What does this equation mean?

There is a further reason the borrowed vocabulary fits with unusual precision here, and it runs backward through the history of the metaphor itself. Sewall Wright introduced the image of a landscape of gene combinations, graded by adaptive value and read by its topography of peaks and valleys, in a 1932 paper to the Sixth International Congress of Genetics. Wright’s own diagram is worth reading in his own words: having reduced an intractably high-dimensional “field of gene combinations” to a two-dimensional contour plot for the sake of illustration, he wrote that “there may be innumerable other peaks which are higher but which are separated by ‘valleys,’” and that “the problem of evolution as I see it is that of a mechanism by which the species may continually find its way from lower to higher peaks” across exactly such a landscape [ 12 ] . The mathematical object Wright drew — a single scalar quantity, contoured over a combinatorial space too large to visualize directly except through a reduced two-dimensional slice — is not a biological object first and a physical one by analogy. It is a general mathematical construction, the same one that describes a potential-energy surface in any physical system with more than a few degrees of freedom, applied by a geneticist to a space of gene combinations because no more specific tool existed for the purpose in 1932. Reading it back onto the chart of nuclides, ninety-four years later, is not exporting a biological insight into physics; it is returning a general-purpose mathematical device to a domain where it was always structurally at home; the peaks and valleys of (N,Z) -space were there in the binding-energy data whether or not anyone had a name for the shape they made.