A Model of Cultural Evolution of Chaffinch Song Derived with the Meme Concept

Abstract

The cultural evolution of song in populations of chaffinches from New Zealand was analyzed using the meme concept. Songs were broken down into their constituent memes (individual syllable types, their variants, or groups of linked syllables), and patterns of geographical variation for memes of different length were assessed. Multivariate spatial-autocorrelation analyses revealed a clinal pattern of variation, with close populations sharing more memes than would be expected at random and the inverse for populations that are far apart. Univariate spatial autocorrelation of syllable variants showed a random pattern attributable to high levels of mutation and random drift at this level. Syllable types, by contrast, are significantly correlated in their frequencies across populations and therefore have similar patterns. This is probably due to a major migrational event during colonization. Using Slatkin's method of rare alleles, we estimated that approximately 10 syllables per generation enter each population as a result of immigration. Rates of mutation vary for different units of cultural transmission, being highest for larger memes of sets of syllables and lowest for single syllable types. Approximately 20-30 new variants of syllables arise in a population each generation. Overall, the rate of cultural mutation and drift of apparently neutral song memes in New Zealand populations of chaffinches is high enough to swamp any homogenizing effect of meme flow, thus allowing the meme pools of these populations to diverge significantly in about the last 100 years. This is the first quantitative attempt to explain the process of cultural evolution of bird song in terms of the interaction of the mutation, flow, and random drift of memes within and among populations, and the task for the future is to assess the generality of this model in other species.