The three-spine sticklebacks are one group of fish that inhabits lakes in the northern regions of North America, where lakes were formed after ice sheets that covered the region retreated. This group of fish has gone through a rapid period of speciation, with the species that inhabits the lakes of coastal British Columbia being the youngest species (Smith & Smith, 2009). Beren Robinson of the University of Guelph Department of Zoology primary focus was to find out if constraints posed by the difficult environments were partly the cause of the evolution of the three-spine sticklebacks.
Robinson’s field study in in Cranby Lake in the coastal areas of British Columbia was an important study in the field of evolution, and will be further discussed here in. In the lakes where a pair of three-spine stickleback species is found, one species is found in open waters and feeds on the plankton in the open water zone, limnetic form, while the second species is found in shallower waters eating larger prey from the sediments and vegetation, benthic form.
In the lakes where only one species occurs, the species are usually found to be intermediate in morphology and habitant to the limnetic and benthic (Smith & Smith, 2009). In Cranby Lake, Robinson was studying an intermediate species, the Gasterosteus aculeatus, when it was observed that individuals sampled from the open water habitat were morphologically different than those from the shallower waters (Smith & Smith, 2009). From this observation, Robinson hypothesized that “these individuals represented distinct phenotypes that are products of natural selection promoting divergence within the population” (Smith & Smith, 2009).
To test his hypothesis, Robinson had to first discover if the morphological differences between the two was heritable and if the differences influenced the foraging efficiency. Robinson tested the first condition by setting up a lab study. In this study, he reared the two forms under identical environmental conditions, having the only variable being the different forms of the fish. Results showed that showed that although there was some phenotypic variation, the differences were heritable (Smith & Smith, 2009).
With data to show that the differences were indeed heritable, Robinson was able to test the second condition of his hypothesis. To test this, he conducted feeding trials in the lab with variables being the two different forms of fish and two different food types, brine shrimp larvae and larger amphipods. The fish were reared in lab, with diets differentiated by the different forms of three-spine sticklebacks, open water and shallow water. The test consisted of introducing a fish into the aquarium for the period of observation time.
The data that was collected was converted into two measures of foraging success, intake rate and capture effort (Smith & Smith, 2009). Results from the foraging success showed very distinct differences in the foraging success of the different morphological forms. The limnetic form individuals were more successful at foraging on the brine shrimp larvae; they had a higher consumption rate and needed half the number of bites to consume the shrimp as compared to the benthic form. It was able to then show that the limnetic form’s success was related to the greater number of gill rakers and larger mouth width.
This suggested trade-offs in relation to the distinct habitat and diet (Smith & Smith, 2009). Robinson’s study is just one example of how natural selection, evolution, and ecology work together. Natural selection is essentially the struggle of life, were evolution is the change of a one species into two or more new species. Natural selection and evolution work hand in hand, as natural selection can force change and adaptation, and evolution is these adaptations that are heritable going through several generations eventually creating new species.
Ecology is the study of the organismal unit and it’s interaction with the environment. Ecology allows researchers to observe natural selection and evolution in a very close way. Robinson’s study show that the morphological structure of the two intermediate three-spine sticklebacks was showing a form of divergent selection, which indicates the species, could be experiencing the early stages of speciation (Smith & Smith, 2009).
This study was carried out in a efficient and repeatable way, but for a follow up study, there would be a few variables that I would like to change. First would be introducing the morphologically different fish with the two different food sources well before the initiation of observation. In doing this, it could possibly reduce a feeding preference that could have been produced by the different diets. Also, since the feeding preference also seemed to be based off the mouth size, I would use fish with similar mouth sizes.
This could just reduce the variation more. Studying the species of three-spine stickleback that are not of the intermediate species could be useful for behavioral observation purposes. Beren Robinson showed natural selection at work within a population. It is one of the few studies that has been able to quantify trade-offs in a population, in relating to exploitation of the habitat. With studies like this, we are coming closer to completely understand the process of evolution, in an ever changing environment.
Courtney from Study Moose
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