The traditional approach to taxonomy relies on general impressions of shape differences among creatures, these differences then being organized into hierarchal categories for the purposes of classification. Such a classification will then reflect overall similarity of organisms. The usual arrangement of terrestrial vertebrates into four classes (Amphibia, Reptilia, Aves, and Mammalia) is an example of this approach. For example, turtles, lizards, and crocodiles are all seen as 'scaly tetrapods', and are therefore placed in the same class, Reptilia. Birds, in contrast, are 'feathered flyers', and are placed in a separate class, Aves.

A more modern approach is phylogenetic taxonomy (also known as cladistics), in which shared derived characters are used to reconstruct the pattern of evolutionary history (phylogeny) of organisms. These cladistic patterns are used to construct a classification, which will then reflect degree of relationships (recency of common ancestry) among organisms. For example, among "reptiles", when crocodiles are recognized as only distantly related to turtles, and as much more recently related to birds, then crocodiles and birds (along with dinosaurs) are classified together as Archosauria.

Rather than relying on general impressions, shape differences among organisms can also be analyzed by the phenetic method, which uses precise quantitative measures and mathematical techniques to describe patterns of overall similarity and difference among individuals, populations, and species. Phenetic differences will reflect the evolutionary relationships among organisms, if evolution is primarily divergent and proceeds at a more or less uniform rate. In other cases, adaptive convergences between species cause them to be more similar than their degree of relatedness would predict (e.g., bats and birds). Alternatively, closely related species may diverge markedly due to contrasting ecologies and lifestyles (e.g., T. rex and a canary).

Wyles et al. (1983) used a simple phenetic method (morphological distance) to study the correspondence between shape difference and the degree of taxonomic relatedness for a pair of bird species and a pair of mammal species, in order to test a specific hypothesis about evolutionary constraints on bird evolution suggested in the scientific literature: that bird species cannot diverge widely, because they are constrained by the requirements of flight [see summary on the webpage].

In this laboratory, we will combine this method with a second method (cluster analysis) to test a similar type of hypothesis. We are concerned with contrasts between the evolutionary relationships of organisms, as indicated by their phylogenetic taxonomy, and their "shape" differences, as a consequence of their adaptation to habitat and/or niche. The analysis of Wyles et al. (1983) can be used as a model.

You will be assigned a set of at least five mammalian species. Identify the whole skeletal mounts corresponding to these five species. The null hypothesis will ordinarily be that phenetic differences among species reflect their evolutionary (cladistic) relationships. Species sets have generally been chosen to illustrate some principal of adaptive evolution in the form of an alternative hypothesis: the instructors will discuss the general nature of these hypotheses with you.

For each of the five species in your set, take the following measurements, as indicated. Use digital calipers whenever possible, or bow calipers for longer measurements. You will have to exercise some ingenuity in taking some of the measurements on whole mounts: consult the instructors. In your Materials and Methods, state exactly how each measurement was taken.

1. Head width: Measure the greatest possible width across the head.

2. Head length: Measure the greatest length of skull.

3. Eye to nostril distance: Consider the nostril to be the posterior-most point of the nasal opening. Measure the distance from this point to the nearest point on the eye orbit.

4. Nostril to lip distance: Measure the distance from the posterior-most point of the nostril to the midpoint between the middle two upper incisors.

5. Shank length: Measure the length of the tibia (longer shank bone).

6. Forearm length: Measure the length of the ulna (longer forearm bone).

7. Toe length: Measure the maximum length of the innermost hind toe, not including the claw or nail.

8. Backbone length: Measure the length of the articulated vertebral column, from anterior end of atlas to end of sacrum (i.e, excluding tail). [Measure the length of a piece of string run along the dorsal surface.]

Calculate the Relative Trait Length of each trait for each species. The relative trait length is defined as:

Calculate the Manhattan Distance (H) between each pair of species. For each trait, find the absolute value of the difference between the relative lengths for each pair of species. The Manhattan Distance for a pair of species is defined as the sum of the differences, for all traits.

Calculate a Group Average Cluster analysis based on Manhattan Distance, as described [Cluster analysis].

Draw a phenogram of your UPGMA analysis.

Include tables of your original measurements, the relative trait lengths, the Manhattan Distance matrix, and the UPGMA phenogram.

Evaluate your hypothesis (you may wish to use the analysis of Wyles et al. 1983 as a model). Ascertain the phylogenetic (taxonomic) relationships (to order and family) of the species under consideration; consult the available vertebrate biology and mammalogy texts. How do the morphological distances correspond with these relationships? Do the calculated morphological differences correspond to your expectations of organismal differences based on niche (feeding habits, locomotion, substrate usage, etc.; consult the available texts)? If there are differences between phylogenetic and phenetic assessments of relationships, why? Is it meaningful to talk about "shape differences" between distantly related organisms such as these?

See the discussion of phenetics & cladistics on pp. 92-94 in Futuyma (1997)

Wyles, J. S. et al. (1983). Birds, behavior, and anatomical evolution. "Proceedings of the National Academy of Sciences of the USA," 80:4394-4397.