Ecological exchangeability versus neutral molecular markers: The case of the great tit

Neutral genetic markers, such as mitochondrial DNA (mtDNA) sequences, are commonly used to discover independently evolving groups of populations in nature. These groups are often considered to be units of conservation because they preserve distinct organismal histories. However, because of the time lag between the isolation of populations and the evolution of diagnostic neutral markers, adaptive traits could be unrepresented by units defined by neutral markers. The concept of ecological exchangeability potentially provides a way to preserve populations possessing local adaptations that lack diagnostic neutral markers. Populations are surveyed for differences in fitness traits, either directly, or by examining morphological or genetic traits that could indirectly serve as proxies for them. The purpose of this paper is to compare the nature of units of conservation defined by neutral gene surveys and ecological exchangeability, using data recently available for the great tit Parus major, a wide-ranging Palearctic species. mtDNA surveys reveal a lack of differentiation across thousands of kilometer. In contrast, studies of body size and clutch size show locally adapted differences between populations separated by a few kilometer, meaning that these populations could be classified as ecologically in exchangeable. These two types of markers have dramatically different consequences for the geographic and evolutionary scale of conservation units. The concept of ecological exchangeability might be an inappropriate way to diagnose units of conservation in birds owing to the time required to document local adaptations and their potential ubiquity. Neutral genetic markers continue to provide a theoretically sound way of identifying units of conservation, and these units ought to be integrated into conservation plans without delay.