Simulations of flowering time displacement between two cytotypes that form inviable hybrids

Abstract

The evolution of reproductive isolation by flowering time displacement between two cytotypes that produce inviable hybrids was studied by computer simulations in an isolation-by-distance model. Flowering time distribution was stabilized by mass-action, both by the mating procedure and by pollen-limited seed-production in early or late flowering plants. Coexistence had to last long enough for flowering time divergence to evolve. This could only be achieved in a mosaic of local patches or parapatry. The model showed that flowering time displacement occurred despite the stabilizing effect of mass-action and the restricted width of the interaction zone. Initially an inverse dine for flowering time was formed but after 200 generations this dine had become monotonic. In contrast to the possible swamping effect by gene flow from outside the zone, there was a spread of genes into the allopatric ranges.

Genetic variation and gene flow in Alpine diploid and tetraploid populations of Lotus (L. alpinus (D.C.) Schleicher/L. corniculatus L.). I. Insights from morphological and allozyme markers

“…Any between-cytotype pollination may also be countered strongly by an efficient triploid block, as suggested by the absence of triploid plants both in situ and in experimental crosses between diploid and tetraploid plants (Urbanska-Worytkiewicz et al, 1979). In Alpine Lotus, such flowering period discrepancy between cytotypes is very similar to that described in the Pyrenees for diploids and autotetraploids of Plantago media (Van Dijk et al, 1992) and Arrhenatherum eliatus (Petit et al, 1997) and may fit the evolutionary model proposed by Van Dijk & Bijlsma (1994).…”

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

“…The first multilocus simulations of flowering phenology (Crosby 1970) focused on reproductive character displacement between pre-existing subspecies with reduced hybrid viability, and did not observe splitting within populations, in part owing to the short time scale of the simulations (less than 100 generations) and the lack of new mutations. van Dijk & Bijlsma (1994) and Kondrashov & Shpak (1998) simulated assortative mating for a quantitative character and found rapid origin of bimodal or disjunct distributions; their findings are an unrealistic artefact of initial conditions involving equal frequencies of two alleles at each locus, and the lack of new mutations, as explained further in the discussion of our results. Reproductively isolated groups originated in models of finite populations with mutation and either sexual selection ( Wu 1985) or inbreeding (Higgs & Derrida 1992).…”

“…Our model demonstrates how polygenic mutation and random genetic drift interact with non-selective assortative mating. 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).…”

Habitat differentiation in a narrow hybrid zone between diploid and tetraploid Anthoxanthum alpinum

“…Tbe proportion of tbe minority cytotype in a population would tben progressively decbne until it disappeared. However, Van Dijk & Bijlsma (1994) demonstrated tbat reproductive isolation tbrougb differences in fiowering time alone migbt be sufficient to allow tbe coexistence of tbe two cytotypes. Tbus, as Fowler & Levin (1984) demonstrated, nicbe differentiation may allow, under particular conditions, tbe coexistence of botb cytotypes.…”