There is a kind of snake that can be born with red, yellow, or green skin. Eagles hunt these snakes, but snakes that are the same color as their environment are harder for the eagles to see and catch. A population of snakes once lived in an environment where there was sand and dirt, but no grass. The image above shows what the population looked like then. The environment changed so that green grasses and plants now grow there. The diagrams below show two predictions for what the population could look like after many generations. Which prediction best shows what the population could look like after many generations? What caused it to change?

• Prediction 1 is best. Two yellow or red snakes had a baby with a mutation in its genes for the green-skin trait. Because green snakes are more likely to survive, that baby survived long enough to pass on its mutation, so the green trait became more common over generations.

• Prediction 1 is best. Green snakes are more likely to survive, so yellow and red snakes began to have offspring with mutations in their genes for the green-skin trait so that their offspring would have a better chance of surviving.

• Prediction 2 is best. A snake could have been born with a mutation in its genes for the green-skin trait and lived for a little while. Because green snakes are more likely to die, it would have been more likely to die before it had offspring, so the final population will only have yellow and red snakes.

• Prediction 2 is best. A snake could have been born with a mutation in its genes for the green-skin trait, but having a mutation in its genes would have caused it to die when it was born, so the final population will only have yellow and red snakes.

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Guppies are small fish that live in South American rivers. They can have different-sized spots on their bodies. The river bottoms are covered in rocks. Guppies with spots that are the same size as the rocks on the bottom are harder for bigger fish to see and catch. The diagram below shows a population of guppies that live in a river. At time 1, the population had the same number of guppies with small and large spots. At time 2, after many generations, there were many more guppies with small spots and fewer guppies with large spots in the population. How did the environment change between time 1 and time 2? How did the population change?

• You cannot tell how the environment changed. With each generation, more guppies passed on the gene for small spots to their offspring.

• The rocks became smaller. With each generation, more guppies with small spots survived long enough to pass on the gene for small spots to their offspring.

• The rocks became smaller. Guppies with small spots are more likely to survive, so the guppies with large spots changed to have small spots.

• The rocks became smaller. Guppies with small spots are more likely to survive, so both kinds of guppies passed on the gene for small spots to their offspring.

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Northern foxes can have different fur colors. The foxes hunt squirrels for food. Foxes that have fur the same color as their environment are better at getting closer to squirrels and catching them. A population of foxes once lived in an environment where there was not very much snow. The image above shows what the population looked like then. The environment changed so it is now very snowy. The diagrams below show two predictions for what the population could be like after many generations. Which prediction best shows what the population could look like after many generations? What caused it to change?

• Prediction 1 is best. Two brown or gray foxes had a baby with a mutation in its genes for the white-fur trait. Because white foxes are more likely to survive, that baby survived long enough to pass on its mutation, so the white-fur trait became more common over generations.

• Prediction 1 is best. White foxes are more likely to survive, so brown and gray foxes began to have offspring with mutations in their genes for the white-fur trait so that their offspring would have a better chance of surviving.

• Prediction 2 is best. A fox could have been born with a mutation in its genes for the white-fur trait and lived for a little while. Because white foxes are more likely to die, it would have been more likely to die before it had offspring, so the final population will only have brown and gray foxes.

• Prediction 2 is best. A fox could have been born with a mutation in its genes for the white-fur trait, but having a mutation in its genes would have caused it to die when it was born, so the final population will only have brown and gray foxes.

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Brown anoles are lizards that live on islands in the Bahamas. They can have legs of different lengths. Lizards with shorter legs are better at climbing trees than lizards with longer legs. This makes lizards with shorter legs better at escaping from the floods that sometimes cover the islands. The diagrams below show three possible lizard populations. If their environment changes so that there are many floods each year, which of the following lizard populations will most likely survive?

• All the populations will survive because the lizards will change the length of their legs if they need to.

• Populations 1 and 2 will most likely survive.

• Populations 2 and 3 will most likely survive.

• Only Population 2 will survive because it is the only population with variation.

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Tree squirrels can have different fur colors. Squirrels live in forests where foxes also live. Foxes hunt squirrels for food. Squirrels that have fur that is the same color as the trees are harder for the foxes to see and catch. The diagram below shows a population of squirrels that live in a forest. At time 1, the population had the same number of black and gray squirrels. At time 2, after many generations, there were many more black squirrels and fewer gray squirrels in the population. How did the environment change between time 1 and time 2? How did the population change?

• You cannot tell how the environment changed. With each generation, more squirrels passed on the gene for black fur to their offspring.

• There were more black trees. Squirrels with black fur are more likely to survive, so the squirrels with gray fur changed to have black fur.

• There were more black trees. Squirrels with black fur are more likely to survive, so both types of squirrels passed on the gene for black fur to their offspring.

• There were more black trees. With each generation, more squirrels with black fur survived long enough to pass on the gene for black fur to their offspring.

