What does this equation mean?

Indifference to subject matter is a strong claim, and Ronald Fisher made the first attempt to cash it out for biology. His 1922 paper “On the Dominance Ratio” treated the frequency of a gene in a finite population as a continuously distributed random variable obeying a differential equation of exactly this drift-and-diffusion form — a genuine, if flawed, first pass at what a physicist would already have recognized [ 4 ] . The flaw was real: Fisher assumed the mean change in his transformed frequency variable was zero across generations, which produced a rate of loss of genetic variability of 1 in 4N per generation rather than the correct 1 in 2N [ 4 ] . Sewall Wright found the error around 1925 and, in his own 1931 paper “Evolution in Mendelian Populations,” worked out the joint stationary distribution of gene frequencies under mutation, migration, selection, and drift acting together, in essentially the form still used today [ 5 ] . Wright’s paper is also where the diffusion picture picked up the parameter that lets it apply to a real, structured population rather than an idealized one: effective population size, a count of breeding individuals corrected for unequal sex ratios, variable family size, and population subdivision, substituted for N wherever the equations call for the census size [ 5 ] . Fisher corrected his own treatment in 1930, this time building explicitly on the Fokker-Planck equation borrowed from physics [ 4 ] .