Understanding Evolution

Using parsimony

After studying some major vertebrate lineages and limiting the data to characters that are likely homologous, you might end up with the following evidence (note that there are many vertebrate lineages and many characters excluded from this example for the sake of simplicity):

Chart showing the presence or absence of certain characters in various vertebrates

Amniotic egg and skull with two post-orbital fenestrae

From studying fossils and lineages closely related to the vertebrate clade, we hypothesize that the ancestor of vertebrates had none of these features:

Chart showing characters

To build a phylogenetic tree from these data, we must base our clades on shared derived characters — not shared ancestral characters. Since we have a good idea of what the ancestral characters are (see above), this is not so hard. We might start out by examining the egg character. We focus in on the group of lineages that share the derived form of this character, an amniotic egg (A, below), and hypothesize that they form a clade (B):

Focusing on the amniotic egg

Clade based on presence of amniotic egg

If we go through the whole table like this, grouping clades according to shared derived characters (C, below), we get the following hypothesis (D):

Looking at all shared derived characters

Clade based on all charted shared derived characters

Download these graphics (four in a series of five) from the Image library.

Of course, this was just an example of the tree-building process. Phylogenetic trees are generally based on many more characters and often involve more lineages. For example, biologists reconstructing relationships between 499 lineages of seed plants began with more than 1400 molecular characters!

 

View this article online at:
http://evolution.berkeley.edu/evolibrary/article/side_0_0/usingparsimony_01

Understanding Evolution © 2021 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California