Case studies and mathematical models of ecological speciation. 2. Palms on an oceanic island

Abstract

A recent study of a pair of sympatric species of palms on the Lord Howe Island is viewed as providing probably one of the most convincing examples of sympatric speciation to date. Here we describe and study a stochastic, individual-based, explicit genetic model tailored for this palms system. Overall, our results show that relatively rapid (<50,000 generations) colonization of a new ecological niche, and sympatric or parapatric speciation via local adaptation and divergence in flowering periods are theoretically plausible if (i) the number of loci controlling the ecological and flowering period traits is small; (ii) the strength of selection for local adaptation is intermediate; and (iii) an acceleration of flowering by a direct environmental effect associated with the new ecological niche is present. We discuss patterns and time-scales of ecological speciation identified by our model, and we highlight important parameters and features that need to be studied empirically in order to provide information that can be used to improve the biological realism and power of mathematical models of ecological speciation.

Incipient allochronic speciation due to non-selective assortative mating by flowering time, mutation and genetic drift

“…Random genetic drift has previously been shown to accelerate the evolution of reproductive isolation by facilitating strong linkage disequilibrium between ecological and mating strategies (Dieckmann & Doebeli 1999;van Doorn & Weissing 2001) or fostering local adaptation to adjacent habitats differing in flowering time (Stam 1983). Previous models of sympatric speciation involved disruptive natural or sexual selection (Kondrashov & Kondrashov 1999;Drossel & McKane 2000;Doebeli & Dieckmann 2003;Bü rger et al 2006;Doebeli et al 2007;Gavrilets & Vose 2007;Leimar et al 2008). In those models, disruptive selection and reproductive isolation are caused by resource competition among similar phenotypes, displacement into distinct ecological niches or selective assortative mating.…”

“…Examples of allochronic speciation are also expected in plants because assortative mating by flowering time is inevitable as mating can occur only among simultaneously open flowers ( Fox 2003;Weis et al 2005;Savolainen et al 2006;Gavrilets & Vose 2007). Although most studies on plant speciation have focused on prezygotic isolating mechanisms such as changes in self-fertilization rate, sex expression and pollinator or habitat specialization (Conner 2006), evidence also exists for temporal isolation.…”

“…Gegear & Burns 2007) nor adaptation to ecological spatial gradients (e.g. Stam 1983;Gavrilets & Vose 2007). We model flowering time as a multilocus quantitative trait undergoing non-selective assortative mating and spontaneous mutation in a finite population to investigate the conditions promoting or inhibiting the emergence of disjunct clusters in the population flowering phenology.…”

Historical connectivity, contemporary isolation and local adaptation in a widespread but discontinuously distributed species endemic to Taiwan, Rhododendron oldhamii (Ericaceae)

“…Flowering time differences between populations have also been reported in other Rhododendron species (Kudo, 1993;Kameyama et al, 2001;Mejías et al, 2002). The occurrence of reproductive isolation or reproductive incompatibility is related to assortative mating, because immigrants that flower at different times from those of local individuals cannot reproduce, and as a consequence no gene flow occurs (Gavrilets and Vose, 2007). Therefore, geographic variations in flowering rhythms of R. oldhamii suggest population isolation caused by reduced gene flow among populations.…”

What, if anything, is sympatric speciation?

“…The authors emphasized the importance of an environmentally induced shift in flowering for facilitating genetic divergence. However, if this effect was indeed present, then speciation was not sympatric but parapatric at least according to some definitions (Gavrilets & Vose, 2007).…”