Lacey Nay and Dr. Jerry Johnson, Biology
A species’ life history traits, including growth and reproduction, follow patterns of natural selection. Evolution is driven by natural selection which, in short, can be described as the ability of an organism to conform to its environment (Pianka 2000). Individuals possessing those traits that are more favorable in a specific environment tend to have higher rates of survival and reproduction, causing those traits to become more prevalent in the population. In this manner, a trait is “selected for” by nature. Life history evolution is based on the concept of trade-offs. The more energy an individual expends on growth, the less it will expend on reproduction, and vice versa. Hence, the amount of energy expended for growth versus reproduction or other traits can vary depending on selection pressures and thus be molded by natural selection (Reznick 1983).
The livebearing fish Alfaro cultratus found in Costa Rica proves an interesting organism for study of life history evolution. Livebearing fishes, the family Poeciliidae, have a unique reproductive strategy in that females give birth to free-swimming offspring, rather than lay eggs. This characteristic is essential in our study, allowing us to examine the unborn offspring of pregnant females and determine the energy expended in reproduction versus growth. A characteristic that makes this particular species different from other members of the Poeciliidae family is its unique body shape. Alfaro cultratus is referred to as the “knife-edged” livebearer due to its sharply keeled ventral surface. Other females in this family, when pregnant, grow a large, distended abdomen to accommodate for the growing embryos. Alfaro cultratus females do not appear to do so. We proposed this constrained morphology to be a restricting factor on reproduction which, we hypothesized, would limit the amount of variation across populations within the species. In other words, we expected to find very little variation of reproductive traits between populations within this species despite the fact that each population experiences different selection pressures.
For this research, we dissected females from ten different populations. Populations were defined by the different river drainages from which they were collected. When it came time to present the results, I mapped out these population locations and found that two of the populations overlapped with others already included in the results. Thus, in reality, we had results from only eight populations (see populations 5,6 and 8,9 on Figure 1), a problem that is solved with our continuing research by performing more dissections on individuals from drainages not already included in our study.
In performing these dissections, we were interested in five life history traits: male size at maturity, female size at maturity, reproductive allotment by females (total mass of the brood), number of offspring, and size of offspring (detailed in Johnson and Belk 2001). We hypothesized that due to the constrained nature of the abdomen, we would not see significant variation between populations, assuming that individuals collected from one drainage would experience different selection pressures from their counterparts in a another drainage.
The results turned out to be very different from what was hypothesized with regards to reproductive traits. The only reproductive trait that was not found to vary significantly across populations was the size of the offspring. Brood mass and number of offspring varied significantly, an unexpected outcome. Thus, despite the highly constrained morphology of the species, life history traits showed considerable variation within the species.
Essentially, body shape does not appear to limit the expression of life history in Alfaro cultratus. The result of this research seems to ask more questions than it answers. The question arises regarding what factors cause this variation, despite the limiting body shape of the species.
Results of this research were presented in a poster session at the Joint Meetings for Ichthyologists and Herpetologists in Portland, OR in July of 2009. Special thanks go to Tessa Smith, Alan Tanner, Megan Phinney, Melissa Todd, Katie Bulkley, Andrew Lybbert, and Zach Panter who helped with dissections and photographing as well as to Dr. Johnson and Justin Bagley for their supervision of the project.
References
- Johnson, J. B. and M. C. Belk. 2001. Predation environment predicts divergent life-history phenotypes among populations of the livebearing fish Brachyrhaphis rhabdophora. Oecologia 126:142-149.
- Pianka E. R. 2000. Evolutionary ecology. Benjamin Cummings, San Francisco.
- Reznick, D. 1983. The structure of guppy life-histories: the tradeoff between growth and reproduction. Ecology 64:862-873.