5.1 Essential ideas
Cladistics, or phylogenetics, is a method of studying genetic and evolutionary relationships by looking at the number of shared characteristics in a group of species.
Unlike traditional taxonomy, which focuses on external characteristics, cladistics is interested in molecular similarities and differences – either gene sequences (DNA base pairs) or gene products (amino acids and proteins). Differences in gene sequence accumulate gradually over time, so there is a correlation between the number of differences and the time of divergence of two species.
All the species in a group that share a common ancestor are together called a clade. A clade can contain very large or very small numbers of organisms depending on the number of characteristics under consideration.
Activity: How to read a cladogram
- For all cladograms, species names are represented by letters A through J. Species names are always written at the distal end of a branch. All extant species’ names are aligned. Extinct species, when considered in the cladogram, are written at nodes.
Print out a copy of Figure 5.1.4b and refer to it as you read and do the following:
- The points where two groups diverge (i.e. the branches) are called nodes. The nodes represent a common ancestor for all the species distal to that node. In Figure 5.1.4a, there are 8 nodes (excluding the outgroup). All the species that follow a node are part of the same clade.
- On your worksheet, locate all 8 nodes.
- Determine how many common ancestors D and F share.
- Highlight the node that represents the most recent common ancestor of A and H.
- Circle all the clades represented in this cladogram. How many are there?
- What do you notice about the number of clades and the number of nodes in a cladogram?
Homologous vs Analagous structures
You probably recall from Page 5.1.1 that homologous structures are those derived from a common ancestor, having a similar structure though not necessarily a similar function. By contrast, analogous structures do not share a common ancestor, and have similar functions but not necessarily similar structures.
Figure 5.1.4c – An example of analogous structures
The fins on a penguin and a fish have the same function, but their evolutionary histories are very different. Similarities like this are not included in cladograms.
Cladistics is the study of homologies. As genetic sequences change in a predictable way, scientists can be quite sure that similarities in structure are a result of sharing a common ancestry.
Figure 5.1.4d – Vertebrate cladogram
A simplified cladogram showing derived homologies for the clade Chordata (animals having a spinal cord). It would not be useful to group species according to analogous structures (e.g. scaly skin of fish and reptiles).
Food for thought
In most cases, the study of cladistics confirms the relationships described by traditional taxonomic classifications. This is not always the case, however: a number of groups have been reclassified using the taxonomic system in light of molecular evidence. What are the implications for knowledge when certain types of evidence have more authority over another?