Ancestor-descendant relationships and the reconstruction of the Tree of Life

Abstract

The reconstruction of the Tree of Life has been a primary goal in biology since Darwin coined the expression, the “great Tree of Life” in On the Origin of Species : “The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species….As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications” (Darwin 1859: pp. 129–130). Ernst Haeckel was the first biologist who attempted to reconstruct the Darwinian Tree of Life. However, he could not reach his goal because his method of tree reconstruction was deeply anchored in the pre-Darwinian scala naturae (Dayrat 2003). A segment of the Darwinian Tree of Life is illustrated in the unique figure published in On the Origin of Species . This figure, which Darwin referred to as a “branching diagram,” represents ancestor-descendant relationships (ADR). Pointing out that Darwin's Tree of Life represents ADR is not controversial. It simply is a fact. However, it might be more controversial to point out a paradoxical situation: why does an overwhelming majority of the trees currently published represent sister-group relationships (SGR)? Given that reconstructing the Darwinian Tree of Life is one of today's foremost goals in systematic biology, one would expect that the reconstruction of ADR would arouse greater interest among biologists. Certainly, only SGR can be …