Family trees help show how people are related to each other. Similarly, scientists use cladograms and phylogenetic trees to study the relationships between organisms.
Cladograms and Phylogenetic Trees
We sometimes use family trees to show relationships between individuals. Those who are closely related are located closer together than those who are only distantly related.
A family tree can show the distance between genetic relationships
This same idea of relationships can be used in science. Biologists use cladograms and phylogenetic trees to illustrate relationships among organisms and evolutionary relationships for organisms with a shared common ancestor.
Both cladograms and phylogenetic trees show relationships among organisms, how alike, or similar, they might be. We can see a typical cladogram and phylogenetic tree here.
An example of a typical cladogram and a phylogenetic tree
First, a cladogram can look at trees that may have been derived from a common ancestor to arrange organisms on different branches. But those branches used aren't representative of the relative amount of change or evolutionary time that has occurred between organisms. Plus, a cladogram doesn't necessarily show exact relationships between ancestors and descendants.
On the other hand, the branches on a phylogenetic tree can be proportional to the amount of change or evolutionary time. So, you can also track how species have changed over time. Species are still grouped according to similarities and physical or genetic characteristics - for example, the presence or absence of gills. But, a phylogenetic tree describes an evolutionary history by showing how ancestors are related to their descendants and how much those descendants have changed over time.
There are further distinctions, and to further complicate matters, different analyses and new information can yield different possible evolutionary relationships. Luckily, for the scope of this lesson, you can think of a phylogenetic tree as a cladogram with a few added bells and whistles.
The phylogenetic tree shown here might help illustrate this concept a little better.
A phylogenetic tree can be used to chart evolutionary relationships
Types of Clades
A clade is a group of species used in cladograms (and phylogenetic trees), which consists of one ancestor and all its descendants. The term clade comes from the Greek word klados, which means branch. Relating this back to our family tree - one clade would consist of the great-great grandparents, all the way down to the siblings.
Just like there are different types of families, there are different types of clades. The three major types are: monophyletic, paraphyletic and polyphyletic.
Monophyletic refers to just one clade; meaning these terms are interchangeable. 'Mono-' means 'one,' making this easy to remember. As stated before, a monophyletic clade includes one ancestor and all of its descendants. An example of this would be the genus Homo. This genus includes all the species from ancestral humans up to modern-day humans, or Homo sapiens. We can see an example of a monophyletic clade here.
A monophyletic clade includes all the members of a species from one common ancestor
A group is said to be paraphyletic if it consists of all the descendants of an ancestor minus one or two small groups. The prefix 'Para-' means 'around' or 'surrounding.' A good example of a paraphyletic clade is the reptiles. Reptiles, mammals and birds all share a common ancestor. However, the reptiles form a paraphyletic clade to the other two groups. We can see an example of a paraphyletic clade here.
A paraphyletic clade includes one ancestor group and its surrounding groups
Polyphyletic groups are characterized by the presence of homoplasies, or characteristics which appear similar, giving the impression they were inherited from a common ancestor, but in actuality, they weren't. This means that instead of sharing one common ancestor, these groups have multiple origins. Polyphyletic groups can be confusing, and phylogenists and cladists rarely use them. The best examples of a polyphyletic group are the pachyderms, or elephants, rhinos and hippopotamuses. These animals each have their own common ancestor, but are grouped together as a polyphyletic group because they all share a similar thick, wrinkled skin and hooves. ('Pachy' means 'thick,' and 'derm' refers to the skin.) We can see a polyphyletic clade here.
Polyphyletic groups include animals evolutionarily independent but sharing common traits
Building & Interpreting Cladograms
Cladograms show possible evolutionary relationships, and in order to build and interpret these visual tools, we first need to understand key differences. Generally, each branch or separation on a cladogram represents an evolutionary change or difference. This feature may or may not be indicated on the diagram.
In order to build a cladogram, you first need to take an inventory of characteristics of the species that you wish to classify. Let's look at a small group to illustrate how to build a characteristic chart and then an evolutionary tree. Let's say we have four organisms: A, B, C and D. Organism A has one eye, while the other three have two eyes. Only organism D walks on two legs, while A, B and C walk on four legs. Organism A has a tail but none of the others do - though organism B somewhat has a short nub resembling a tail. Now that we know some characteristics of each organism, we can fill in our characteristic table. We can see here that we have checked off when traits are present or absent. This will help us decide who is most likely evolutionarily close together and who is further apart.
We can now use this chart to help construct a cladogram. Also, notice that we're building a cladogram here and not a phylogenetic tree here because our branches don't represent evolutionary time or amount of change. We will start by identifying the organism that looks least like the others. In this case, A is the only organism with a full tail and with one eye. A will be our 'out' group. We can then look at our other options to see what may be next evolutionarily. B is most similar to A because it has a short tail. Next will be C as it still walks on four legs - just like B. Our last organism is D as this species has two legs, unlike the others. Here is what this may look like as a cladogram.
How our cladogram might look
Both phylogenetic trees and cladograms help show the relationships between different organisms, but only phylogenetic trees have branches that represent evolutionary time and amount of change. These were formerly based on physical characteristics, but are more accurately based on genetic relationships. Organisms that are closely related are found closer together than those that are not closely related. We related this to a family tree in which people who are closely related are near each other and those who are not as closely related are further apart.
We then looked at three different groupings or clades. The three major types are: monophyletic, paraphyletic and polyphyletic. Monophyletic means one clade, paraphyletic means around one clade and polyphyletic means many clades.
Lastly, we talked about how to build a cladogram based on a characteristic chart. The most important thing is to identify the organism that stands out the most and then identify the evolutionary possibilities from there. The benefit of using cladograms and phylogenetic trees is that we can visualize relationships in order to better understand our world.
By the end of this lesson, you should be able to:
- Define cladogram and phylogenetic tree and recall the differences between the two
- Define clade and identify the three major kinds
- Tell whether a clade is monophyletic, paraphyletic, or polyphyletic
- Build a cladogram