One hundred and fifty years after the publication of On the origin of species, it seems that one of Darwin's most fruitful ideas, that the course of evolution can be represented in the shape of a tree, is in need of updating. Evidence is mounting that what we should be thinking of instead is a web.

Reconstructing relationships between species from genetic information is a mathematical exercise, as well as a biological one. Darwin's tree of life, as a mathematical object, is an example of a graph, a collection of nodes connected by edges, with the special property that there's only one route between any two nodes. There are no circuits in the graph because it was assumed that genes can only be transferred between parents and offspring. But as Plus reported last year, there is evidence that genes can also be transferred between species horizontally, for example through hybridisation or swapping of genetic material between unrelated species. An article in this week's New Scientists brings together many strands of evidence for horizontal gene transfer. According to the article, the evidence doesn't just concern strange unicellular beasts, but also humans and other mammals: it is estimated that "40 to 50 per cent of the human genome consists of DNA imported horizontally by viruses"!

As a result, many scientists now think that a more general form of graph, a network, is what is needed to describe life. Given the potential complexity of networks, genetics may soon be in desperate need of mathematicians.