Biology ETDs


Rhiannon West

Publication Date



Sympatric speciation, the divergence of one lineage into two or more lineages within one geographic range, is often driven by disruptive selection on niche utilization. While niche specialization is found in sympatrically speciated groups, it is often reinforced by other strong selective pressure. This dissertation examines the role of predation, morphology, mate choice, and parasite load in maintaining reproductive isolation in two sympatrically speciated species flocks of pupfish (Cyprinodon). The role that predation, morphology and male and female mate choice play in reproductive isolation is examined in the Bahamian species flock. To examine the role of predation, we focus on two species, a generalist detritivore and a predator/scale-eater. We show that premating isolation is based primarily on visual cues and is asymmetrical. It is well developed in the less abundant scale-eater but is much less well developed in the more abundant detritivore. Responses of detritivore females from three lakes to conspecific and scale-eater males suggest that abundance of the predator also affects development of premating isolating mechanisms in the prey species. These results highlight the importance of frequency-dependent selection and predators in the evolution of premating isolating mechanisms. The three species, a detritivore, an ostracod-eater, and a scale-eater which preys upon the other two, inhabit distinct trophic niches. We use geometric morphometrics to examine variation in body shape among these three species. Results show that there are significant differences in male, but not female body shape. These results suggest that male body shape, specifically nuchal hump height, is under sexual selection and may contribute to reproductive isolation between species. Female mate choice reinforces reproductive isolation in sympatrically speciated species. Male mate choice, while not as strong a selective pressure as female mate choice, could also act as a reproductive isolation mechanism in sympatric species. Using a binary choice design, we examine the importance of visual and olfactory cues in female mate choice and focus on the ostracod-eater, the least abundant of the three sympatric species of pupfish. We also examine male mate choice in the scale-eater and the detritivore. Females use visual and not olfactory cues and prefer conspecific males. Males also preferentially associate and court conspecific females. Thus, mutual mate choice acts a strong premating isolation mechanism in these sympatrically speciated pupfish. Recognition and behavioral avoidance of parasitized conspecifics or individuals with slightly different immunities, either due to differences in genotype and/or regional parasite load, would be favored by natural selection to maintain a healthy phenotype. This avoidance of parasitized conspecifics may play a role in species differentiation in two ways: 1) rapidly changing genes that encode for immune function, such as the Major Histocompatibility Complex may lead to small, localized differences in immune function. If small differences in immune function and parasite load drive behavioral avoidance of conspecifics, then reproductive barriers within a population are effectively formed through non-random mating. If such nonrandom mating barriers persist, they may lead to isolation of gene pools and subsequently, sympatric speciation; 2) Changes in parasite resistance may be tightly coupled with trophic differentiation as parasitic infection often occurs when organisms are foraging. To address this question, we performed two experiments to examine how parasites affect female choice in sympatrically speciated pupfish (Cyprinodon spp.) from the Yucatan, Mexico. We infected and sham infected male C. labiosus and C. maya with Ichthyophonus hoferi. In a series of binary choice trials, we presented female C. labiosus and C. maya with olfactory cues from infected and non-infected conspecific males. Then we presented the females with odors from infected conspecific males and non-infected heterospecific males. Results indicate that infection was successful in both species but varied in infectivity. Our findings show that females preferred non-infected conspecific males and suggest that reproductive barriers may be formed by female preference for healthy male conspecifics. Additionally, reproductive barriers between species are strong and prevent mating with heterospecific males. Overall this dissertation shows that multiple selective pressures contribute to and reinforce reproductive isolation. Future studies should take care to examine the roles of sexual selection, frequency dependence, ecological pressures beyond trophic niche usage, mate choice, morphology and other common methodologies such as genetic inference. Realistic models of sympatric speciation likely include a non-random collection of these and similar variables.

Project Sponsors

Gerace Research Centre and the University of New Mexico: Grove Research Scholarship; Office of Graduate Students Research Allocations Committee; Biology Graduate Student Association, Graduate Research Allocations Committee; and Office of Graduate Students Research, Project, Travel award.




pupfish, character displacement, reproductive isolation, sympatric speciation, male and female mate choice, visual cues, parasite, predation, morphology

Document Type


Degree Name


Level of Degree


Department Name

UNM Biology Department

First Committee Member (Chair)

Thornhill, Randy

Second Committee Member

Watson, Paul

Third Committee Member

Bolnick, Daniel