Birder's Guide is the American Birding Association's newest publication. Each issue focuses on a key subject, providing tips from experienced birders on a wide variety of topics like Travel, Listing & Taxonomy, Gear, and Conservation & Community.
Issue link: http://bg.aba.org/i/574960
17 October 2015 | Birder's Guide to Listing & Taxonomy North Easton, Massachussetts email@example.com Nicholas Block F • Outgroup: A taxon closely related to but out- side of the taxonomic group of interest. The outgroup is included to help researchers deter- mine which traits are ancestral (versus derived) in the taxa of interest; any trait shared with the outgroup typically is presumed to be ancestral. To determine evolutionary relatedness, re- searchers focus on shared derived traits, which is why the outgroup is needed to "polarize" traits as ancestral or derived. G • Scale bar: The amount of evolutionary change represented by this length on a branch (e.g., the bar in Figure 1 represents 5% change in DNA sequence). H • Node support values: The numbers next to each node represent how well-supported that particular node is. These numbers are com- puted through various statistical methods (e.g., bootstrapping) that will be stated in a phylogeny's caption or legend, and they range 0–1 or 0–100. The closer to 1 or 100, the better the statistical support for the node (i.e., the better the support for the hypothesis that all taxa descendent from the node are more closely related to each other than to any other taxa on the phylogeny). Before you begin the next section, I'd like you to answer this question using the phylogeny in Figure 1: Is Species 2 more closely related to Species 1 or to Species 7? Think about it for a minute. Got it? Okay, write your answer in the margin and continue on. The ultimate goal of a phylogeny is to show relationships among a set of taxa, but understanding how to "read" those relationships is often not an intui- tive skill. Almost every semester, at the beginning of the lectures about under- standing phylogenies ("tree-thinking"), I present my students with a question very similar to the one you just answered. On aver- age, I'd say that at least 60% give the incorrect an- swer. And the number one reason for their incorrect responses, by far, is that they focus on the linear or- der of the taxa across the tips of the tree. However, the linear order of the tips is almost meaningless when it comes to communicating evolutionary re- lationships. The exact same evolutionary relation- Outgroup Species 1 Species 2 Species 3 Species 4 Species 5 Species 6 Species 7 100 100 100 98 85 64 0.05 A F E B D C G H A A A A B B B B C C C C D D D D Grouses Flamingos Grebes Loons Blackbirds Grouses Blackbirds Loons Grebes Flamingos Grouses Flamingos Grebes Blackbirds Loons Loons Blackbirds Grebes Flamingos Grouses Figure 1. A hypothetical phylogeny showing the relationships among seven species of interest and an outgroup species. Figure 2. Identical evolutionary relationships—based on Jarvis et al. (2014)—are presented in these four phylog- enies, with the only differences being the arrangement of the nodes. Using the phylogeny outlined in red as a refer- ence, each of the other three phylogenies is the result of the blue node(s) simply being rotated. Although the linear order of taxa changes, the evolutionary relationships de- picted do not (e.g., Grebes are always sister to Flamingos).