Lecture 2: Describing Microbial Diversity: the ... - MCD Biology

4
understand the evolutionary process. But, you need a metric – some property that provides a quantitative
measure of the extent of relatedness of different organisms.
A. Note that phylogenetic relationships can be “predictive”, not only “descriptive”: We can predict (some)
properties of organisms based on the properties of their relatives – the principle: representatives of a
particular phylogenetic group are expected to have the properties that are common to the group.
e.g. -- We know a lot about Escherichia coli, but relatively little about Chromatium vinosum:
1) E. coli is a chemoheterotroph, whereas C. vinosum is a photoheterotroph. (What do these terms
mean?) Classical physiological treatment would have considered these organisms wildly different. From
gene-sequence comparisons we know they are fairly close relatives (!-group of proteobacteria).
2) Thus, even though one eats glucose and the other light, we can predict that the biochemical underpinnings
are similar -- protein-synthesizing machinery (antibiotic-sensitivity), DNA replication machinery, amino acid-
synthesizing machinery, nucleotide biosynthesis, etc. Because of the close phylogenetic relationship, we
expect to be able to swap many genes between these two superficially disparate organisms.
3) The predictive value of phylogeny is essential for interpreting environmental sequence surveys.