Chapter 2, Problem 1PDT

Summary Introduction

To estimate: The phylogeny of the given taxa by plotting the changes on each of the possible unrooted trees and determine the tree with fewest evolutionary changes.

Introduction: Three species named 1, 2, and 3 are endemic to a group of islands. They all share a common ancestor named species 4. Species 4 serves as an out group. It has a huge population size which indicates that since islands colonized, no mutation become fixed in its population. The genes were sequenced and ten nucleotide sites were located that differed among these four species. The nucleotide bases at these sites are as follows:

$\begin{array}{l}\text{Species 1: GCTGATGAGT}\hfill \\ \text{Species 2: ATCAATGAGT}\hfill \\ \text{Species 3: GTTGCAACGT}\hfill \\ \text{Species 4: GTCAATGACA}\hfill \end{array}$

Explanation of Solution

Deoxyribonucleic acid consists of four nucleotides, these are: adenine (A), thymine (T), guanine (G), and cytosine(C). These nucleotides can be paired and labeled for the given condition as follows:

• GC - a
• AG - b
• GT - c
• AC -d
• AT - e
• TG - f
• CA - g

The data of the nucleotide bases at the ten sequenced sites in the given four species are as follows:

Pictorial representation: Fig.1, Fig.2 and Fig.3 represent the possible unrooted trees for phylogeny of the given taxa.

Fig 1: Seven changes observed in this unrooted tree

Fig 2: Six changes observed in this unrooted tree

Fig 3: Seven changes observed in this unrooted tree

Therefore, the tree with fewest evolutionary changes is the second unrooted tree represented by Fig.2. This tree has only six changes, whereas the other two have seven changes which are higher than the second one.