The very issue of whether complexity is a trend has been controversial (e.g., McShea 1996). Moreover, some might complain that this smacks too much of orthogenesis or progressionism for their tastes. And what do we mean by complexity, anyway? These issues have been and will continue to be debated in the literature. But one of the neatest things about the Adamowicz paper is that they provide a possible mechanism for a trend in complexity. They found that newly originated higher taxa had greater limb differentiation than their contemporaries, and that taxa going extinct had lower degree of limb differentiation. Moreover, limb complexity turns out to be one of the strongest predictors of species richness in extant crustacean clades. Together, these suggest the possibility that the trend in complexity might be driven in part by differential speciation and extinction of lineages based on complexity. What if lineages with higher complexity diversified at greater rates than lineages with reduced complexity? Over time, traits associated with complexity might increase simply because of this connection to diversification. This research thus raises some intriguing levels of selection issues, because there is – in principle – no reason why complexity could only be favored by selection at the individual level.
How might limb complexity fuel the diversification process? The authors speculate that increased limb complexity might increase ‘evolvability’ (the meaning of which is even more fun to discuss than ‘complexity’!) and possibly promoting niche specialization. They also note that new limb types might amplify the intensity of sexual selection, possibly serving indirectly to enable that supposed ‘engine of speciation.’ Anyway, don’t expect this paper to end with a case-closed feeling – after all, questions like these are on par with the biggest unresolved issues in biology. But there are lots of things to think about here!