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A. Some stickleback populations that swam to freshwater lakes to spawn (breed) got trapped at the end of the last ice age.
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.
B. Some stickleback populations that have always lived exclusively in fresh water migrated into lakes that formed at the end of the last ice age.
Most stickleback species live in salt water, migrating to fresh water annually to spawn. Try again.
C. Some stickleback populations evolved the traits necessary to live in freshwater environments. Afterward, they moved into lakes that formed after the last ice age.
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.
D. Some stickleback populations that swam to freshwater lakes to spawn stayed because there were no predators at the end of the last ice age.
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.
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A. Pelvic spines are homologous to legs in four-legged animals, and freshwater stickleback fish don't need hind limbs to move.
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.
B. Freshwater stickleback fish have evolved different types of protective armor to keep them safe from predators.
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.
C. A marine stickleback will shed its spines when trapped in a freshwater lake.
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.
D. Although no large predatory fish live in the lakes, there are freshwater predators that grab stickleback fish by their pelvic spines.
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.
A. To find the location of the gene(s) causing the differences among stickleback populations with and without spines.
Correct! Geneticists use genetic crosses to map the location of genes. Select another correct answer. You selected all the correct answers.
B. To test whether Pitx1 is involved in the formation of pelvic spines.
Their crosses did suggest that Pitx1 is involved in the formation of spines, but Kingsley and his team didn't know that when they started. Try again.
C. To compare the Pitx1 protein-coding sequence from fish with and without pelvic spines.
Their crosses did suggest that Pitx1 is involved in the formation of spines, but Kingsley and his team didn’t know that when they started. Try again.
D. To understand the timing of the expression of gene(s) for making pelvic spines in embryonic development.
Genetic crosses do not yield information about the timing of development, though changes in form do arise from changes in development. Try again.
A. It is found in the coding region of the Pitx1 gene.
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.
B. It results in a protein that is no longer functional.
A stickleback without pelvic spines still makes a functional PITX1 protein that is expressed in other regions of the body during development. Try again.
C. It is always found in one of the regulatory "switches" near the coding region of the Pitx1 gene.
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.
D. It occurred once and has spread to many different populations around the world by natural selection.
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.
A. Initially (time A), fish with full pelvises (red line) were most common in the lake population.
Initially, fish with reduced pelvises (blue line) were far more common in the lake population at time A. Try again.
B. At time B, the frequency of fish with full pelvises was very high. At time C, this population started to evolve reduced pelvises.
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.
C. In this population, having pelvic spines seems to have provided a selective advantage to stickleback fish beginning at time C.
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.
D. The evolution of pelvic reduction seen in this fossil record is different from the phenomenon occurring in the lakes of Alaska today.
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.
A. Molecular data from present-day stickleback populations are not as helpful as fossil data because evolution takes millions of years.
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.
B. Genetic evidence reveals the precise molecular mechanism responsible for the change in pelvic structures in stickleback populations.
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.
C. The fossil data show an ancient pattern of evolution that is repeating in the present day.
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.
D. Fossil data are not as helpful as molecular data from present-day stickleback populations because the fossil record is incomplete and unreliable.
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.