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Dr. David Morgan

Associate Professorphoto
Phylogenetic Evolution and Plant Systematics
dmorgan@westga.edu
678-839-4044


 

Education   

 

Research Interests

My primary research is in phylogenetic evolution and systematics, in which the goals are to reconstruct the evolutionary history (phylogeny) of a group of organisms and to find out how the organisms and their characteristics have originated and changed over time. The plants that I concentrate on are in the families Asteraceae (the sunflower family) and Rosaceae (the rose family).

Asteraceae

For many years it was thought that ancient interspecific hybridization (reticulate evolution) had little impact on the evolution of plants. Recent evidence, however, is showing that reticulate evolution has had a more substantial role than previously thought. My work in Asteraceae involves a group of plants that appears to have experienced a significant amount of reticulate evolution. Machaeranthera and several related genera make up a group in Asteraceae that is closely related to familiar plants such as asters. I have been studying the systematics of Machaeranthera with a variety of approaches, incorporating evidence from morphology, cytology, secondary chemistry, and DNA sequences. My first investigation, employing evidence from the chloroplast DNA (cpDNA), suggested many relationships that conflicted with other characteristics, raising new questions about the evolution of Machaeranthera and its relatives. I have subsequently studied the group with nuclear sequence data from the nrDNA regions: the internal and external transcribed spacers (ITS and ETS). The nrDNA data supported several relationships that conflicted with those supported by cpDNA, suggesting that reticulate evolution has been a factor in many lineages. These results indicate that the evolutionary history of Machaeranthera and its relatives has been extraordinarily complex, with as many as seven occurrences of reticulate evolution. There are several evolutionary processes that could have produced these results, such as chloroplast DNA introgression and hybrid speciation. Resolving these questions requires more thorough characterization of nuclear DNA, and my plans involve further study of the nuclear genome. One approach that I am taking is to obtain sequence data from other parts of the nuclear genome, such as the 5S ribosomal RNA gene spacers. This year, one of my students completed a preliminary investigation of this spacer, finding that it contains useful sequence variation. My plans for the next few years are: 1) to generate a complete set of 5S spacer sequences from Machaeranthera, including sequences of multiple clones from most of the species; 2) analyze these data phylogenetically, and use the results to help further resolve the evolution of Machaeranthera. My ultimate objective is to determine what specific evolutionary process produced each instance of reticulate evolution in Machaeranthera. This work should help provide a better understanding of the role of interspecific hybridization in the evolution of plants. The relationships suggested by my research do not conform to any previous classification of Machaeranthera. Therefore, a new one is necessary, and (with coauthor R. L. Hartman of the University of Wyoming) I produced a revisionary synthesis of the genus, which was published in 2003. I have also been asked to contribute identification keys and descriptions ("treatments") to the Flora of North America for some of the genera that I work on.

Rosaceae

Investigations of large plant groups involve contributions from a number of people because they involve large numbers of species and often use data from many different sources, including multiple chloroplast and nuclear genes. I am collaborating with several other Rosaceae researchers on a project to produce a phylogeny of the entire family to facilitate more detailed study of the origin and evolution of specific lineages and specific characteristics. As our primary source of characters, we are using DNA sequence data from several nuclear and chloroplast genes. I am contributing in two ways. My first contribution consists of sequence data from the chloroplast gene rbcL. These sequences were generated during the past several years. My second contribution consists of fruit characters. There is a great deal of variation in the morphology and anatomy of fruits in Rosaceae, and one of our goals is to identify patterns of fruit evolution in the family. I am responsible for putting together a detailed data set of the variation in many fruit characters. I am currently working on this project, and most of the data thus far have been generated by undergraduate students working with me. We anticipate that this collaboration will result in at least one major publication, which will include our findings on the phylogeny of Rosaceae, conclusions about the evolution of specific aspects of the family (such as fruit characteristics, chromosome number, and secondary chemistry), and a new classification for Rosaceae.   

Student Researchphoto

For students who are interested in working on research with me there are a number of different possibilities. Those who are interested in lab work may want to help with my plant systematics research, which involves DNA amplification, cloning, and sequencing. There are also several possibilities in field research; some field research topics include floristic inventory or plant community ecology work.

 

Courses Taught   

Biol 3223 is designed to be a thorough introduction to the biology of plants. It includes an evolutionary survey of the plant phyla, in which students learn about the origin of the land plants from algae, how plants have changed over time, and what strategies they have developed to deal with living on land. The course also deals with how plants are built and how they function, including how they grow and develop, how they transport things internally, and how they respond to the environment.

Biol 3221 is all about flowering plant systematics. In the lab, students learn how to identify plants using keys, both in winter and in spring. In class, students learn about the most important and most interesting families of flowering plants: how they are related to one another, how they and their characteristics have originated and evolved, and how the economically important families are used by people.

 

Selected Publications