Stickleback quiz

4. The first clue into the genetics of pelvic spine loss in sticklebacks came from researchers working in a different area of science. Those researchers found that mice genetically engineered to lack the function of the Pitx1 gene did not develop complete hind limbs. The mice without a functional Pitx1 gene had just the remnants of a pelvis and hind limbs, and what was left of these structures was larger on the left than on the right side. Since then, researchers have discovered that pelvic reduction in most freshwater stickleback populations is associated with mutations that inactivate Pitx1 in the pelvis. Based on this knowledge and your findings in this exercise, would you agree or disagree with the following statement: Left-biased pelvic asymmetry appears to be a signature of the loss of function of the Pitx1 gene.

Agree

Living populations tell us about mechanisms of selection; fossils provide a record of change over time. Based on what you have learned so far in this virtual lab, would you agree or disagree with this statement?

Agree

9. If you conducted the analysis portion of Experiment 2, you calculated the rate at which the percentage of fish with a complete pelvis decreased in that ancient lake. Do you think the same rate of change would apply to modern fish populations?

Although we did not measure this directly, it is possible that the same rate of change observed in the fossil record applies to modern populations.

How do your data compare to those obtained by Dr. Bell and colleagues?

Answers will vary, but if students scored the fish accurately, their data should be similar to Dr. Bell's data.

3. How did ancestral populations of ocean-dwelling fish come to live in freshwater lakes?

At the end of the last ice age, new freshwater lakes formed as glaciers melted. These lakes provided new breeding grounds for stickleback swimming to these lakes from the ocean. As more and more ice melted, the land that had been depressed by the weight of the massive ice sheets became elevated, and the connections between some of the lakes and the ocean were cut off. Researchers think that stickleback populations became trapped in these lakes and adapted to living exclusively in fresh water.

In a population, what happens to organisms that are better adapted to the environment in which they live?

At the population level, individuals that are better adapted for a particular environment are more likely to live longer and reproduce, and pass their genetic variants to the next generation. Over many generations, these genetic variants and the associated traits increase in frequency in the population.

From a researcher's perspective, what is the benefit of having access to hundreds of postglacial lakes?

Because there are hundreds of postglacial lakes in the Northern Hemisphere, researchers can compare a trait like pelvic reduction in many different stickleback populations living in these lakes. The great number of lakes allows researchers to make predictions about which environmental conditions influence the trait.

What can we infer from the stickleback fossil record about evolutionary processes occurring today?

Evolutionary patterns observed in the fossil record are consistent with evolutionary processes occurring today.

Is the following statement true or false? As they adapted to life in fresh water, all stickleback populations living in lakes in Alaska underwent exactly the same evolutionary changes.

False

Explain why the stickleback fish in Frog Lake are more similar to ocean and sea-run stickleback than they are to the stickleback fish in Bear Paw Lake.

Fish in marine and sea-run populations have complete pelvises; the pelvic spines protect stickleback fish from larger fish that prey on them in the ocean. Most stickleback fish in Bear Paw Lake have either a reduced or an absent pelvis, whereas those in Frog Lake have a complete pelvis. Thus, the stickleback population in Frog Lake is more similar to marine and sea-run stickleback populations.

In addition to predators, what other environmental factors might be responsible for the differences between Bear Paw Lake and Frog Lake stickleback populations? How would you test your prediction? Answers will vary.

For example, students could mention fresh water is not as rich in calcium and other minerals needed to form pelvic spines. Students could test this prediction by measuring the mineral content of fresh water and comparing it to that of ocean water. Another example would be that unlike the open ocean, freshwater lakes have more vegetation and more places where stickleback can hide, in which case having less armor may make stickleback more agile and better able to quickly dart into hiding places. Students may come up with other ideas.

Name two other vertebrate animals whose evolutionary histories included the loss of hind limbs

Hind-limb reduction has occurred in several vertebrate lineages, including manatees, snakes, and whales.

11. What is the purpose of looking for evidence of left or right bias in pelvic asymmetry in stickleback populations?

If the pelvic vestiges are larger on the left side in most stickleback in a population, the Pitx1 gene is probably involved in pelvic reduction.

How can evolutionary processes that we measure today inform our interpretations of evolutionary changes seen in the fossil record?

If the same morphological changes occur in the fossil record as in living populations, we can infer that the genetic mechanisms discovered in living populations might be responsible for the changes observed in fossils.

Why do some stickleback populations lack pelvic spines?

In lakes where there are no predatory fish, there is no advantage to having pelvic spines.

