I was inspired several months ago by this tweet from Drew Lab in which they made a phylogeny of candy bars. Besides thinking, “what a delicious lab project,” I also thought it was an intuitive way to understand the basic evolutionary concept of how species (or candy bars as it may be) are related to each other. Candy bars are accessible in a way species are not; kids and adults, scientists and non-scientists alike understand candy and can debate how the candy bars are related to each other based on their tasty traits. Bringing us to an important point about phylogenies: they are debatable! Phylogenies represent hypotheses about how species are related to each other. Some phylogenies are well supported and even have multiple lines of evidence to support the tree topology such as molecular sequence data, morphological data, and fossil records. However, some phylogenies are not well supported and scientists gather new data to resolve these phylogenies.
To understand phylogenies let’s start with some basic terminology (Figure 1).
Taxon (plural: taxa): the species/candy bars we will infer relationships between in a phylogeny
Tips: the terminal unit in the phylogeny. There will be one tip for every taxon included in the phylogeny.
Node: point that joins two groups together. Nodes represent the most recent common ancestor of the two groups that were joined.
Branch: the portion of the tree that joins tips and nodes, or nodes with other nodes. In a cladogram, branches are equidistant between nodes; while in a phylogeny representing time, the length of the branch estimates the amount of time that has passed between evolutionary events.
Clade: A group of similar taxa. Figure 1 represents the peanut clade.
Traits: information about the taxa that may be unique to a single taxon or shared by multiple taxa.
Biologists describe how clades are related to each other using the following terms:
Sister taxa: Two taxa (could be one or multiple species/candy bars) that arise from a common node. (Example- Mounds and Almond Joy in Figure 2)
Monophyletic: A clade formed by all of the species/candy bars sharing a common ancestral node. (Example- M&Ms are monophyletic)
Paraphyletic: A group of taxon descended from a common ancestral node that does not include all of the taxa descended from that node.
Polyphyletic: A group formed by shared traits even when a single common ancestral node is not shared. (Example- In Figure 2 peanuts independently arise three times on the candy bar phylogeny)
The candy bar phylogeny uses American candy bars. If you recreate with your country’s candy, please tweet a picture to WildlifeSNPits! We would love to see candy phylogenies from around the world.
You can download the data here: CandyBarPhylogenyData.