Just about all of evo psych is based on the simple proposition that the reason we do what we do (why we dress little girls in pink or lose our temper, or who we vote for) is based on what spread our earliest human ancestors’ selfish genes. So reproductive success must be pretty important, right? It must equal evolutionary fitness. Otherwise, we wouldn’t be here, or so the Darwin tale runs.
Here are science blogger Greg Laden’s exceptions to whether reporeductive success equals evolutionary fitness:
RS is a measure applied to an individual while fitness is ideally applied to alleles for a gene or some other genetic construct;
The offspring-fertility link can be misleading. A queen bee with an allele that allows her to produces more sterile offspring may also produce more fertile offspring;
RS is fitness plus or minus random effects;
RS usually does not consider indirect fitness;
RS is selected to be optimized while fitness is selected to be maximized.
Equating RS and fitness is therefore only a rough approximation. When initially learning about Natural Selection students are often led to believe that RS and fitness are the same, which is only true with these (and possibly other) caveats. Equating RS and fitness in pedagogy risks skipping past and perhaps never understanding the caveats, and these caveats are very far from trivial. They are, in many cases, the point of specific evolutionary research projects.
In short, there are so many exceptions that no one knows very much about what “evolutionary fitness” is.
Hence the importance of jamming the idea into schoolkids’ heads before they can think carefully about it, via teachers who know their job could be at stake if they think carefully about it.