David Reznick, an ecologist at the University of California at Riverside, studied the process of natural selection in populations of guppies (small freshwater fish) on the island of Trinidad. Reznick found that populations at lower elevations face the assault of predatory fish, whereas the populations at higher elevations live in peace because few predators can move upstream past the waterfalls. The average size of individuals in the higherelevation waters is larger than the average size of guppies in the lower-elevation populations. Reznick hypothesized that the smaller size of individuals in the lower-elevation populations was a result of increased rates of predation on larger individuals; in effect, predation was selecting for smaller individuals in the population. To test this hypothesis, Reznick moved individuals from the lower elevations to unoccupied pools upstream, where predation was not a factor. Eleven years in these conditions produced a population of individuals that were on average larger than the individuals of the downstream populations. Is the study by Reznick an example of natural selection (does it meet the necessary conditions)? If so, what type of selection does it represent (directional, stabilizing, or disruptive)? Can you think of any alternative hypotheses to explain why the average size of individuals may have shifted through time as a result of moving the population to the upstream (higherelevation) environment?

Step by step solution

01

Identifying conditions for natural selection

First, let's establish the conditions necessary for natural selection, which are: 1. Variation - There must be genetic variation among individuals in a population. 2. Inheritance - Traits must be inheritable, i.e., passed on from parents to offspring. 3. Differential survival and reproduction - Individuals with favorable traits have higher chances of survival and reproduction than those with less favorable traits. 4. Time - The change in the population's traits must happen over a few generations.

02

Evaluating Reznick's study against conditions

Now let's evaluate Reznick's study against these conditions: 1. Variation - There is variation in guppy size among the individuals. 2. Inheritance - The size of guppies can be a heritable trait. 3. Differential survival and reproduction - In lower-elevation waters, the predation is higher and selects for smaller individuals with higher survival chances. 4. Time - The study took place over eleven years, which is enough time for multiple generations of guppies to be born. Since the study meets all the necessary conditions, it can be considered an example of natural selection.

03

Identifying the type of selection

Next, let's identify the type of selection happening in this case. There are three types: 1. Directional selection - This occurs when one extreme phenotype is favored. 2. Stabilizing selection - This occurs when the intermediate phenotype is favored. 3. Disruptive selection - This occurs when both extreme phenotypes are favored. In Reznick's study, the smaller guppies in the lower elevations were being favored by predators, while larger guppies had an advantage in the higher elevations with fewer predators. This represents directional selection as one phenotype (smaller or larger size) is favored depending on the environment (presence of predators).

04

Alternative hypothesis

As for alternative hypotheses to explain the shift in average individual size when moving the population to the higher-elevation environment, some possibilities are: 1. Resource availability - In the higher-elevation environment, there might be more food resources available, allowing the guppies to grow larger. 2. Competition - There could be less competition for resources in the higher-elevation environment, leading to an increase in the average size of guppies. 3. Environmental factors - Differences in water temperature or water flow in the higher-elevation environment could influence the average size of guppies, contributing to their size increase. These alternative hypotheses would need further investigation and experiments to determine their validity compared to Reznick's hypothesis.

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