Let's Review 1 of 2

How did some stickleback populations come to live exclusively in fresh water?

Correct! Most stickleback fish live in salt water but migrate to fresh water annually to spawn. As ice retreated at the end of the last ice age, new connections formed between the ocean and inland bodies of fresh water that were well suited to stickleback fish. As the land rose, however, the connections dried up, trapping some stickleback fish in isolated freshwater lakes. Select another correct answer. You selected all the correct answers.

Most stickleback species live in salt water, migrating to fresh water annually to spawn. Try again.

Evolution cannot predict the future. Populations can only adapt to changes in the environment after the environmental changes occur. Certain adaptations became common in freshwater populations of stickleback fish only after they moved into the freshwater environment. Try again.

Predators, such as other fish and large aquatic insects, were (and are still) common in the freshwater lakes inhabited by the stickleback fish. Try again.

Let's Review 2 of 2

Which of the following helps to explain why some freshwater stickleback populations lack pelvic spines?

Though pelvic spines are homologous to legs in four-legged animals, their primary function in stickleback fish has to do with protection from predation, not movement. Try again.

Though stickleback fish do have other forms of armor that protect them from some predators, this does not explain why some freshwater stickleback populations have lost their pelvic spines. Try again.

An individual stickleback cannot spontaneously lose pelvic spines; the fish's genes determine whether it will or will not develop pelvic spines. Evolutionary change occurs in populations over generations, not in an individual over one lifetime. Try again.

Correct! Different environments can provide different selective pressures. For threespine stickleback fish, pelvic spines provide a selective advantage in environments with large predatory fish (making the stickleback fish hard to swallow), but are a liability in environments with dragonfly larvae (making the stickleback fish easier to catch). Select another correct answer. You selected all the correct answers.

Let's Review 2 of 2

What did researchers discover about the genetic mutation causing the loss of pelvic spines?

When researchers compared the sequences of the Pitx1 coding region in stickleback fish with and without pelvic spines, they surprisingly did not find any differences. Try again.

A stickleback without pelvic spines still makes a functional PITX1 protein that is expressed in other regions of the body during development. Try again.

Correct! Many mutations of evolutionary importance are found in regulatory regions. The gene remains intact, but the location or timing of its expression changes, conveying a new phenotype without losing existing capabilities. Reduced pelvic spines have evolved many times in many different stickleback populations, but the mutation is always in the same regulatory region. Select another correct answer. You selected all the correct answers.

Different mutations causing a loss of pelvic spines have increased in frequency in stickleback populations around the world due to natural selection, but this suggests that spinelessness has evolved independently many times, not just once. Try again.

Let's Review 1 of 2

A line graph showing relative frequency for stickleback fish with a full pelvis, represented by a red line, and for stickleback with a reduced pelvis, represented with a blue line, relative to the passage of time. Relative frequency is measured on the y-axis and ranges from zero to one and elapsed time is measured in years on the x-axis ranging from zero to 20,000 years in intervals of 5,000 years. There are points along the x axis marked with an arrow and letters A through D. The letter A is at 0 years, the letter B at 10,000 years, the letter C at 12,000 years, and the letter D at 20,000 years. The red line representing the full pelvis stickleback starts and hovers near zero frequency until 10,000 years when the line quickly rises to a relative frequency of one. The blue line representing the reduced pelvis stickleback starts and hovers near a relative frequency of one until 10,000 years when the line quickly drops to a relative frequency of zero. The blue and red lines intersect at 10,000 years at point B. The blue line plateaus near zero frequency from 10,000 years until approximately 12,000 years at point C and then steeply increases to near 0.75 relative frequency at approximately 15,000 years and then more gradually increases to a relative frequency of one at 20,000 years at point D. The red line plateaus near a relative frequency of one from 10,000 until approximately 12,000 years at point C and then steeply decreases to near 0.25 relative frequency at approximately 15,000 years and then more gradually decreases to near 0 relative frequency at 20,000 years at point D.

Bell and collaborators painstakingly documented the prevalence of stickleback fish with full and reduced pelvises over a 20,000-year fossil record. What can be concluded from their data, shown in the graph?

Initially, fish with reduced pelvises (blue line) were far more common in the lake population at time A. Try again.

Correct! At time C, no fossils of stickleback fish with a reduced pelvis (blue line) were found, so the steady increase in the prevalence of a reduced pelvis after time C must have resulted from a new mutation spreading in the population. Select another correct answer. You selected all the correct answers.

Having a reduced pelvis (blue line) must have been advantageous for this ancient stickleback population, given the steady shift toward reduced pelvises between time C and time D. Try again.

Millions of years later, the pattern of change in Alaska is very similar to the changes documented in this data. It is even happening on the same timescale (about 10,000 years), perhaps due to similar genetic changes. Try again.

Let's Review 2 of 2

Multiple lines of evidence (from the field, the fossil record, and molecular genetics) help to illustrate stickleback evolution. Which of the following statements about the data are true? Select all that apply.

Evolution can happen relatively quickly. In freshwater North American stickleback populations, the loss or reduction of pelvic spines has evolved within the past 10,000 years. Moreover, genetic data from living organisms can be used to estimate when evolutionary changes occurred—even if they were millions of years ago. Try again.

Correct! Mutations identified in freshwater stickleback populations around the world are all in the same region of DNA that controls pelvic expression of the Pitx1 gene. Select another correct answer. You selected all the correct answers.

Correct! The 10-million-year-old fossil record of stickleback evolution analyzed by Bell and his colleagues illustrates a very similar pattern of evolution to what is observed in present-day freshwater stickleback fish. Evolution can repeat itself when selection pressures are similar. Select another correct answer. You selected all the correct answers.

The fossil record is indeed incomplete, but it is the only physical evidence we have of Earth’s history and there are many examples of extraordinarily good preservation that provide detailed documentation of evolution. Try again.