bioscience I explained Vol 61 No 2 How to draw a phylogenetic tree In this example, there are six beetle-like organisms, labelled A to F A table has been completed, listing the features of each organism Spots on Colour of ture Body HeadArteninae presenk? Colour visible? back? Black Oval Ovat Yes Black Round Orange Black No None Oval Round RoudOrange Pink Black Whte Orange 1-Black しーour_ Using the records from the first table, a data matrix, recording the number of differences be- tween pairs of organisms, has also been produced: A 1 Currently living (extant) organ isms will be placed at the top of the tree – all of the organismsE analysed live in the present. Coming down the tree, we are moving into the past. The last common ancestor of the or- ganisms will be shown on the very bottom of the tree. Last common ancestor Copyright by the authar, 201 bioscience | explained Vol 6 | No 2 To create the tree, proceed as follows: 1. Start by finding the closest relatives- the pair or pairs of organisms that have the fewest differences en them. In this example, beetles A and B have only one difference, so they are the closest relatives. C and F also have just one difference so they are the closest relatives as well. Place these pairs at the top of the tree. The last common ances- tor of A and B lived relatively recently. Mark a dot under A and B, showing the separation of evolution- ary lines leading to the contemporary forms of A and B. Repeat this for C and F (Figure 1). 2. Next, find organisms that have more than one dif- ference. B differs from C by two features, so the last common ancestor of this pair lived before the com mon ancestor of A and B and C and F. Draw a dot to represent the common ancestor of both of these pairs and link them together (Figure 2, left) 3. Organisms D and E also differ by just two features so they are closely related to each other, but not to the other organisms. D and E are paired up by link- ing them with a common ancestor (Figure 2, right). Fig. 1 Fig. 2 Fig. 3 A BC 4. Establish which organisms differ by three or four characteristics. In this way, we can confirm the ear- lier assumption of the relationship between A, B, C and F Next, look for organisms that differ by five features. These are beetles A and D, C and E and E and F When there groups are linked by a common ances- tor, the tree is complete (Figure 3) 5. www.biosclence -explained org Copyright by the authot, 200