Demonstrating The Theory Of Natural Selection Essay

, Research Paper

In 1976 two biologists, David Reznick of the University of California at Riverside and John Endler of the University of California at Santa Barbara, traveled to the West Indies island of Trinidad to begin the first in their inclusive series of studies concerning evolution. Their studies focused on the vast population of guppies. After over a decade of researching the guppies, the two biologists came to the conclusion that the guppies were able to evolve a great amount in a small duration of time. This not only supported Darwin s theory of evolution but more specifically his idea of natural selection. There was also one other theory which was very important, that was the life-history theory. This theory predicts that reduced adult survival will select for earlier maturation and increased reproductive effort. (Impact of predation) On other hand, reduced adolescent survival will select the opposite. This theory has only been proven in laboratory studies and cannot assess that of natural populations. In the experiment conducted by Mr. Reznick and Endler they support the predictions of the life-history theory with guppies.

Natural populations of guppies (Poecilia reticulata), along the El Cedro and Aripo rivers, are found in simple communities and live in one of two predation areas, high or low. (Evaluation of the rate) In the high predation areas the main predator is the Pike Cichlid (Crenicichla alta), other predators include different types of cichlids and characins. The Crenicichla alta mainly prey on the larger more sexually mature guppies.(Impact of predation) The predators which reside in high predation areas are excluded from parts of the upstream because of rapids and waterfalls. The low predation areas are located in the upstream of the rivers. In these areas there is only one predator, and that is the Killifish ( Rivulus hartii). The Rivulus hartii is an omnivore; it therefore does not prey on guppies exclusively, but only sometimes. When the Rivulus hartii do prey on the guppies they prefer the small and sexually immature guppies. (Life history evolution .Phenotypic)

There are many differences among the guppies in each area. Guppies in the high predation areas tend to mature at an earlier age, have higher reproduction effort, and smaller offspring. In addition to that they devote more resources to each litter and produce more litter more frequently. These characteristics are all contrary to the guppies which reside in the low predation areas. All of these differences were the basis for the scientists hypothesis. They believed that predation was the cause for different patterns in offspring size, number, and the age at which they sexually mature. They also believed that that the age at which the guppies died was due to the fact that the predators were size-specific. (Bryga) To prove that predation was the cause of all of the differences they altered the natural habit of the guppies by changing predation against adults to predation against adolescent.

Their experiment took place in a rare river system in Trinidad where most of the fish are separated by waterfalls and rapids. They first moved the guppies from a high to low predation area. (Evaluation of the rate) They then took the guppies from locations below waterfalls on the Aripo and El Cedro rivers which contained numerous predators, and placed them in a tributary (experimental site) where only one predator existed, the Rivulus hartii. The high predation areas on the rivers were used as the controls. By doing that the biologists exposed the guppies to selective predation on adolescents instead of on the adults, like they were familiar with before. (Life history evolution . Parallelism) The guppies used in the experimental site on the El Cedro river and the control site on the Aripo river, were brought up in a three-generation, extended-lineage, paternal half-sibling design. (Evaluation of the rate)

Mr. Reznick and Endler used different food levels at certain times in the experiment. The first four years of the experiment were done at two food availability levels. There was an interaction involving food availability and population, such that the difference between the control and the experimental sites was more pronounced at low levels of food availability. (Evaluation) After seven and a half years the experiment changed to a single, high level of food availability. That resulted in a changes of size in the male guppies. Nine years into the experiment they added the conditions similar to that of the fourth year. The results were alike, but they lacked the interaction amongst position and food availability. (Bryga)

At the same time Mr. Reznick and Endler were also conducting the same experiment in laboratory. By doing the same experiment simultaneously they were minimizing the chance of any errors. Another way they minimized errors was by conducting reduced versions of the same experiment six other times. These experiments were taken between March 1981 and March 1988. One last step to the experiment took place after they had gathered all of the results. They wanted to see whether or not some or the variations between the guppies in the control site and experimental site were heritable. To do this they gathered guppies from each location in March 1987, then brought up their offspring through two generations in a common environment. By bringing them up in a common environment they reduced the environmental influences on life-history, which only leaves the possibility that the variations have a genetic base. (Bryga)