Why are random samples, rather than entire populations, used in most research studies?

In most cases, it would take too much time and money to study all individuals in a population. As a result, scientists study randomly collected subsets of individuals from a population, or random samples. Scientists study random samples to estimate properties of the population and infer conclusions about the population as a whole.

Which statement below best describes what happened to the stickleback population in the ancient lake that is now the Truckee Formation?

Initially, the stickleback population in the lake consisted mostly of fish with pelvic spines, but over 15,000 years the population changed to fish without pelvic spines.

10. The Pitx1 gene is involved in which of the following?

It is the gene identified to be involved in the formation of the pelvic spines in stickleback fish, as well as hind limb development in other vertebrates, such as the mouse.

What is one advantage of studying larger-sized samples?

Larger samples are usually more representative of the population as a whole than are smaller samples.

Why was it appropriate to use a line graph to summarize the results from this experiment?

Line graphs are used to represent continuous data.

What is the difference between marine, sea-run, and freshwater stickleback fish populations? Be specific

Marine stickleback populations live and breed strictly in the ocean. Sea-run stickleback fish are anadromous, meaning that they are born in fresh water, spend most of their lives in the ocean, and return to fresh water to breed. Freshwater stickleback fish live entirely in fresh water. (Students may also mention that marine and sea-run stickleback have heavy armor, whereas freshwater stickleback populations have reduced armor.)

Which of the following is a definition of the process of natural selection?

More advantageous traits in a particular environment allow individuals with those traits to have more offspring; as a result their heritable traits become more common in succeeding generations of the population.

How do spines protect ocean stickleback fish?

Most stickleback fish living in the ocean sport heavy armor in the form of bony plates and spines projecting from the back and pelvis. When threatened, a stickleback can simultaneously flare out its pair of pelvic spines and three dorsal spines, making it difficult for predators to swallow them.

Give an example of sampling bias. Answers will vary.

One example of sampling bias would be if researchers collected fish only in shallow water because it was easier to place traps there; as a result, they might miss individuals living in deeper water, which could have different traits because, for example, deep-water predators are different from those in shallow water. Another example would be that by using traps to catch fish, researchers might catch only the most daring and inquisitive individuals in a population; this might result in a sampling bias if researchers are studying traits associated with behavior.

What happened to these fish as they adapted to living exclusively in fresh water?

Over many generations, populations of fish changed in many different ways, including in their skeletons.

Which lake is located between Bear Paw Lake and Frog Lake just to the north?

Prator Lake

12. Which of the following is a reason for collecting specimens by random sampling when conducting the type of research in this virtual lab?

Random sampling increases the probability that the specimens selected for the research study are representative of the population as a whole.

Why is the threespine stickleback a model organism for studies in evolution? (List at least two reasons.)

Stickleback fish are small and have short generation times. These two characteristics make them easy to keep in a lab and useful for conducting genetic studies, since researchers can follow several generations of fish in a relatively short time. Also, stickleback fish populations occur throughout the Northern Hemisphere in a wide range of environments, so researchers can compare different populations and study how they have changed over time in response to different environmental pressures. Another reason that stickleback are useful model organisms for studies in evolution is that there are many stickleback fossils, so researchers can compare living populations to their ancestors.

Which of the following is an explanation for why the stickleback is a model organism for studying evolution?

Stickleback populations have evolved recently and repeatedly in postglacial lakes. Researchers have identified some of the genetic mechanisms involved in the evolution of stickleback populations. The evolution of stickleback populations can be detected in the fossil record.

Explain how the stickleback armor protects the fish from some predators.

The back spines stick up and the pelvic spine out at almost a perpendicular angle, making it hard for a predatory fish that catches a stickleback to swallow it. The plates also make stickleback fish difficult to eat. (Students may also mention that in lakes with different types of predators, such as dragonfly larvae, the pelvic spines actually make stickleback easier to catch.)

Why do you think it is important to empty the used stain, destaining solution, and water under a fume hood?

The hood has a built-in vacuum, which helps limit a scientist's exposure to potentially toxic or hazardous fumes from the different chemicals used in the staining procedure.

Why is it important that the labels included in specimen jars be made of special paper that does not disintegrate in alcohol over time?

The labels record critical information about the specimens, such as the location (e.g., Bear Paw Lake) and date of collection. The labels are placed in the jars along with the specimens to make sure that they are not lost. Because fish are stored in 50% isopropyl alcohol, which disintegrates most types of paper, the labels have to be made of a special paper that does not disintegrate. If the label is lost or destroyed, scientists would not know what population they were studying, and the data they collected on this sample would be useless.

