FUTURE Seminar by Andrew Berry

“Tangling the Tree of Life: Horizontal Genetic Transfer in the History of Life”

Andrew Berry

Central to the theory of evolution is the idea of the tree of life: the branching pattern generated by the splitting of evolutionary lineages to produce, over the 3.9 billion years since the origin of life, the spectacular array of forms we see alive today or in the fossil record.  Key to our ability to reconstruct this tree is vertical inheritance.  Traits are passed down from generation to generation: your DNA comes from your parents, just as, on a grander time scale, humans and chimpanzees have inherited their DNA from the common ancestor of both species that existed about 7 million years ago.  We reconstruct the evolutionary past by comparing the sequences of this passed-down DNA because mutation causes DNA sequences to change slowly over time.  The DNA sequences of species with a relatively recent common ancestor are more similar to each other than the DNA sequences of species with a more ancient common ancestor.  This vertical vision of evolution has been challenged by the finding of horizontal genetic transfer (HGT): instances in which distantly related species may acquire DNA from each other.  If a human has acquired via HGT a piece of DNA from a cat, say, and we happen to use that gene region for our evolutionary analysis, we would conclude that humans are more closely related to cats than they are to chimpanzees.  HGT can occur in a number of ways, including through viral transport of DNA between disparate hosts, or, more prosaically, through hybridization between species (such as between our species and Neanderthals).  If HGT is pervasive, then the tree of life becomes a tangled mess: the horizontal "noise" would swamp the vertical "signal".  Ultimately this is an empirical issue: is HGT so common that evolutionary tree reconstruction is impossible?  The conclusion is clear.  While HGT definitely occurs, especially in simple organisms, it does not occur at rates high enough to overwhelm our attempts, based on vertical transmission, at reconstructing the tree of life.