Molecular Phylogenies

As already mentioned, a phylogeny is an evolutionary history of a group of organisms, usually represented as a tree (< Figure 23.23). The branches of the phylogenetic tree represent the ancestral relationships between the organisms, and the length of each branch is proportional to the amount of evolutionary change that separates the members of the phylogeny.

Before the rise of molecular biology, phylogenies were based largely on anatomical, morphological, or behavioral traits. Evolutionary biologists attempted to gauge the relationships among organisms by assessing the overall degree of similarity or by tracing the appearance of key characteristics of these traits. The first phylogenies constructed from molecular

"Zebras

Horses

Sequence divergence (%) przewalski

"Zebras

Horses

Sequence divergence (%) przewalski

23.23 A phylogeny is the evolutionary history—the ancestral relationships—of a group of organisms. This branching diagram shows the phylogeny of horses based on mitochondrial DNA sequences. DNA of the extinct quagga was extracted from skins from preserved museum specimens.

Chicken

Newt

Carp

The number 1 indicates an invariant position in the cytochrome c molecule (i.e., all the organisms have the same amino acid in this position). The position is probably functionally very significant.

Side chains marked by red arrows interact with the heme group.

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