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Blue jays are birds that live in the forest. They can have thick beaks, medium beaks, or thin beaks. Blue jays use their beaks to get to the seeds they eat. Blue jays with thinner beaks can easily reach and eat the seeds inside pinecones. Blue jays with thicker beaks can easily open and eat seeds with hard shells. The population of blue jays shown above lives in an environment that has always had seeds with hard shells. Could there ever have been blue jays with thin beaks in this population?

• No blue jays could have been born with a thin-beak trait because none of the adult blue jays had that trait to pass down.

• No blue jays could have been born with a thin-beak trait in the past, but some with that trait could be born in the future if the environment changes to have pine cones with seeds.

• A blue jay could have been born with a mutation in its genes for the thin-beak trait and lived for a little while, but it would have been more likely to die before it had offspring.

• A blue jay could have been born with a mutation in its genes for the thin-beak trait, but having a mutation in its genes would have caused it to die when it was born.

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Northern foxes can have different fur colors. The foxes hunt squirrels for food. Foxes that have fur the same color as their environment are better at getting close to squirrels and catching them. The diagrams below show three possible fox populations. If their environment changes to have only brown dirt, which of the following fox populations will most likely survive?

• Populations 1 and 2 will most likely survive.

• Populations 2 and 3 will most likely survive.

• Only Population 2 will survive because it is the only population with variation.

• All the populations will survive because the foxes will change the color of their fur if they need to.

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Harbor seals live in northern oceans and have a layer of blubber (fat) that keeps them warm. Seals can have blubber of different thicknesses. Seals with thicker blubber are more likely to stay warm and survive in cold ocean waters. But, in warm ocean waters, thicker blubber can make the seals overheat and die. The diagram below shows a population of seals that live in an area of ocean. At time 1, the population had the same number of seals with thin and thick blubber. At time 2, after many generations, there were many more seals with thick blubber and fewer seals with thin blubber in the population. How did the environment change between time 1 and time 2? How did the population change?

• You cannot tell how the environment changed. With each generation, more seals passed on the gene for thick blubber to their offspring.

• The water became colder. Seals with thick blubber are more likely to survive, so the seals with thin blubber changed to have thick blubber.

• The water became colder. Seals with thick blubber are more likely to survive, so both kinds of seals passed on the gene for thick blubber to their offspring.

• The water became colder. With each generation, more seals with thick blubber survived long enough to pass on the gene for thick blubber to their offspring.

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Guppies are small fish that live in South American rivers. They can have different-sized spots on their bodies. The river bottoms are covered in rocks. Guppies with spots that are the same size as the rocks on the bottom are harder for bigger fish to see and catch. The diagram above shows a population of guppies that lived in a river. Scientists had previously only seen guppies with medium or large spots in this river; they had never seen any guppies with any other spot sizes. The diagram below shows how the population changed after many generations. Karina says the change in the guppy population was caused by a mutation. Miles says the change was caused by a change in the environment. Zora thinks both Karina and Miles are correct. Who do you think is right? Explain what happened to the guppies over many generations.

• Miles is correct: Karina's is not accurate which makes Zora's answers inaccurate also. Guppies with small spots become less common because they become less common because they couldn't survive very long and don't have a lot of offspring.

• Zora is correct: Both Karina's and Miles’s answers are accurate. Miles is correct because the environment must have changed to have small rocks. Guppies with medium or large spots became less common because they could not survive very long and could not have a lot of offspring. Karina is also correct because, at some point, a guppy must have been born with a mutation for the small-spot trait since there were no guppies with small spots before. The guppy with small spots survived in the environment with small rocks long enough to reproduce. Some of its offspring were also born with small spots. Over many generations, a greater number of guppies with small spots had offspring, so that trait became most common.

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Tree squirrels can have different fur colors. Squirrels live in forests where foxes also live. Foxes hunt squirrels for food. Squirrels that have fur the same color as the trees are harder for the foxes to see and catch. The diagram above shows a population of squirrels that lived in a forest. Scientists had previously only seen brown and gray squirrels in this forest; they had never seen squirrels of any other color. The diagram below shows how the population changed after many generations. How did the color of the trees in the squirrels’ environment change? Explain what happened to the squirrels over many generations.

• The squirrel's environment will did not change, the trees stayed the same color. New Squirrels were born, with the black fur on squirrels reproducing's more rapidly.

• The squirrels’ environment changed to have lots of black trees. At some point, a brown or a gray squirrel had a baby with a mutation for the black-fur trait. It was easier for that baby to survive, so it lived a long time and reproduced. It passed down its mutant black-fur trait to some of its offspring. Over many generations, these black squirrels had more offspring with black fur, so black fur became the most common trait. Most brown and gray squirrels died before they could have offspring, so, over time, fewer brown and gray squirrels were born, and those traits became less common.

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