Theses and Dissertations

Issuing Body

Mississippi State University


Counterman, Brian A.

Committee Member

Outlaw, Diana C.

Committee Member

Hoffman, Frederico G.

Committee Member

Brown, Matthew W.

Date of Degree


Document Type

Graduate Thesis - Open Access


Biological Sciences

Degree Name

Master of Science (M.S.)


College of Arts and Sciences


Department of Biological Sciences


The X and Z chromosomes have unique characteristics that lead to unique evolutionary consequences. Lepidopterans have a well-known, disproportionately large-Z effect for behavioral and morphological traits that distinguish closely related species. A potential explanation for the Large-X effect is the faster evolution of the sex chromosome (Faster-X evolution). We use whole genome re-sequencing of Heliconius erato races and of the incipient species H. himera to test for faster-Z evolution between hybridizing populations at different reproductive isolation levels, by calculating divergence and nucleotide diversity. We show evidence for Faster-Z evolution in Heliconius butterflies at the early stages of speciation and along the speciation continuum. Evidence of higher divergence and lower nucleotide diversity suggests not only selection but also nonaptive process, like demographic changes, may be driving faster-Z evolution, especially in the incipient species.