title: Climate niches of milkweeds with plesiomorphic traits (Secamonoideae; Apocynaceae) and the milkweed sister group link ancient African climates and floral evolution.
abstract: OBJECTIVE
  Climate change that increases mortality of plants and pollinators can create mate-finding Allee effects and thus act as a strong selective force on floral morphology. Milkweeds (Secamonoideae and Asclepiadoideae; Apocynaceae) are typically small plants of seasonally dry habitats, with pollinia and high pollen-transfer efficiency. Their sister group (tribe Baisseeae and Dewevrella) is mostly comprised of giant lianas of African rainforests, with pollen in monads. Comparison of the two groups motivated a new hypothesis: milkweeds evolved in the context of African aridification and the shifting of rainforest to dry forest. Pollinia and high pollen-transfer efficiency may have been adaptations that alleviated mate-finding Allee effects generated by high mortality during droughts. We formally tested whether milkweeds have a drier climate niche by comparing milkweeds with plesiomorphic traits (Secamonoideae) and the milkweed sister group in continental Africa.
  
  
  METHODS
  We georeferenced specimens of the milkweed sister group and Secamonoideae in continental Africa, extracted 19 climatic variables from the Worldclim model, conducted factor analysis to identify correlated suites of variables, and compared the frequency distributions of the two lineages relative to each factor.
  
  
  RESULTS
  The distributions of Secamonoideae and the milkweed sister group differed significantly relative to four factors, each correlated with a distinct suite of climate parameters: (1) air temperature (Secamonoideae: cooler), (2) total and (3) summer precipitation (Secamonoideae: drier), and (4) temperature seasonality and isothermality (Secamonoideae: more seasonal and less isothermal).
  
  
  CONCLUSIONS
  Secamonoideae in continental Africa inhabit drier, cooler sites than do the milkweed sister group, consistent with a shift from rainforests to dry forests in a cooling climate.
authors.affiliation: The Academy of Natural Sciences of Drexel University, Philadelphia, Pennsylvania 19103, USA. livshultz@ansp.org
authors.forename: Tatyana
authors.initials: T
authors.lastname: Livshultz
authors.forename: Jerry V
authors.initials: JV
authors.lastname: Mead
authors.forename: David J
authors.initials: DJ
authors.lastname: Goyder
authors.forename: Michelle
authors.initials: M
authors.lastname: Brannin
mesh_terms: D000349:Africa; D029062:Apocynaceae / Q000235:genetics*; D031108:Asclepias / Q000235:genetics*; D044822:Biodiversity; D005075:Biological Evolution; D002980:Climate; D005163:Factor Analysis, Statistical; D035264:Flowers / Q000235:genetics*; D010802:Phylogeny; D019655:Quantitative Trait, Heritable; D011891:Rain; D013696:Temperature; D013997:Time Factors