In this monograph, the author shows that it is possible to mathematically combine ecology with evolutionary biology to construct a quantitative theory of biodiversity, which he calls the unified neutral theory of biodiversity. Since its publication, Hubbell's theory has come under sustained attack from traditionalists who object to the theory, e.g. mistakenly identifying its main driver to be genetic drift, or claiming that the theory removes the need for ecology. A careful reading of the book shows this criticism to be unwarranted, as he explains the limits of application and the challenges some people may have in dealing with the concepts.

One extraordinary theoretical insight arrived at in this book is that biodiversity is of fractal dimension, which is directly related to a "fundamental biodiversity parameter" (denoted by 𝚯). This is a deep and profound discovery, as "the more extensively and finely we look, the more biodiversity we find". At a stroke, it also radically dethrones the typological concept of species as a standard representative of biodiversity and suggests that lower levels are more important, such as that of individuals or their DNA.

The theory allows quantitative comparisons to be drawn between highly diverse communities. One such study, published in 2005, shows that speciation rates in the Cape Floristic Region is more than twice that of the Amazon rainforest. The CFR, has, in fact, higher values of 𝚯 (and therefore too of fractal dimension), than any previously examined plant system. See Latimer, Silander and Cowling: "Neutral Ecological Theory Reveals Isolation and Rapid Speciation in a Biodiversity Hot Spot". ( )