by the Understanding Evolution team

Evolutionary trees (also called phylogenetic trees) represent the relationships among different lineages, but how do we know what these relationships are in the first place? Are evolutionary trees set in stone? Are they based on facts that are simply obvious to scientists? On the contrary, biologists have to put a lot of scientific study into figuring out how different species and groups of species are related to one another. They make detailed anatomical observations and measurements, travel all over the world to collect DNA samples, and run programs on computers that may take days or weeks to finish — all for the purpose of better understanding the Tree of Life. Even very broad patterns — such as the now well-established idea that robins are more closely related to Tyrannosaurus than they are to mice — may be the result of years of scientific study and debate.

Most learners don’t need a complete understanding of how scientists construct phylogenetic trees, but knowing even the basics of how trees are built will help anyone get more out of the trees they see. In this module, we’ll review those basics:

• Trees as hypotheses. Evolutionary trees represent hypotheses about evolutionary history.
• Evidence for building trees. Evolutionary trees are based on many different types of evidence.
• Comparing characters. To build a tree, biologists compare the characteristics of different organisms.
• Not-so-shady characters. Biologists aim to use only reliable and informative characteristics to build trees.
• Tree-building basics. A few key steps are involved in any phylogenetic analysis.
• Building trees using parsimony. One method for tree building involves minimizing the number of evolutionary changes proposed by the tree.
• Other ways of building trees. Trees can be built using mathematically complex methods.
• Confidence in connections. Not all relationships on a tree have equal degrees of support from the evidence.
• Tangled trees. The usual ways we build trees can’t account for hybridization.
• True trees. These different approaches all aim to reconstruct the tree that represents the true evolutionary history of its lineages.

Click “next” to get started.