Transcript
NAME_________________________________________ DUE: FEB 12TH
SPECIAL ASSIGNMENT- EVOLUTION IN GUPPIES (10 points)
(Modified from Broder et al., National Center for Case Study Teaching in Science)
Part I- Discovering Guppies
544893517056100054514754889500Trinidad is a tropical Caribbean island off the coast of Venezuela, best known for being the larger of the two islands that make up the country of Trinidad and Tobago. While exploring the numerous streams emerging from Trinidad's Northern Range Mountains, Professor John Endler observed diverse fish communities in the lowlands. These lowland stream communities were made up of many predatory species like the wolf fish and numerous types of cichlids, which were all preying on an abundant small fish. Endler noticed these small colorful fish occurred in the thousands, forming “shoals” or social groups, along the edges of the streams. The locals called them “millionfish” because of their abundance and the number of bright colors on their bodies, but we know them as guppies.
The Trinidadian guppy, Poecilia reticulata, is usually less than two centimeters long, with females being larger than males. Males are brightly colored to attract females while females are drab, camouflaging into their environment. Males display their coloration in elaborate courtship displays to attract females. Guppies have internal fertilization, and females give birth to live young. As females age and grow, they are able to carry more babies and produce more offspring.
Endler noticed that the guppies in the lowland stream grew quickly, matured early, and had many small babies so that they could reproduce as soon as possible before they were eaten by the larger predatory fish. Endler then hiked upstream and climbed waterfalls expecting that these natural barriers prevented upstream fish movement. Instead he found many thriving guppy populations that lived without the large predatory fish. The only other fish living with guppies was the slightly larger killifish, which preys on juvenile guppies. He described these streams and the fish living there as “low predation”. He noticed that the guppies were very different from those that lived with many large predators in the downstream rivers, which he called “high predation” environments. The low predation guppies were larger, the males were more colorful, and their behavior differed. For example, the low predation males performed more courtship displays than high predation males. These differences seemed intuitive based on differences in the potential for predation in these environments. There is a risk of being spotted and eaten if a male performs flashy courtship displays in a stream with lots of predators. Similarly, higher mortality in high predation environments should favor individuals that grow quickly and become sexually mature at a young age (Reznick and Endler 1982). Endler knew that different color patterns in guppies had a genetic basis because breeders had generated a diversity of fancy ornamental guppies that were sold in pet stores around the world. What Endler wanted to know was whether predators in nature could be driving the differences in color between high and low predation male guppies (Figure 4). To test this (Endler 1980), Endler introduced guppies from a high predation location to a site above a waterfall where there were no other fish, except the killifish. He came back two years later and examined the guppies at the experimental introduction site. He discovered that many traits had changed.
1. What is the basic question of Endler’s study?
2. In Endler’s experiment, where did the introduced population of guppies originally come from? At the start of the experiment, did the introduced population look like the guppies in the high or low predation population?
3. What hypothesis was Endler testing with this experiment?
4. What did he expect to find if his hypothesis was supported?
Figure 1. Results from a field experiment comparing a high predation (HP) population of guppies and a low predation (LP) reference population of guppies to a population that was introduced (Intro) from a HP stream to a LP stream two years before. Source: adapted from Endler (1980).
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5. Based on the graphs above, is the introduced population more similar to the high predation source population or the low predation reference population?
6. Is it possible that evolution explains the differences between the three populations? Why or why not? Include a definition of evolution in your answer. Describe the selective force that is likely driving the changes.
Part II – Guppy Diets
Following Endler’s work, scientists continued to study the evolution of color patterns in guppies. One important discovery was that females are particularly attracted to orange body and tail coloration on male guppies (Endler and Houde, 1995). Several years later, Professor Greg Grether was especially interested in how orange color was obtained by males. Like earlier scientists, Grether saw that color patterns differed from stream to stream, but he noted differences among streams that could not be explained by predators since all had only one minor predator, the killifish. One thing that was different in these streams was the amount of sunlight that made it through the rainforest canopy. This in turn affected the amount of algae available for the guppies to eat, specifically a unicellular alga that contains carotenoid compounds (Grether et al., 2000). (The word “carotene” comes from carrots, which get their orange color from these compounds.) To better understand how color on the body might relate to the amount of carotenoid-rich unicellular algae they consumed, Grether manipulated the amount of carotenoids in the diet of guppies. Once he was able to determine the sex when they were about six weeks old, Grether separated brothers into different tanks and reared them on three different diets: high levels, low levels, and trace levels of carotenoids (Grether, 2000). See Figure 2 to the right and answer the questions below.
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What hypothesis was Grether testing with this experiment?
What did he expect to find if his hypothesis was supported?
Briefly state the independent and dependent variables in Grether’s experiment.
Why did Grether use brothers in the three treatments instead of unrelated guppies?
What differences do you observe among the three brothers in Figure 2? How might you explain the difference in color between the brothers?
Is this an example of evolution? Why or why not? If not, what is it?
Given the data presented in this experiment, has your answer to Question 6 from Part I changed? Why or why not?
