example, be present in spiders as well.
Or if they're derived, a trait that has risen
within the evolutionary history of insects that you're trying to study.
So what you're gonna do then is we're going to construct a tree
using the minimum number of changes from the ancestral form to derived form.
Right the minimum number of possible changes, you'll be using for actually
looking at relationships, you'll be using what we refer to as shared derived traits.
The technical term for this is synapomorphies, but
don't worry about that.
You're looking for shared derived traits.
In this tree right here, the ancestral trait is black, right?
Black body color.
The derived trait is red.
So, which individuals share the derived trait?
The derived trait is red, the individuals that share it are Taxon C and Taxon D.
So the assumption here is that the simplest tree
is correct in terms of these derived characters.
I'm looking specifically at those.
So that places C and D as being close relatives.
This tree also uses the minimum number of changes because it only requires
one change.
So what would have happened instead?
So we have A-B-C-D-E-F and you see there's the common ancestor.
What would happen if instead C was more closely related to F, for example?
Well, let's take a look at what that tree would have to look like.