Natural Selection Mid Unit Review

Hummingbirds can have different beak lengths. They use their beaks to reach to the nectar (their food) at the bottom of flowers. Hummingbirds with longer beaks can get food from long flowers. Hummingbirds with shorter beaks cannot reach the nectar in long flowers. If a hummingbird can’t easily reach its food, it will die. The diagrams below show three possible hummingbird populations.

If their environment changes to have only long flowers, which of the following hummingbird populations will most likely survive?

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

• b. All the populations will survive because the hummingbirds will change the length of their beaks if they need to.

• c. Populations 1 and 2 will survive.

• d. Populations 2 and 3 will survive.

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Eider ducks live in the ocean and have a layer of warm feathers underneath their regular feathers. They can have feathers of different thicknesses. These feathers help the ducks stay warm in cold water. Ducks with thicker feathers are more likely to stay warm and survive in cold water. But, in warm water, thicker feathers can make the ducks overheat and die. The diagram below shows a population of ducks that live in an area of an ocean. At time 1, the population had the same number of ducks with thin and thick feather layers. At time 2, after many generations, there were many more ducks with a thin feather layer and fewer ducks in the population with a thick feather layer.

How did the environment change between time 1 and time 2? How did the population change?

• a. The water became warm. As a result, ducks with thin feathers were more likely to survive, so the ducks with thick feathers changed to have thin feathers.

• b. The water became warm. As a result, ducks with thin feathers were more likely to survive, so both kinds of ducks passed on the gene for thin feathers to their offspring.

• c. The water became warm. With each generation, more ducks with thin feathers survived long enough to pass on the gene for thin feathers to their offspring.

• d. You cannot tell how the environment changed. With each generation, more ducks passed on the gene for thin feathers to their offspring.

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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.

Which prediction best shows what the population could look like after many generations? What caused it to change?

• a. 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.

• b. 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.

• c. 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.

• d. 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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Tree squirrels can have different fur colors. They can be black, brown, or gray. 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 population of squirrels shown above lives in an environment that has always had black trees. Could there ever have been squirrels with gray fur in this population?

• a. No squirrels could have been born with the gray trait because none of the adult squirrels had that trait to pass down.

• b. No squirrels could have been born with the gray trait in the past, but some with that trait could be born in the future if the environment changes to have gray trees.

• c. A squirrel could have been born with a mutation in its genes for the gray trait and lived for a little while, but it would have been more likely to die before it had any offspring.

• d. A squirrel could have been born with a mutation in its genes for the gray trait, but having a mutation in its genes would have caused it to die when it was born.

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Eider ducks live in the ocean and have a layer of warm feathers underneath their regular feathers. They can have feathers of different thicknesses. These feathers help the ducks stay warm in cold water. Ducks with thicker feathers are more likely to stay warm and survive in cold water. But, in warm water, thicker feathers can make the ducks overheat and die. The diagrams below show three possible duck populations.

If their environment changes to have very cold water, which of the following duck populations will most likely survive?

• a. Populations 1 and 2 will most likely survive.

• b. Populations 2 and 3 will most likely survive.

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

• d. All the populations will survive because the ducks will change their feather thickness if they need to.

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Harbor seals live in northern oceans and a have 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 diagrams below show three possible seal populations.

If their environment changes to have warm water, which of the following seal populations will most likely survive?

• a. Populations 1 and 2 will most likely survive.

• b. Populations 2 and 3 will most likely survive.

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

• d. All the populations will survive because the seals will change the thickness of their blubber if they need to.

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Guppies are small fish that live in South American rivers. They can have large spots, medium spots, or small spots. 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 population of guppies shown above lives in an environment that has always had large rocks. Could there ever have been guppies with small spots in this population?

• a. No guppies could have been born with the small-spot trait because none of the adult guppies had that trait to pass down.

• b. No guppies could have been born with the small-spot trait in the past, but some with that trait could be born in the future if the environment changes to have small rocks.

• c. A baby guppy could have been born with genes for the mutant small-spot trait and lived for a little while, but it would have died before it had any offspring.

• d. A baby guppy could have been born with genes for the mutant small-spot trait, but its mutant trait would have caused it to die when it was born.

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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?

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

• b. Populations 1 and 2 will most likely survive.

• c. Populations 2 and 3 will most likely survive.

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

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Harbor seals live in northern oceans and a have 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?

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

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

• c. 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.

• d. 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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Eider ducks live in the ocean and have a layer of warm feathers underneath their regular feathers. They can have thin feathers, medium feathers, or thick feathers. These feathers help the ducks stay warm in cold water. Ducks with thicker feathers are more likely to stay warm and survive in cold water. But, in warm water, thicker feathers can make the ducks overheat and die.

The population of ducks shown above lives in an environment that has always had very cold water. Could there ever have been ducks with thin feathers in this population?

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

• b. No ducks could have been born with the thin-feather trait in the past, but some with that trait could be born in the future if the environment changes to be warmer.

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

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

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