Wednesday, August 8, 2012

Direction is important when showing history

In an earlier post I presented an evolutionary network showing the history of the various software and hardware components of the revolutionary Xerox 8010 "Star" computer. This network is shown below.

This network summarizes how various systems related to the Star have influenced one another over the years. Time progresses downwards (as indicated), double arrows indicate direct successors (i.e. follow-on versions), while single arrows indicate "influence" on the subsequent ideas. It is thus interpretable as a network showing hybridization and possibly horizontal transfer. It is thus a valid evolutionary diagram, and it neatly and succinctly displays the historical patterns of "descent with modification".

Unfortunately, a recent attempt to update this diagram is not successful at all.

Click to enlarge.

Note that this version loses most of the important components of an evolutionary diagram:
(i) there is no indication of the time direction
(ii) the edges are not directed
(iii) the bold lines are not explained as being direct descent.
Furthermore, in the accompanying text the (internal) nodes are referred to as "leaves". The authors have thus turned a directed graph into an undirected one, and have thereby created something with only ambiguous interpretation. This network no longer makes evolutionary sense, although it intends to display evolutionary information.

The authors are apparently well-intentioned, and they are trying to use an appropriate form of information display, but they have failed to appreciate the importance of using a directed versus an undirected graph when displaying historical relationships.

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