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 phenologies. 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.

Asynchrony of Seasons: Genetic Differentiation Associated with Geographic Variation in Climatic Seasonality and Reproductive Phenology

“…Because information on local breeding schedules is lacking for our study species, we used precipitation seasonality as a proxy for food availability, which is expected to correlate with reproductive timing (Martin et al 2009 Jetz, unpublished data). We also used monthly precipitation data extracted from the WorldClim database at a projected resolution of 30 arcseconds (Hijmans et al 2005).…”

“…We searched the literature for studies describing intraspecific genetic variation for which geographic information on the location of collecting sites was accessible. Our sampling was focused in the New World tropics, where precipitation asynchrony is most likely to influence population differentiation (Martin et al 2009; see "Discussion"). Our initial database comprised mitochondrial DNA sequences of 1,586 individuals of 74 nonmigrant New World species (table S1, deposited, along with all supplementary tables and figures, in the Dryad Digital Repository: http://dx.doi.org/10.5061/dryad.40d6h [Quintero et al 2014]; fig.…”

“…Thus, due to spatial variation in precipitation regimes, different locations may exhibit temporal asynchrony in pulses of food supply, which may result in distinct reproductive schedules among populations reflecting either local evolutionary adaptation or phenotypic plasticity (Hahn and MacDougall-Shackleton 2008;Charmantier and Gienapp 2014). If asynchronous reproduction in relation to spatial variation in climatic seasonality (hence, resource availability) leads to divergence between populations, then increasing seasonal asynchrony should be positively correlated with population differentiation and, eventually, reproductive isolation (Martin et al 2009). This postulate, known as the asynchrony of seasons hypothesis (ASH; Martin et al 2009), has not been formally tested but is based on evidence suggesting that spatial variation in precipitation seasonality may lead to breeding allochrony and population differentiation.…”

“…Because precipitation is strongly related to food availability (Ahumada 2001;Wright and Calderó n 2006), tropical animals are expected to adjust their reproductive timing to match local precipitation patterns (Hau et al 2008;Shine and Brown 2008;Martin et al 2009;Jahn et al 2010), which may be variable even over small spatial scales (Legates and Willmott 1990). Thus, due to spatial variation in precipitation regimes, different locations may exhibit temporal asynchrony in pulses of food supply, which may result in distinct reproductive schedules among populations reflecting either local evolutionary adaptation or phenotypic plasticity (Hahn and MacDougall-Shackleton 2008;Charmantier and Gienapp 2014).…”

“…If asynchronous reproduction in relation to spatial variation in climatic seasonality (hence, resource availability) leads to divergence between populations, then increasing seasonal asynchrony should be positively correlated with population differentiation and, eventually, reproductive isolation (Martin et al 2009). This postulate, known as the asynchrony of seasons hypothesis (ASH; Martin et al 2009), has not been formally tested but is based on evidence suggesting that spatial variation in precipitation seasonality may lead to breeding allochrony and population differentiation. For example, two tropical populations of rufous-collared sparrows (Zonotrichia capensis) located only 25 km apart experience different rainfall patterns that trigger asynchronous breeding seasons, which, combined with female preference for local songs, likely drive their observed genetic and cultural differentiation (Moore et al 2005;Danner et al 2011; see also Jacobs and Wingfield 2000).…”