Fig. 1.

Simplified depictions of two competing hypotheses about the genetic history of life. (a) The tree model characteristic of traditional phylogenetic thought. (b) An example of a network model [as advanced by McCarthy (23) in the context of hybridization and stabilization of recombinant genotypes]. Uppercase letters indicate different species or phenotypically recognizable life forms, arrows indicate historical pathways of descent, and successively lower rows in the diagrams represent more recent horizons in evolutionary time. When viewed backward in time, lineages shrink or coalesce to particular ancestors in a tree, but they expand to multiple ancestors in a network (because each new species is of hybrid origin).