Step 1. Pick your organisms (I’m going to pick a lemur, deer, cow, chimpanzee, human, and lizard). The taxon (group) which all of these organisms have in common is the phylum
Chordata. If we removed the lizard – hypothetically speaking – the taxon would be the class Mammalia.
Step 2. List as many possible characteristics the organisms possess (five is typically ideal, but you could pick >5 if you really wanted to; the more you choose, the more accurate your cladogram). I’m going to pick:
- Five digits
- Hair
- Tail
- Opposable Thumbs
- Placenta
This goes on the first row, while the organisms we chose go into a column on their own.
Here is a list of things you can choose:
Also: | |
Step 3: Use some logic and pick which organism in the group would be considered the outgroup. In other words, which one,
in your opinion, is least common to the rest. It’s really important that you think this through because it will significantly determine how your cladogram will turn out. Using some logic, it seems like the lizard, which is a reptile, is least common with all the rest (the rest are obviously mammals).
Now that we’ve distinguished the reptile as our outgroup, let’s look at the five characteristics we decided to focus on, namely, five digits, hair, tail, opposable thumbs, and placenta.
Make a small list like this:
Using this information, we can judge which trait is the more recently derived trait. This can usually be done by comparing the traits with the outgroup (our lizard). What this means is that since the lizard is our outgroup – the one that is least related – the
shared derived characteristics we’re going to choose will be the opposite of what our lizard possesses. For example, the lizard has 5 digits (a primitive condition), so the shared derived characteristic will be “Does not have 5 digits”. The lizard has no hair (a primitive condition), so a shared derived characteristic will be “Has hair”. Having a tail is a primitive condition, so a shared derived characteristic is “Has no tail”. Has no opposable thumbs is a primitive condition, so a shared derived characteristic is “Has opposable thumbs”. And finally, Has no placenta is a primitive condition, so “Has a placenta” is a shared derived characteristic. Remember that shared derived characteristics are called
synapomorphies. We can start building our table.
Now fill it in. If the organism has the shared derived trait, put a plus (+); if the organism does not have the shared derived trait, put a minus (-).