Vestigial Organs as Opportunities for Functional Innovation: The Example of the Penstemon Staminode

Abstract

Abstract. Vestigial organs arise commonly during morphological evolution, but they need not be destined for elimination. Instead, vestigial organs may facilitate functional innovation because their freedom from functional constraints allows them to assume novel roles that their functional progenitor could not perform. We illustrate this vestigial transition between functions with an experimental study of the staminode of Penstemon flowers. Previous phylogenetic and developmental studies indicate that this staminode represents a stamen that was lost phenotypically, but not genetically, and then reappeared in the tribe Cheloneae, to which Penstemon belongs. To assess whether the Penstemon staminode has adopted a novel function(s), we compared pollination of flowers with and without staminodes for two bee-pollinated species, P. ellipticus and P. palmeri, and two hummingbird-pollinated species, P. centranthifolius and P. rostriflorus. The staminode acts differently in the two bee-pollinated species. For P. ellipticus, which represents the basal Penstemon lineage, the staminode hinders pollinator access to nectar, which increases visit duration and pollinator contact with sexual organs, thereby increasing pollen receipt by stigmas and controlling pollen removal from anthers. In contrast, in P. palmeri, the staminode acts as a lever that enhances stigma contact with the pollinator, so that staminode removal reduced pollen receipt, but did not affect pollinator attraction, visit duration, or pollen removal. Unlike in bee-pollinated species, staminode removal from hummingbird-pollinated species did not affect any measured aspect of pollination, indicating that the staminode serves no function in this derived pollination system. These results illustrate several features of vestigial organs: loss of function can be temporary; loss of function facilitates the evolution of novel roles; and functional reinvention will often involve only a single role, thus increasing the chance of both secondary loss of function (bird-pollinated penstemons) and functional switches (P. palmeri).Key words. Adaptive radiation, floral evolution, functional evolution, Penstemon, pollen deposition, pollen removal, pollination, staminode, vestigial organs. Natural selection operates within a historical context because it depends on the variation present in the preceding generation and the genetic relationship of offspring to their parents. As a result, morphological adaptation to altered environments typically modifies structures to serve new functions, rather than inventing new structures. Such functional evolution must balance the value of new functions against any reduced performance of a structure's original roles. The historical, genetic context of natural selection also imposes inertia on evolution, so that nonfunctional traits can persist in populations for many generations.As the preceding quote from Darwin (1859) indicates, the evolutionary inertia experienced by vestigial organs creates an opportunity for functional innovation. Because ve...

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