Type of Culminating Activity
Master of Science in Biology
James F. Smith, Ph.D.
Determining the specific factors that played a role in speciation previously took extensive resources that made such studies nearly intractable. Despite the difficulties presented by speciation studies, we are still interested in determining what forces drive the process of evolution to gain a better understanding of divergence among species. Advances in technology allow for a new approach to speciation studies, beginning with molecular phylogenetic analyses that identify the species within a monophyletic clade and generate a species-level phylogeny. Molecular data are an independent source of data and provide a phylogeny to map both morphological characters and ecological parameters. Identifying patterns among phylogenetic studies, morphological characters, and ecological variables highlight possible forces driving speciation.
Morphological characters have previously provided the foundation for phylogenetics. However, convergence among characters has made phylogenetic studies difficult based on morphology alone. Molecular phylogenetic analyses provide better insight into relationships across the family and as a result, better classification systems that are a reflection of ancestral evolutionary history rather than convergent evolution.
As the largest Neotropical genus in the family Gesneriaceae, with over 200 species, Columnea has had a complex taxonomic history with classification systems based on morphological characters including vegetative, nectary, and floral characters. Most recent classification systems had classified the species of Columnea into five sections: sections Columnea, Collandra, Ortholoma, Pentadenia, and Stygnanthe. Section Stygnanthe encompassed eighteen species based on a similar floral morphology, characterized by small corollas that are slightly ventricose and constricted at the base, ranging in size from 1.4 to 5.2 cm in length.
A well-supported topology of the species of Columnea can test the previous subgeneric classifications. The phylogenetic analyses presented here sampled 129 accessions representing 90 species within Columnea. Of the 90 species included, fifteen of the eighteen species of Stygnanthe were sampled along with an additional three species, Columnea grisebachiana, C. moorei, and C. ulei, which had not been placed in Stygnanthe but share a similar corolla morphology. Based on five chloroplast DNA (cpDNA) gene regions (trnQ-rps16 spacer, rpl32-trnLUAG spacer, rps16 intron, trnS-G spacer, and trnH-psbA spacer) and nuclear ribosomal internal transcribed spacers (ITS1 and ITS2, hereafter referred to as ITS) the results of this study indicated that the species of section Stygnanthe and the three additional species all with similar corolla morphologies, belong in three separate clades within Columnea. Three of the species from Stygnanthe (C. moesta, C. ultraviolacea, and C. xiphoidea), including the type species and one of the additional species (C. moorei), were separated from the remaining twelve species of Stygnanthe that were tested. Because the type species (C. moesta) moved to another clade, most of the species of the former section Stygnanthe and one of the additional tested species (C. ulei) belong within section Angustiflorae. The other tested species, C. grisebachiana, fell out into a third separate clade. The results indicate that within Columnea small corollas that are slightly ventricose and constricted at the base are the result of convergent evolution.
Molecular phylogenetic analyses with five cpDNA gene regions and ITS provided phylogenetic support for seven monophyletic clades within Columnea but failed to resolve species-level relationships within clades. Additional molecular phylogenetic analyses were conducted to resolve species level relationships with 36 accessions, representing thirteen of fifteen species within Angustiflorae. The external transcribed spacer (ETS) was added to the sequences from the five cpDNA gene regions and ITS from the genus wide analysis to generate a well-resolved species level phylogeny. In addition, two low-copy nuclear gene regions, glyceraldehyde 3-phosphate dehydrogenase (G3pdh) and NADP-dependent isocitrate dehydrogenase (idh) were included to boost phylogenetic support of the major branching events within the section.
This study used these molecular phylogenetic analyses along with morphological characters and climatic variables to determine driving forces of speciation within section Angustiflorae. The species in section Angustiflorae have morphological variation and cover nearly the full geographic and climatic range of Columnea, making speciation studies interesting and possible. Studying evolutionary and ecological parameters approaches speciation from a new angle, identifying patterns among phylogenetic studies, morphological characters, and ecological parameters. Correlation analyses between parameters identify possible forces that have driven evolutionary divergence by highlighting relationships between character states over the phylogenetic history. Character state shifts may indicate the larger forces that are driving evolutionary divergence. This study mapped fourteen morphological characters, phenology, and nineteen climate variables onto the species level phylogeny of Angustiflorae, and patterns were identified with ancestral state reconstructions and correlation analyses. Defining patterns among morphological characters (including phenology) and climatic variables showed evidence for allopatric speciation, changes in photosynthetic ability, nectar robbing, pollinator shifts, and climate changes in temperature and precipitation as possible forces driving evolutionary divergence within Angustiflorae.
Schulte, Lacie Janelle, "Phylogenetic Relationships within Columnea Section Angustiflorae: Insights into Forces Driving Speciation" (2012). Boise State University Theses and Dissertations. Paper 277.