A major part of comprehending the ecological theory of adaptive radiation is observing and analyzing scientific experiments that illustrate such theory. Scientists have studied Gasterosteus aculeatus, also known as the threespine stickleback fish. Until 12,000 years ago, these fish only lived in marine environments. After the last ice age, hundreds of populations of these fish were isolated in freshwater environments and evolved in a parallel manner due to their environment change. By following these fish, scientists have been able to gain an insight into the mechanics of adaptive radiation. G. aculeatus fish aid scientists in better understanding the process of adaptive radiation through observation of parallel evolution among stickleback populations.
The ecological theory of adaptive radiation can be described as a period of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill vacant ecological roles in their communities (Campbell Reece, 2009). Large-scale adaptive radiations commonly occur after events such as ice ages. After these events, isolated survivors are forced to adapt to their newly formed ecological niches (Campbell Reece, 2009). Scientists have been studying various populations of organisms in order to better understand this theory of adaptive radiation and how it works. Through scientific study and observation of the parallel evolution of G. aculeatus, also known as threespine stickleback fish, the mechanics of adaptive radiation are revealed.
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Scientists including Dr. Schluter studied G. aculeatus, also known as three spine stickleback fish, to explain the genetics of adaptation within this species. These scientists have determined Eda to be the major gene underlying reduced number of lateral plates in freshwater populations of sticklebacks (Schluter et al., 2010). Homozygous individuals for the low-armour allele typically have fewer plates than the marine form. Heterozygous individuals usually have full plates or an intermediate number. In freshwater populations, the low-armour Eda allele has been repeatedly selected from standing genetic variation present in the ancestral marine populations (Schluter et al., 2010).
By studying the specific gene, Eda, underlying the reduction in lateral plates in freshwater populations, Schluter and colleagues were able to illustrate how these fish contribute to our understanding of adaptive radiation through parallel evolution (Schluter et al., 2010). These stickleback inhabit the lakes and nearby coastal waters of British Columbia, Canada. Once the postglacial lakes formed 12,000 years ago, marine sticklebacks that were trapped in these lakes had to adapt to the freshwater. Scientists observed that the direction of evolution was largely parallel in each freshwater population. Furthermore, they observed that the magnitude of divergence from the marine form varies from lake to lake according to various local conditions (Schluter et al., 2010). Each population of stickleback has small differences from other populations. Throughout every population of stickleback fish living in freshwater lakes, reduced external body armor is present. Also, freshwater stickleback fish differ from their ancestral saltwater fish in medial fin size, overall head size, body depth, and rotation of the jaw (Schluter et al., 2010). These evolutionary differences are thought to be associated with reduced predation by birds and fish in freshwater and greater predation by aquatic insects. Scientists also believe that an increase in littoral habitat and benthic invertebrate prey along with the loss of a migratory lifestyle in freshwater play a role in the adaptive radiation observed (Schluter et al., 2010). The observation of this parallel evolution, occurring independently in hundreds of lakes across Canada, illustrates evidence of natural selection and adaptive radiation at work.
Schluter and colleagues summarized that parallel genetic change between populations in similar environments provides evidence for the effects of natural selection at the genetic level (Schluter et al., 2010). Scientists attempted to determine why fish in many different freshwater lakes have evolved in a parallel manner and have significantly less body armor than their saltwater ancestor. They carried out field experiments to test hypotheses about the causes of fitness differences between alternative alleles. These experiments may help the scientific community understand why the low-armour Eda allele reached fixation in every freshwater population (Schluter et al., 2010). This approach was applied so that scientists could have a better understanding on the mechanisms that drive parallel evolution of the reduced lateral plate armor in freshwater sticklebacks.
Scientists studying threespine sticklebacks, including Dr. Schluter, measured the selection on the Eda locus that underlies the phenotypic difference in the populations. They measured this in order to test a hypothesis about the causes of natural selection, specifically adaptive divergence (Schluter et al., 2010). The scientists began by measuring the selection on Eda in transplanted populations. Their aim of this specific experiment was to comprehend the mechanisms of natural selection on lateral plates in freshwater. The scientists also aimed to understand why the reduced lateral plate allele was favored in each of these freshwater environments (Schluter et al., 2010).
