Cladograms and Phylogenetic Trees: Evolution Classifications

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  • 0:02 Cladograms &…
  • 2:50 Types of Clades
  • 5:10 Building &…
  • 7:20 Lesson Summary
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Lesson Transcript
Instructor: Laura Enzor

Laura has a Master's degree in Biology and is working on her PhD in Biology. She specializes in teaching Human Physiology at USC.

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
example of a family tree

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
An example of a typical cladogram and 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
A phylogenetic tree depicting the genetic relationship between eels, fish, sharks, and birds

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
An example of a monophyletic clade

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
An example of a paraphyletic clade

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
An example of a polyphyletic group

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