Latitudinal variation in the asynchrony of seasons: implications for higher rates of population differentiation and speciation in the tropics

Abstract

Speciation rates for some taxa increase from the poles towards the equator, augmenting the number of species in the tropics. The causes of latitudinal variation in speciation rates are presently unknown. Here we present and discuss the Asynchrony of Seasons Hypothesis that invokes latitudinal differences in the spatial asynchrony of climate, and the corresponding phenologies of organisms, to explain increased rates of population differentiation and speciation in the tropics. At high latitudes, most organisms time their phenologies to coincide with seasonal fluctuations in solar radiation and temperature that are synchronous across broad regions. High synchrony of phenologies across high latitudes should facilitate the establishment of gametes and immigrants moving into new populations because they are likely to find themselves in the same life history stage as local individuals, with similar phen-ologies. In contrast, many tropical species time their phenologies to coincide with seasonal variation in precipitation, which can vary over short geographic distances due to variation in patterns of airflow and topography. Greater asynchrony in the phenologies of tropical organisms could reduce survival and reproductive success of gametes and immigrants moving between asynchronous populations, impeding gene flow and increasing rates of local adaptation and speciation.

Keywords

• latitude
• seasonality
• synchrony of life history stages
• population differentiation
• ecological speciation
• diversity