Inferring punctuated evolution in the learned songs of African sunbirds

Abstract

36Signals used in animal communication, especially those that are learned, are thought to be 37 prone to rapid and/or regular evolution. It has been hypothesized that the evolution of song 38 learning in birds has resulted in elevated diversification rates, as learned song may be subject to 39 especially rapid evolution, and song is involved in mate choice. However, we know little about 40 the evolutionary modes of learned song divergence over timescales relevant to speciation. Here 41 we provide evidence that aspects of the territorial songs of Eastern Afromontane sky island 42 sunbirds Cinnyris evolve in a punctuated fashion, with periods of stasis, on the order of 43 hundreds of thousands of years or more, broken up by strong evolutionary pulses. Stasis in 44 learned songs is inconsistent with learned traits being subject to constant or frequent change, 45 as would be expected if selection does not constrain song phenotypes, or if novel phenotypes 46 are frequently advantageous. Learned song may instead follow a process resembling peak 47 shifts on adaptive landscapes. While much research has focused on the potential for rapid 48 evolution in bird song, our results suggest that selection can tightly constrain the evolution of 49 learned songs over fairly long timescales. More broadly, these results demonstrate that some 50 aspects of highly variable, plastic traits can exhibit punctuated evolution, with stasis over fairly 51 long time periods. 52 53 Introduction: 54