The results reached by the biologists were extensive. Within two years the

phenotype females varied from those in the control site by offspring size (see figure 1) and reproductive allowance see figure 2). After seven more years the females were also larger in size. In addition to their enormity, the guppies also lived longer. This suggests a reaction to modified predation. (Bryga) The female guppies also had smaller numbers of offspring per litter and smaller reproductive allowance in four dry seasons. These took place in March of 1981, February of 1984, April of 1987, and March of 1988. Even though the females produced fewer offspring, the offspring were significantly larger per litter (see figure 1), the females also initiated reproduction at a larger size. Another noticeable result was that the females provided a lesser amount of their consumed resources to reproduction. (Evaluation of the rate) There also was a difference in each of the litter sizes, the females produced fewer offspring in the first litter than usual, but there was no difference in the second and third litters. One of the most important changes took place among both the males and females, and it was that they matured at a later age and larger size. That change started to occur within four years of the experiment. A result immediately noticed amongst the males was that they were significantly larger than those from the control site(see figure 3). This change only took place up until 1984, and after that the male sizes were smaller than those of the control site. Reproductive effort was also lowered after eight to ten weeks of controlled feeding (see figure 5). With the exception of the male size, the phenotypes of all variables support the prediction of the life-history theory. As a result their life-history had evolved to be comparable to the guppies which naturally occur in low predation populations. (Impact of predation)

The biologists then determined, through units called the darwin, the rate of evolution for the changes in the guppies. A darwin is a proportional amount of change over time. While most rates of evolution are recorded at 0.1 to 1.0 darwin, in the fossil record, the guppies’ rate of evolution was from 3700 to 45,000 darwins. This proves very well that evolution can occur very fast. (Evaluation of the rate)

After four years there was one noticeable result in the El Cedro river. They observed that there was a 10-fold difference between the sexes for the rate of change in the age at maturity. In addition to that there was also a 3 to 5-fold difference for the size at maturity. When the similar evaluations were made three and a half years later, the comparative rate of change for male age was still higher than that for female age, but the discrepancy was far less prominent. (Bryga)

Even though a new species was not dealt with in the study, a significant increase in body size was. Body size is one of the principal traits used in paleontology to distinguish evolutionary changes and also was a major characteristic adapted by the guppies which were observed. Mr. Reznick and Endler believe that even though the rate of change seemed somewhat slow, there were significant changes which took place. They also said that if the study were to have gone for at least a thousand years we would have experienced a new species. (Evaluation of the rate) They also believe that the study shows that it is possible to use short term experiments of natural selection to gain a superior insight of evolutionary changes which occur over millions of years. They have also come to the conclusion that evolution by way of natural selection can occur very fast. In light of fossil records, it shows that natural selection is four to seven times faster. (Dawson)

These results demonstrate significant evolutionary changes in the life histories of the evolved guppies. They provide direct experimental evidence of the importance of predation in molding life history evolution in guppies. In addition to predation there were many other important factors involved. On a more general level, these results provide evidence that mean differences in age-specific survival will mold the evolution in lfe-history patterns. (Bryga) The extensive evidence for size-specific predation in other species suggests that this could be a common factor in life-history evolution.

Their work did not deal with the effectiveness of methods other than natural selection, but it did expand our knowledge and understanding of what is possible through this process. It was a part of a growing body of evidence that the rate and patterns of change possible through natural selection to account for the patterns observed in fossil record. (Maugh)

There were many positive results directly from this experiment, not only scientific but also historical. First of all it gives us a much deeper insight and understanding of evolution and its complicated ways. Secondly because we know how fast evolution can take place, we are able to better assist other animal species, such as the endangered, to adapt to their environmental surroundings. Thirdly it shows us that we are able to understand the evolutionary changes that took place over millions of years by way of short-term experiments about natural selection. Finally we are able to learn more about the ways of previous peoples and how they developed, we can then incorporate that in solving common debates among science and religion.

Works Cited

1. Bryga, H., Endler, J. A., Reznick, D. A. Experimentally induced life history

evolution in a natural population. Nature. 26 July 1990. 357-359

2. Dawson, Jim. Guppy study suggests that natural selection can work out in


Minneapolis Star Tribune. Mar. 28, 1997 late ed.: C3

3. Maugh, Thomas. Lowly guppy helps scientists validate Evolutionary Theory.

Los Angeles Times. 27 July, 1990. late ed.: A4

4. Reznick, D. A., Endler, J. A. The impact of predation on life history evolution in

Trinidadian guppies (Poecilia Peticulata). Evolution. Mar. 1982: 160-177

5. Reznick, D. A., Bryga, H. Life history evolution in guppies. 1. Phenotypic and

Genotypic, Changes in an introduction experiment. Evolution. July 1987: 1370-1385

6. Reznick, D. A., Shaw, F., Rodd, F. Helen, Shaw, R.G. Evaluation of the rate of

evolution in Natural populations (Poecilia Peticulata). Science. Mar. 28, 1997.

7. Reznick, D. A., Rodd, F. H. Life history evolution in guppies (Poecilia

Peticulata: Poeciliidae), IV Parallelism in life history phenotypes. American Naturalist. July 1996: 319-338.


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