Evolutionary trees are collections of hypotheses about the genealogical relationships between taxa (species or groups of species). Using different data, different tree building techniques, or different taxonomic representatives can lead to lots of different trees that need to be compared with one another. This module will introduce you to some ways to compare trees and provide you with some practice problems.

We will be working with data and trees used to study the phylogenetic placement of whales (Cetacea). As highly modified mammals it is not immediately apparent where they fit in the hierarchy of the tree of life.

There is currently little debate that whales, are ungulates. That is, whales arose somewhere within the collection of past species that gave rise to the extant hoofed mammals (including deer, rhinoceroses, horses, camels, pigs and hippopotamuses). What is less clear is where in the tree they belong.

Figure 1: Which of these groupings is correct? Are the Cetacea more closely related to the even-toed or the odd-toed ungulates?

The two trees below represent different hypotheses about the evolutionary relationships among the whales and various ungulates. Within the hoofed mammals there is a well defined split between those with an even number and those with an odd number of toes. The even-toed hoofed mammals are often labeled as the Artiodactyla and include pigs, hippopotamuses, llamas, cattle, deer and goats. The odd-toed hoofed mammals go by the label Perissodactyla and include horses, zebras, tapirs and rhinoceroses. Due to the extensive morphological adaptation that occurred in the whales evolutionary lineage it is difficult to place them within the ungulates simply by counting their toes.

Figure 2

Figure 2: Two alternative phylogenetic trees. Describe how the placement of whales is different in these hypotheses.

One of the powerful implications of descent with modification relationships is that even when looking across different lines of evidence (e.g., behavioral, molecular, biochemical, morphological) the same general pattern of evolutionary relationship among species should be found. Any one line of evidence may have "noise" – similarities or differences between taxa that don’t show simple descent with modification patterns. In these situations more similar may not necessarily imply more closely related. However, looking across many lines of evidence should overcome the messiness of particular characters and make it possible to identify the evolutionary "signal" that defines the true historical relationships between taxa.

So, we have the very real problem of looking at multiple evolutionary trees and trying to decide how similar they are. The following set of activities will allow you to puzzle through some of the details of comparing trees and determine the phylogenetic position of whales among the ungulates.