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The high-armour allele is thought to be favored in marine environments because many lateral plates aid in defense against predators. The plates interfere with the predator's ability to ingest the stickleback fish and reduce injury after the stickleback's escape (Schluter et al., 2010). Little is known about the advantages of the low-armour allele in freshwater environments. This transplantation experiment investigated a hypothesis that illustrates the advantages to having the low-armour allele in freshwater. The hypothesis states that juvenile sticklebacks with reduced lateral plate armor have a growth advantage in water (Schluter et al., 2010). A subsequent laboratory experiment illustrated that homozygotes for the low-armour allele grow faster than high-armour homozygotes. This finding suggested that faster growth permitted by a reduction of lateral plates explains the spread of the low-armour allele in freshwater lakes (Schluter et al., 2010). In the freshwater populations, faster growth of the sticklebacks translates into a lower mortality rate by predatory insects, which prey on the smallest sticklebacks. Low-armour fish have increased nutritional reserves because they do not have to put forth energy to form plates. Therefore, they have more energy in the winter than those fish that have to make plates. Enhanced reproductive success is also prevalent because more fish that are homozygous for the low-armour allele survive winters and are able to reproduce the next year (Schluter et al., 2010).
In the experiment conducted by Dr. Schluter and his colleagues, adult marine sticklebacks that were heterozygous at the Eda locus were transplanted to freshwater ponds. The scientists were attempting to test the hypothesis that there is a growth advantage for reduced armor in freshwater sticklebacks (Schluter et al., 2010). Schluter and colleagues tracked selection on Eda genotypes in the offspring of these heterozygous fish over subsequent years. A total of 180 heterozygous sticklebacks were transplanted into four separate ponds. Over time, the scientists sampled 50 random offspring and recorded their growth rates and changes in Eda frequencies (Schluter et al., 2010). Once these offspring reached sexual maturity, scientists observed that the low-armour allele frequency increased from 33% to 51%, which represents a 1.5-fold survival advantage compared to the high-armor allele (Schluter et al., 2010).
These findings are consistent with and verify the hypothesis that there is a growth advantage for reduced armor in freshwater sticklebacks. They also support the idea that faster growth of sticklebacks translates into a lower mortality rate and an enhanced reproductive success. If the sticklebacks with the growth advantage are able to survive the winter, they will subsequently spread their genes to their offspring. This explains the 18% increase in low-armor allele frequency over a short period of time. From the data collected in this experiment, one can infer that fixation would occur fairly quickly for the low-armour allele because it has a higher fitness for this specific environment. This experiment illustrates how observation of G. aculeatus helps scientists better understand the mechanisms that push parallel evolution and adaptive radiation to occur in nature.
After analysis of the data collected from the experiment, scientists detected almost equally strong selection in the opposite direction in very young fish (Schluter et al., 2010). This initial selection against the low-armour genotypes early in life counteracted the gains by the low-armour allele later in life. This yielded weaker net selection over the lifespan of the sticklebacks than previously anticipated (Schluter et al., 2010). In a similar experiment, fish raised in a laboratory did not show the same pattern of selection in the opposite direction. This implies that the reasons for selection both early and later in life are environmentally specific (Schluter et al., 2010). Through observation of the threespine stickleback fish, scientists have found support for the hypothesis that a growth advantage is responsible for the parallel evolution of reduced armour in freshwater sticklebacks throughout hundreds of Canadian lakes (Schluter et al., 2010). The growth advantage gave fish the ability to survive and spread their genes on to their offspring, making them more fit to survive in their environment.
These various observations and scientific experiments by Schluter and colleagues help to illustrate how the increased knowledge of the genetics of phenotypic evolution in stickleback fish is aiding our studies of natural selection on traits. They also show how measuring the effects of selection on a specific locus, for example Eda, in a population can help us to understand the mechanisms of phenotypic selection. Due to freshwater environments that emerged after the last ice age, freshwater sticklebacks were forced to adapt in order to survive. The "most fit" individuals, those being the sticklebacks with low-armour alleles, survived through winters and were able to pass their genes on to their offspring. This explains how fixation was able to happen in various freshwater lakes across Canada. Scientists who studied the stickleback fish in various freshwater populations have found enough support to form a hypothesis that explains the parallel evolution of reduced lateral plate armour in freshwater sticklebacks.
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Through scientific study and observation of Gasterosteus aculeatus, the mechanics of adaptive radiation are revealed. In various freshwater populations of sticklebacks throughout Canada, the low-armour Eda allele was "most fit" for its environment and, therefore, favored over the marine high-armour allele. Threespine stickleback fish with the low-armour allele that lived in freshwater all adapted to their different environments in order to survive. The driving force of adaptive radiation between all freshwater stickleback fish into slightly different forms from the marine form is due to the organisms need to fill all available niches. Every freshwater lake that the marine sticklebacks were forced to occupy had minor differences. These small differences in the environments help us understand why all of the freshwater sticklebacks have very small differences in morphological structure. All scientific study of parallel evolution in G. aculeatus helps to reveal how adaptive radiation occurs in nature.