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Graduate Defense: Cara Hastings

December 18, 2023 @ 1:00 pm - 3:00 pm

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Dissertation Information

Title: Relationships, Species Boundaries, And Speciation Of Piper Section Enckea

Program: Doctor of Philosophy in Ecology, Evolution, and Behavior

Advisor: Dr. James Smith, Biological Sciences

Committee Members: Dr. Allan Bornstein, Biological Sciences; Dr. Stephen Novak, Biological Sciences; and Dr. Sven Buerki, Biological Sciences


Species are fundamental biological units but defining what a species is has been debated by biologists, philosophers, and other scholars. Our concept of what constitutes a species has evolved over time, reflecting progress and advances in technology as well as increased understanding of the complexity of the speciation process. To understand how extant species came to occur, it is useful to evaluate their relationships using molecular data in a phylogenetic framework. By pairing phylogenetic data with other types of data including morphological and fossil data as well as topographical and climatic history, we gain understanding of what factors led to speciation, dispersal, and general evolutionary patterns associated with speciation.
Genera with over 500 species are considered “giant genera”. Given their size, giant genera can be difficult to study but provide a valuable opportunity for understanding speciation and other evolutionary processes. Piper L. is a pantropically distributed giant genus. Piper species form three geographically structured lineages in the Neotropics, in the South Pacific, and in the Asian tropics. Although most of the economically important Piper species are from the Asian tropics, the Neotropical region is the center of diversity of the genus. Within the Neotropics there are seven phylogenetically resolved sections (Enckea, Macrostachys, Ottonia, Peltobryon, Pothomorphe, Radula, and Schilleria).
Enckea L. is a mid-sized lineage (~120 species) of Piper and is distributed in most Neotropical regions. The actual number of species in Enckea is difficult to determine and taxonomic inflation, mostly due to synonymy, is suspected to be relatively common. Most species within Enckea occur as narrow endemics, but there are a few broadly distributed species in the lineage as well (e.g., Piper amalago L. and Piper reticulatum L.). Morphological features used to characterize Piper in Enckea include leaf venation and flower structure, but identification of species within Enckea can be difficult because of character overlap. Previous studies failed to resolve the lineages within Enckea and led to questions regarding species delimitation within the section. The goals of this study are to determine which are included in monophyletic Enckea, to gain a better understanding of the relationships within Enckea, and to understand the historical biogeography of the lineage.
To accomplish our goals we used a targeted sequencing approach and recently developed bioinformatic pipeline to produce a nuclear data set. The nuclear data was used to model phylogenetic relationships using maximum likelihood and multispecies coalescent methods. The phylogenetic models were used to determine which species are in Enckea, where species boundaries lie, and to test biogeographic models and reconstruct the historical biogeography. Phylogenetic reconstruction identified 31 nominative species within Enckea and some of those species formed a species complex. We increased sampling in the species complex in an attempt to delimit species and recovered a single well-supported monophyletic lineage, indicating that our complex was actually a single species with a broad geographic distribution and intraspecific morphological variation. Finally, we found that speciation within Enckea is driven by dispersal within Mesoamerica and between Mesoamerica and other regions.
The results of this study increase our understanding of Piper, especially in the Neotropics. Our models help to enumerate the number of species in the giant genus Piper as well as in Enckea. Delimitation of species provided understanding of the degree of intraspecific variation possible within a single species, especially Piper amalago. Our findings help reveal evolutionary patterns in Piper and for the entire Neotropical region.