K. Nicole Schoonmaker and Dr. Jerald B. Johnson, Biology
The process of natural selection drives the evolution of species which results in the survival of individuals whose traits are most adaptable to a specific environment and the death of individuals whose traits are less fit. Thus, populations of a single species that reside in habitats with differing environmental pressures can have different physical (phenotypic) or genetic (genotypic) traits. An understanding of the environmental pressures that create variation in traits and observation of that variation is important to understanding the adaptability and evolution of living organisms.
Life history evolution studies how individuals allocate available energy sources. Examples of life history traits include age at maturation, life-span, body size, and number of offspring or fecundity. Current research suggests that in livebearing fishes life history traits are altered by the presence of a predator in the environment because adult mortality is higher than juvenile mortality. The difference in mortality rate causes fish that live in predator populations to mature at a smaller size, have a higher overall reproductive investment, have smaller offspring, and have a higher number of offspring compared to fish in predator-free populations. This pattern has been consistent across four different species of livebearing fish. My research tests to determine if populations of the livebearing fish Poecillia gillii from Costa Rica follow the same trend.
To perform my research I have used collections of P. gillii from over twenty sites throughout Costa Rica that were collected between the years 2006-2008. I determined whether each site was predator or predator-free based on the presence of a cichlid, Parachromis dovii , known to prey on other fish. Next I determined which populations of fish had enough individuals which could be used to quantify the specified life history traits. So far, I have examined nine populations, five from predator-free sites and four from predator sites, but I hope to continue the research to look at five to seven other populations.
To quantify life history traits, we followed the methods outlined in Johnson and Belk’s paper in 2001. Male fish stop growing at maturity, so I determined male size at maturity by taking the standard length of all adult males in a populations and determining the average length. Females continue to grow even after they mature, so I determined female size at maturity by dividing the fish into size classes. When at least half of the fish in a size class were mature I defined that size as the size of maturity. I measured the reproductive investment by removing all reproductive tissue from the females and taking the mass after drying it in an oven for twenty four hours. Number of offspring was quantified by a count of the number of embryos and offspring size was determined by measuring the dry mass of all the embryos and dividing it by the total number of offspring. To determine if the differences in these life history traits was statistically different between predator-free and predator populations, I used an analysis of covariance.
I hypothesized that Poecillia gillii would follow the same pattern observed when comparing predator-free and predator populations of other livebearing fish. I expected the size at maturity to be smaller, reproductive allotment to be higher, offspring size to be smaller, and offspring number to be higher in predator populations. However, the data doesn’t appear to follow the trend at all. For the population analyzed at this point, there is no significant difference in size at maturity for male or female fish between the predator-free and predator environments. Statistically, there is no difference in reproductive allotment or the number of offspring in predator and predator-free populations. While the differences aren’t statistically different, there is an observable difference in both traits and the difference is exactly opposite of what I predicted. Reproductive allotment was slightly lower in the predator environment as was the number of offspring. Currently the sample sizes are moderate, so the statistical power of these analyses is low, but I hope as we collect more data from the remaining populations to see a more significant trend. The only trait that followed my predictions and was statistically significant was the size of offspring with predator-free populations having larger offspring than predator populations.
These unexpected observations give rise to other questions which provides opportunities for continuing research. When doing the project, we assumed that the mortality rates between adult and juvenile fish differed, but it’s possible that there is really no difference in mortality rate. A field research project could examine the mortality rate of fish at in their natural environment and determine if adult mortality is actually higher than juvenile mortality. Another important consideration is whether the phenotypic traits we observed actually have a genetic basis so they can be passed from one generation to another. Again, we assumed that these traits are genetic, but haven’t collected any data to support that assumption and if the traits are not genetically based, predation environment would not impact them.
In June of this year I attended the Evolution conference in Minneapolis, Minnesota and presented my research in a poster session. Currently, the plan is to have enough data by the end of the year to write a manuscript which will be published in a scientific journal. These experiences have been invaluable to my education. I have learned a great deal about biology and also about research and the time, effort, organization, successes and failures that are necessary to complete a meaningful project.
References
- Basolo, A.L. and Wagner Jr., W.E. 2004. Biological Journal of the Linnean Society 83:87-100.
- Jennions, M.D. and Telford, S.R. 2002. Oecologia 132:44-50.
- Johnson, J. B., and Belk, M.C. 2001. Oecologia 126:142–149.
- Reznick, D. and Endler, J.A. 1982. Evolution 36:160-177.