Presentation Detail

Speciation

DiVittorio, Christopher [1].

Mechanisms of extreme divergent natural selection at an Encelia hybrid zone.

Divergent natural selection is thought to be critical for most speciation events. However, the mechanisms through which adaptation causes divergence are often unclear. Adaptation to environmental gradients and resultant trade-offs in resource use and allocation should be able to initiate and maintain divergence if the magnitude of selection is stronger than the homogenizing effect of gene flow, although mechanistic studies of selection at natural hybrid zones are needed to test theoretical predictions. Using reciprocal transplant and resource manipulation field experiments I tested the hypothesis that extremely strong divergent natural selection between fully interfertile desert shrubs in the genus Encelia (Asteraceae) is maintaining dramatic phenotypic differentiation despite abundant hybrid embryo formation. I planted parental taxa and a range of hybrid phenotypes into both parental habitats and the intervening ecotone and analyzed juvenile fitness categorically as a function of taxon as well as continuously using a multivariate phenotypic hybrid index. Selection coefficients against non-native phenotypes were extremely high, 0.98 for E. palmeri in the dune habitat and 0.76 for E. ventorum in the desert habitat. Hybrids had intermediate fitness in parental habitats but equaled or exceeded the fitness of parental taxa in the ecotone. Theory predicts that selection of this magnitude is sufficient to drive divergence under very high amounts of gene flow, and high hybrid fitness in intermediate habitats suggests that fitness differences are extrinsically rather than intrinsically based. To further test the mechanisms of selection I added water to a subset of parental taxa in each parental habitat. Adding water eliminated divergent natural selection such that E. ventorum performed as well or better than E. palmeri in all habitats demonstrating that divergent natural selection is critically dependent on a gradient in water availability. Other mechanisms of selection were also important. Herbivory selected against E.ventorum and seed germination selected against E. palmeri in all habitats. Burial by sand selected against E. palmeri and hybrids in the dune and ecotone habitats but not the desert habitat, and the watering treatment affected fitness differences in the desert but not the dune habitat. These patterns suggest that the extremely strong divergent natural selection measured between E. palmeri and E. ventorum is not due to any one selective mechanism but that it is the net effect of multiple interacting mechanisms that may not be individually divergent but that together maintain the dramatic phenotypic differentiation observed between these taxa.

1 - University of California, Berkeley, Integrative Biology, 1005 Valley Life Science Bldg. #3140, Department of Integrative Biology, Berkeley, California, 94720, USA

Keywords:
hybrid zone
reciprocal transplant
divergent selection
Speciation
extrinsic postzygotic isolation
resource manipulation.

Presentation Type: Regular Oral Presentation
Session: 98
Location: Cotton C/Snowbird Center
Date: Monday, June 24th, 2013
Time: 9:45 AM
Number: 98006
Abstract ID:1218
Candidate for Awards:W.D. Hamilton Award for Outstanding Student Presentation,Student Travel Awards from the ASN