 Consequently, the adaptive significance of TSD in all but the most recent origins of TSD may have been obscured by the passage of deep time, with TSD potentially being maintained in many amniote clades simply because it works 'well enough' (i.e.has no overall fitness costs along the lines of the phylogenetic inertia explanation).Other work centers on a 1977 theoretical model (the Charnov–Bull model), predicted that selection should favour TSD over chromosome-based systems when "the developmental environment differentially influences male versus female fitness"; this theoretical model was empirically validated thirty years later but the generality of this hypothesis in reptiles is questioned.