homologous structures independently came to have similar functions (Sanderson and Hufford, 1996; Hoekstra and Price, 2004; Scotland, 2011; Wake et al., 2011). This has been suggested for other systems as well. For example, homologous brain nuclei appear to be involved in vocal learning in lineages of birds that evolved song independently (Feenders et al., 2008; Hara et al., 2012). Similarly, interaural coincidence detection circuits arose independently in the brainstem nuclei of birds and mammals (Schnupp and Carr, 2009). Finally, the appearance of similar cortical areas are correlates with the independent evolution of precision hand control in primates (Padberg et al., 2007), suggesting that constraints in cortical organization led to the evolution of similar neural mechanisms underlying dexterity (Krubitzer, 2009).
If homologous neurons are repeatedly incorporated into neural circuits for analogous behaviors, it suggests that these neurons may be part of a more readily achievable state for swimming. Thus, the nervous system may affect the evolvability of behavior because some configurations of existing neurons could be more robust than others. The concept of evolvability first arose from genetics (Kirschner and Gerhart, 1998; Masel and Trotter, 2010), but has since been applied to nervous systems (Airey et al., 2000; Bendesky and Bargmann, 2011; Katz, 2011; Yamamoto and Vernier, 2011). Exploring the aspects of neural organization that lead to repeated evolution of particular behaviors will point to the factors that are most important for behavioral output.
We thank Arianna Tamvacakis for feedback on the manuscript. This work was supported by National Science Foundation Integrative Organismal Systems Grants 0814411, 1120950, and 1011476.