Evolutionary convergence

League dreams of breeding players with titanium knees, they will have to make do with bones and ligaments for the foreseeable future. Constraints come in many forms, from those that limit the range of variation available for natural selection to act upon (elephants with titanium legs seem unlikely to appear) to the physical forces of fluid dynamics, gravity and the like. In 1995, Kurt Schwenk usefully divided constraints into two different classes. For one class, the constraint operates because organisms simply are unable to produce new variants, like titanium bones, that might be useful. Therefore, natural selection has no variation to use in sculpting new solutions. The second class of constraint is one where there is abundant variation, but various forces act through natural selection to limit the range of solutions. Schwenk pointed out that here constraint isn’t even the right term — limitations are due to good old stabilizing selection.