The ultimate phylogenetic network?

One concern with the current move from phylogenetic trees to phylogenetic networks is the increased complexity of a reticulating network versus a dichotomous tree. People fundamentally have trouble with interlinked and overlapping structures, and a network is more complex than a tree, just as a tree is more complex than a chain (see this previous blog post).

However, if we restrict ourselves to a two-dimensional representation, then there is a limit to how complex a network can be and yet still be interpretable. The network shown here, published by the anthropologist Franz Weidenreich, comes close to that limit.

Pedigree of the Hominidae, from Weidenreich (1947) p. 201.

This is usually referred to as a "trellis" or "lattice", for obvious reasons. It first appeared in Weidenreich (1946) and then again in  Weidenreich (1947); and it has recently been re-published several times (eg. by Brace 1981; Templeton 2007; Caspari 2008). It is "an attempt to present graphically the relation between the different hominid forms in time and space", expressing Weidenreich's idea that evolution is "transformation, in close connection with inter-breeding".

The labelled circles refer to named fossil species of the Hominidae. According to Weidenreich, the vertical lines represent different stages of human evolution through time, the horizontal lines represent the morphological differentiation between different geographical regions, and the diagonal lines represent patterns of gene flow ("crossing") between the populations. Thus, the trellis emphasizes continuity of descent (and ancestry) through time within geographical regions (vertically), while also emphasizing gene flow between the regional lineages (horizontally and diagonally). In particular, note that in the figure the horizontal and diagonal lines are just as important as the vertical lines — this is not a tree obscured by vines!

Weidenreich viewed humans as being a single polytypic species throughout the Middle and Late Pleistocene, with nearly continuous gene flow during that time. This gene flow was seen  as an integral part of the evolution of modern humans, dispersing genes throughout the species, so that any one recent human is likely to have had Pleistocene ancestors from different parts of the planet. This has been called a "polycentric model" of human evolution, also known as the "multi-regional model".

From Howells (1959).

However, racial thinking (as discussed in this previous blog post) has led to tree-like models of human evolution, and so Weidenreich's network model of inter-connected groups was either ignored or mis-interpreted (see Brace 1981; Caspari 2003; Templeton 2007). In particular, the trellis was repeatedly re-drawn as a tree, usually referred to as a candelabrum. This mis-representation started with the work of William White Howells (eg. Howells 1959), as shown above, which then became the source for most subsequent discussions of the multi-regional model, rather than Weidenreich's original. (Actually, Howells' mis-interpretation of Weidenreich's multi-regional model dated way back to 1942; see Hawks & Wolpoff 2003.)

Interestingly, the trellis metaphor has been revived as a model for recent human evolution, notably by Alan Templeton, as shown in the final two figures.

The trellis model of recent human evolution,
from Templeton (1999) p.636.

A model of recent human evolution, from Templeton (2007) p. 1517.

References

Brace C.L. (1981) Tales of the phylogenetic woods: the evolution and significance of evolutionary trees. American Journal of Physical Anthropology 56: 411-429.

Caspari R. (2003) From types to populations: a century of race, physical anthropology and the American Anthropological Association. American Anthropologist 105: 63–74.

Caspari R. (2008) "Out of Africa" hypothesis. In: Moore J.H. (ed.) Encyclopedia of Race and Racism, Volume 2 G–R, pp. 391-397. Macmillan Reference, Detroit.

Hawks J., Wolpoff M.H. (2003) Sixty years of modern human origins in the American Anthropological Association. American Anthropologist 105: 87-98.

Howells W.W. (1959) Mankind in the Making: the Story of Human Evolution. Doubleday, Garden City, NY.

Templeton A.R. (1999) Human races: a genetic and evolutionary perspective. American Anthropologist 100: 632-650.

Templeton A.R. (2007) Genetics and recent human evolution. Evolution 61: 1507–1519.

Weidenreich F. (1946) Apes, Giants, and Man. University of Chicago Press, Chicago.

Weidenreich F. 1947. Facts and speculations concerning the origin of Homo sapiens. American Anthropologist 49: 187–203.