Evolutionary Relationships: Definition & Diagram

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  • 0:01 How We Study Relationships
  • 1:05 Reading an Evolutionary Tree
  • 3:40 Humans and Their Relatives
  • 4:15 Lesson Summary
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Lesson Transcript
Instructor: Angela Lynn Swafford

Lynn has a BS and MS in biology and has taught many college biology courses.

Just like you can build a family tree to show the relationships of your ancestors and their descendants, scientists can build trees to show the evolutionary relationships of species. In this lesson, learn how to interpret these evolutionary trees.

How Do We Study Evolutionary Relationships?

Systematics is the part of science that deals with grouping organisms and determining how they are related. It can be divided into two main branches:

  • Taxonomy focuses on classifying, naming, and grouping organisms. A group, or taxon, can be a population, a species, a genus, or a higher-level grouping, such as family, order, class, phylum, kingdom, or domain. The plural of taxon is taxa.
  • Phylogenetics is the study of determining evolutionary relationships, or patterns of descent of organisms.

All of the species of organisms that are alive today have descended from ancestral species. This is due to evolution, or simply change over time. The evolutionary relationships of ancestral species and their descendants can be diagrammed using branching evolutionary trees. Just like your family tree, an evolutionary tree indicates which ancestors gave rise to which descendants.

How to Read an Evolutionary Tree

An evolutionary tree can also be called a phylogenetic tree, or a just a phylogeny. There are many different ways to draw phylogenies, but they do all have certain parts that you must understand before you try to interpret them.

The root of a phylogeny represents the common ancestor of all the descendants in the tree. The descendant taxa are labeled at the tips of the tree. A node splits into two branches and indicates a divergence or speciation event. The node itself represents the common ancestor of any descendants that branch off of it. The two taxa that branch off at a node are called sister taxa. They share an immediate common ancestor. In this phylogeny, taxa B and C are sister taxa. They are both equally related to taxon A.

The branches of a phylogeny can be rotated around a node without changing evolutionary relationships. If you want to determine how two or more taxa are related, it is important to look at the nodes and branches in a tree and not just the ordering of descendant taxa. Sometimes the evolutionary relationships between taxa cannot be determined. This results in a phylogeny with a polytomy, or a node from which more than two groups split.

A clade is a group that includes an ancestor and all of its descendants. Clades, also called monophyletic groups, can be nested in larger clades. For example, mammals are a monophyletic group because they all descended from a common ancestor. Within the mammals, there are also many smaller clades, such as primates or bats.

Not all groupings of organisms qualify as monophyletic. A paraphyletic group consists of an ancestor and only some of its descendants. Reptiles are animals like crocodiles, lizards, and snakes. This is actually a paraphyletic grouping because the ancestor that gave rise to all reptiles also gave rise to birds. If birds are added to the definition of reptiles, then it could be considered a monophyletic group.

A polyphyletic group is made up of various descendants with no recent common ancestor. Marine mammals are polyphyletic. Whales and seals are both marine mammals, but they are not closely related at all. Seals are more closely related to bears than they are to whales. Whales share a more recent common ancestor with deer than they do to seals.

Humans and Their Relatives

Now that you know how to read an evolutionary tree, let's look at a simple example using humans and their living relatives.

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