Parallel Evolution Observed in Populations of Threespine Stickleback

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Parallel evolution is a mechanism of natural selection associated with the well-understood concept of speciation. Observations of organisms such as the three-spined stickleback (Gasterosteus aculeatus) have provided further evidence of this phenomenon. Following the end of glaciation, this species experienced allopatric speciation after marine stickleback colonized thousands of inland lakes in British Columbia, Canada. These freshwater stickleback developed new body armor adaptations despite being divided among these many lake regions, a phenomenon known as parallel evolution. It was later discovered that this morphological change was due to an allele frequency difference in the Ectodysplasin (Eda) gene, which controls armor development. With this empowering knowledge, researcher Dolph Schluter conducted a transplantation experiment aimed at investigating stickleback parallel evolution in more detail. His conclusions provided additional concrete explanations for this influential mechanism of speciation and allowed for further questioning of this phenomenon.

Parallel evolution is a mechanism of natural selection associated with the concept of speciation and has been documented in various environments. Observations of organisms such as the threespine stickleback (Gasterosteus aculeatus) have further enhanced scientific knowledge of this phenomenon. The threespine stickleback is a species of marine and freshwater fish whose lineages can be traced back to a common marine ancestor. Until about 12,000 years ago, glaciers covered the majority of British Columbia, Canada, during the most recent ice age. Following the end of glaciation, land depressions filled with water, forming thousands of inland lakes which were subsequently colonized by marine sticklebacks (Schluter et al, 2010). Over time, allopatric speciation occurred due to the geographical separation of the marine and freshwater populations (Campbell and Reece, 2009). As a result, parallel evolution was observed in freshwater stickleback, all of which developed new body armor adaptations despite being divided among many lake regions (Schluter et al, 2010). Upon further research, it was discovered that the new morphology was due to an allele frequency difference in the Ectodysplasin (Eda) gene, which controls armor development (Colosimo et al, 2005). One study aimed at investigating parallel evolution was conducted by researcher Dolph Schluter, who applied this empowering knowledge in a transplantation experiment with marine sticklebacks (Schluter et al, 2010). His conclusions provided additional concrete explanations for the forces behind the parallel evolution of the sticklebacks as well as for this influential mechanism of speciation itself.

In order to understand parallel evolution in sticklebacks, it is first necessary to investigate the genetics behind this phenomenon. Following the end of glaciation, marine threespine sticklebacks experienced allopatric speciation as they colonized freshwater lakes in British Columbia. Despite separation from the marine species and other freshwater stickleback, they all developed similar adaptations through parallel evolution (Schluter et al, 2010). One such morphological adaptation was a change in the number of lateral plates in these groups of G. aculeatus. In an experiment conducted by Pamela Colosimo, it was found that sticklebacks homozygous for the low-armor allele had fewer armor plates than those homozygous for the high-armor allele. In contrast to these two phenotypic extremes, heterozygous sticklebacks were found to exhibit a range of phenotypes, including full- and medium-plated armor. Examination of allele frequencies also revealed that the high-armor allele was selected for in marine stickleback while the low-armor allele was selected against. The exact opposite was true for freshwater stickleback, in which the high-armor allele appeared in lower frequencies due to favoring of the low-armor allele. These new developments were eventually linked to the Ectodysplasin (Eda) gene, which was found to directly control body armor growth in both marine and freshwater stickleback (Colosimo et al, 2005).

Although the discovery of the Eda gene provided a basic understanding of armor development, it was still unclear why high-armor alleles were selected against in freshwater stickleback. In marine stickleback, lateral plates often serve as a defense mechanism, reducing the probability of injury and ingestion by a predator. Thus it was uncertain why this form of protection would be selected against in freshwater stickleback. Numerous hypotheses cite environmental factors, including a transition from bird predation to insect predation, a littoral habitat with an abundance of benthic invertebrate prey, and the loss of a migratory lifestyle (Schluter et al, 2010). In addition, armor reduction was believed to increase body size among freshwater stickleback, a theory which was confirmed in an experiment performed by R.D.H. Barrett. In this study, since the majority of potential insect predators prey upon smaller stickleback, larger size was found to decrease the probability of predation (Barrett, 2008). Thus, parallel evolution could result from any of these propositions due to the universal lake environment shared by all freshwater stickleback.

In order to affirm these hypotheses, Dolph Schluter and a group of scientists decided to research the effects of transplantation on threespine sticklebacks. They hypothesized that since the Eda alleles were believed to be associated with various armor phenotypes, transplanting sticklebacks from a marine environment into a freshwater environment would result in selection against the high-armor allele. Since marine sticklebacks are known to breed with freshwater sticklebacks due to their migratory lifestyle, the experiment sought to mate heterozygous marine and homozygous freshwater sticklebacks. Thus, Schluter and his team collected about 180 heterozygous marine G. aculeatus, transplanted them into four freshwater ponds on the campus of the University of British Columbia, and monitored the offspring of these species over intervals of four to six weeks (Schluter et al, 2010).

Schluter’s data not only provided further support for parallel evolution, but also allowed for more opportunities to question stickleback adaptations. One significant conclusion from this research was a direct correlation between body size and armor development, which reinforced Barrett’s previous results. In all four experimental lakes, Schluter noted that, after the same duration, sticklebacks homozygous for the low-armor allele typically attained a larger size than those homozygous for the high-armor allele. As a result, the unarmored sticklebacks had more nutrient reserves in comparison to the other sticklebacks, which had used up these reserves during armor growth. Schluter’s conclusion supported the idea that freshwater sticklebacks lacked lateral plates due to environmental influences. This further supports the mechanism of parallel evolution due to the universal lake habitat shared by all freshwater sticklebacks (Schluter et al, 2010). Finally, Schluter hypothesized that the Eda gene potentially has pleiotropic influences, affecting multiple genes including armor development and body size (Campbell and Reece, 2009). All of these genetic implications support the mechanism of parallel evolution in threespine sticklebacks and provide a further understanding of this phenomenon.

Since all freshwater sticklebacks are descended from the same marine ancestor and inhabit similar lake environments, it can be determined that parallel evolution resulted from these analogous influences and other environmental factors. Observations of threespine sticklebacks provide further basis for understanding parallel evolution. This process was witnessed in the transplantation experiment conducted by Dolph Schluter and his colleagues. By looking at known information about G. aculeatus, including the knowledge of how the Eda gene affects armor development and the general history of freshwater sticklebacks, Schluter designed an experiment which further supported the belief that environmental factors influence adaptations and parallel evolution. In addition, the discovery of a direct correlation between body size and armor development in freshwater sticklebacks serves as concrete proof of parallel evolution in this species (Schluter et al, 2010). Thus, research of stickleback evolution supplies further evidence of the process of speciation in all organisms by reinforcing the belief that environmental factors influence evolution.

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