Studies of living stickleback populations indicate that pelvic spines are an effective defense against large predatory fish such as trout. What can we infer based on the data from Experiment 2?

The lake in Nevada was initially populated by stickleback that had complete pelvises, and because the lake did not contain large predatory fish, the frequency of stickleback with pelvic spines decreased over time.

According to Dr. Bell, what is an important difference between Bear Paw Lake and Frog Lake?

The main difference is that Bear Paw Lake is completely enclosed and does not drain at the surface. This means that predatory fish cannot readily enter the lake. On the other hand, Frog Lake is connected to the water system by a small stream, which allows predatory fish to gain access to the lake.

For this analysis, what is your null hypothesis?

The null hypothesis is that a random sample of stickleback from a population would contain roughly the same number of individuals with and without pelvic spines.

Explain in your own words the overall objective of Experiment 1

The objective of Experiment 1 is to analyze the pelvic structures of stickleback fish collected from two lakes around Cook Inlet, Alaska. The data collected will be used along with information about the lakes to suggest possible environmental factors that influence pelvic morphology.

In addition to the spines, what is another component of the "armor" of a stickleback fish?

The row of plates on each side of the body.

Describe where its spines are located

The threespine stickleback has three spines on its dorsal side and two spines projecting from the pelvis on its ventral side. The spines are modified fins.

In the study described in the previous question, Dr. Bell examined more than 100 fish per lake population. When you compare the results you obtained for Bear Paw Lake and Coyote Lake to the results Dr. Bell obtained in the same two lakes, you may find your numbers to be slightly different. What might be a reasonable explanation for the difference?

There might be a difference in results due to a sampling error. In this experiment, you only examined 20 fish, and they might not have been entirely representative of the population.

The study of fossil stickleback offers the advantage of studying evolutionary rates across geologic time scales, but only living stickleback lend themselves to DNA analysis. Thus, one cannot study the DNA of fossil stickleback to determine the gene or genes associated with a particular phenotype. Dr. Bell has looked for left and right asymmetry in fossil stickleback with reduced pelvises (see one example below). He found that in more than 75% of cases, the pelvic vestiges of these fish are bigger on the left. Which of the following statements might be a plausible conclusion for these findings?

These studies suggest that the Pitx1 gene was responsible for pelvic spine loss in the lake population of fossil stickleback that he studied, which existed 10 million years ago.

How did some ancestral sea-run stickleback populations come to live exclusively in fresh water?

They became trapped in lakes from the ice age

How were the fish you will be using in this virtual lab caught?

To catch the fish, scientists throw special traps in the water, which they leave overnight. The traps are tube-shaped with funnel openings at each end. As they are swimming, some fish enter the traps, and don't always manage to swim back out. Researchers simply pull the traps out of the water to collect the live fish.

In this virtual lab, why did you compare pelvic structures of stickleback populations from two different lakes?

To compare the trait in stickleback populations living in two potentially different environments.

Examine the pelvic score data you just collected. Does the pelvic phenotype differ between Bear Paw Lake and Frog Lake fish? Explain.

Yes, it appears that Bear Paw Lake contains mostly stickleback fish that lack pelvic spines, and Frog Lake contains stickleback fish that have pelvic spines.

How would you test whether the ancient Nevada lake contained predatory fish?

You could look for fossils of other fish in the rocks that contain stickleback fossils.

Complete pelvis

a pelvis with a full pelvic girdle with two pelvic spines

Reduced pelvis:

a range of structures from a simplified girdle with no pelvic spines to an oval nub for a girdle.

Important evolutionary insights can...

be gained by comparing fossils of different ages and locations; living populations under different selective pressures; and fossils to living populations.

Natural Selection can...

drive the evolution not just of simple traits like coat color or body size, but also of complex traits like the size and shape of skeletons.

Statistical analysis...

is essential for gauging confidence in conclusions drawn from population data, which can include natural variation, populations in transition, and measurement error by the researcher.

Absent pelvis:

no pelvic girdle and no pelvic spines.

Model Organism

nonhuman species that are extensively studied to understand certain biological phenomena

Careful quantitative analysis of the traits of living populations can...

tell us about mechanisms of selection, including genetic mechanisms, whereas quantitative analysis of populations of fossilized specimens can provide a record of change over time and even the pace of specific adaptive changes.

Different Environments

with different predators, food sources, or resource limitations—apply different selective pressures on the shape of animal bodies.