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