Date of Award


Degree Type

Honors College Thesis

Academic Program

Marine Biology BS


Biological Sciences

First Advisor

Jacob F. Schaefer, Ph.D.

Second Advisor

Jacob F. Schaefer, Ph.D.

Third Advisor

Sabine Heinhorst, Ph.D.

Advisor Department

Biological Sciences


Hybridization of closely related taxonomic groups within the Animal Kingdom has led to complex species interactions that are not well understood. Pre-zygotic or post-zygotic barriers complicate hybridization as a means to diversify; however, diverging hybrids may speciate and expedite evolution of involved taxa. This study pursues understanding of these intricate interactions using topminnow Fundulus notatus, Fundulus olivaceus, and their hybrids from the Fundulus notatus species complex. Persisting hybrids within riverine contact zones were hypothesized to experience barriers of hybrid breakdown from lower fitness in comparison to the pure species. Specimens were collected within four contact zones: Spring River, Pascagoula River, Tombigbee River, and Noxubee River. Each genetically differentiated specimen was phenotypically analyzed based on length, wet mass, spot density, sex, gonad mass, and eviscerated mass. Significant differences in Fulton’s condition index were present in comparison of sex (P 0.001), zones (P 0.0007), and species across zones (P 0.04) due to females dedicating more energy to body condition and extreme variation in contact zones, respectively. Comparisons of gonadosomatic indices (GSI) yielded significant differences between sexes (P ) of each species due to females investing in reproduction more than males. Spot phenotype analyses confirmed genetic analyses of each species and emphasized the intermediate traits of hybrids. Averaged indices across species did not support the original hypothesis since hybrids showed similar or increased Fulton’s condition indices and GSI in comparison to the pure species. Possible explanations lie within the phenomenon of hybrid vigor, and future research should focus on replicating these results and predicting if hybrids will speciate.

Keywords: Hybridization, hybrid breakdown, fitness, reproductive barriers, gonadosomatic index, Fulton’s condition index, Fundulus notatus species complex, hybrid vigor

Included in

Evolution Commons