Evolutionary Trees Can Be Constructed on the Basis of Sequence Information
Figure 7.20. An Evolutionary Tree for Globins. The branching structure was deduced by sequence comparison, whereas the results of fossil studies provided the overall time scale showing when divergence occurred. I. The Molecular Design of Life 7. Exploring Evolution 7.4. Evolutionary Trees Can Be Constructed on the Basis of Sequence Information Conceptual Insights, Sequence Analysis, offers insights into evolutionary trees through interactive analysis of simulated evoluntionary histories. The observation that homology is often manifested as sequence similarity suggests that the evolutionary pathway relating the members of a family of proteins may be deduced by examination of sequence similarity. This approach is based on the notion that sequences that are more similar to one another have had less evolutionary time to diverge from one another than have sequences that are less similar. This method can be illustrated by using the three globin sequences in Figures 7.10 and 7.12, as well as the sequence for the human hemoglobin β chain. These sequences can be aligned with the additional constraint that gaps, if present, should be at the same positions in all of the proteins. These aligned sequences can be used to construct an evolutionary tree in which the length of the branch connecting each pair of proteins is proportional to the number of amino acid differences between the sequences (Figure 7.20). Such comparisons reveal only the relative divergence times for example, that myoglobin diverged from hemoglobin twice as long ago as the α chain diverged from the β chain. How can we estimate the approximate dates of gene duplications and other evolutionary events? Evolutionary trees can be calibrated by comparing the deduced branch points with divergence times determined from the fossil record. For example, the duplication leading to the two chains of hemoglobin appears to have occurred 350 million years ago. This estimate is supported by the observation that jawless fish such as the lamprey, which diverged from bony fish approximately 400 million years ago, contain hemoglobins built from a single type of subunit (Figure 7.21). These methods can be applied to both relatively modern and very ancient molecules, such as the ribosomal RNAs that are found in all organisms. Indeed, it was such an RNA sequence analysis that led to the suggestion that Archaea are a distinct group of organisms that diverged from Bacteria very early in evolutionary history.