In my post below where I try to synthesize The Superficial and The Causes of Evolution I used the term "fitness." Well...as Matt McIntosh pointed out the term itself is problematic, and so using it as a reference of any sort is really sketchy. Evolgen has slammed the use of "genetic load", and I I think the skepticism is warranted to some extent. The originator of the formula for genetic load, JBS Haldane, famously quipped "fitness is a bugger!" Part of the problem is that the term "fitness" has unique connotations in evolutionary biology. Physical fitness, health and longevity, often correlate with reproductive fitness, but the correspondence is not 1:1. In fact, in many species there is likely a correlation between extremely fecund but short-lived individuals and fitness, their reproductive value, while longer-lived individuals within the population would exhibit less fitness. This is antagonistic pleiotropy at work, short-term benefits outweigh long-term consequences, evolution is not "concerned" with long-term designs when weighed against short-term success. Those who fail in the present do not live to fight another day. Ultimately the issue here is the distinction between the fact of evolution and the theory of evolution. The process, the description, is pretty clear via a wide range of inferences. But the specific theoretical models are still being worked out, and many of the constructs of population genetics can be accused of excessive reification. Yet, as I said, evolution is about short-term windfalls, and the utility that abstractions offer us may simply be a necessary aspect of doing the science in the present as opposed to waiting for a future when our empirical methods and analytic techniques catch up to our ambitions. Hubris is in our nature. Charles Darwin formulated his theories before Mendelian genetics, and the Modern neo-Darwinian Synthesis rests upon the tenuous backs of the Wright-Fisher models. Myself, I believe that the work of Haldane, Sewall Wright and R.A. Fisher may finally be brought down from the heavens and tested in the trenches because of rise of the genomics, quantitative evolutionary models necessarily needed an enormous quantity of raw data to explore their assumptions and consequences with sensitivity.