Recent evolutionary divergence in a plant ring-species is not accompanied by floral phenology or pollinator shifts

Background and aims – The study of factors and processes involved in evolutionary divergence can inform how biodiversity is generated and maintained. We evaluate shifts in phenology or in pollination systems as potential barriers to gene exchange and thus promoters of divergence at the population-species boundary in the plant ring-species Euphorbia tithymaloides in the Caribbean. Material and methods – Combining collections-based and field-based observations and measurements, we evaluate evidence supporting that shifts in tempo of reproductive activity (floral phenology) or pollinator guilds (using visitation as a proxy) could be acting as mechanisms promoting divergence in E. tithymaloides. We focus on the geographic region where evolutionary divergence in this species has been documented: Greater and Lesser Antilles. Phenology data were derived from herbaria and online databases, for a total of 376 records across the Greater and Lesser Antilles. We quantified and characterized reward (nectar n = 13 sites) and gathered visitation data using direct observation (n = 12 sites) for a total of over 133 hours of observation/site. Key results – The peak of floral activity of E. tithymaloides is in winter, when days are short (~late October–late May). Under natural conditions, plants in the Antilles produce up to 22.4 µL of nectar, with mean sugar concentrations of ~ 46.5 ºBrix that amount to up to 10.3 mg of total sugars, with no significant differences observed between plants of the Lesser and Greater Antilles. Hummingbirds are the main floral visitors of E. tithymaloides in both areas: Greater Antilles: 61%, Lesser Antilles: 85%, and network analyses support a floral visitor community turnover across islands/countries. Conclusion – Evolutionary divergence in Caribbean E. tithymaloides along the Greater and Lesser Antilles is not accompanied by shifts in floral phenology or pollinator systems. Other factors, like pollinator turnover or pollinator-plant trait matching, might be at play. We outline hypotheses to this effect.