Natural Selection Critical Juncture

Scientists measured the spot size of guppies (small fish) in two different populations. Which population has more guppies with large spots?

• They both have the same number of guppies with large spots.

• The population in Brazil has more guppies with large spots.

• The population in Venezuela has more guppies with large spots.

• These bar graphs do not show which population has more guppies with large spots.

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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 above show three possible hummingbird populations.

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

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

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

• Populations 1 and 2 will survive.

• 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?

• 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.

• 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.

• 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.

• 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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Scientists measured the height of the giraffes in two different populations. Which population has the largest number of tall giraffes?

• The population in Niger has the largest number of tall giraffes.

• The population in Somalia has the largest number of tall giraffes.

• Both populations have the same number of tall giraffes.

• These bar graphs do not show which population has more tall giraffes

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 diagrams above show three possible seal populations.

If their environment changes to have warm water, which of the following seal 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 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 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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Scientists measured the spine size of a population of cactuses found in an area in 1970. They measured the spine size of the cactus population again in 2015. Which of the statements below best describes the difference in the cactuses at the two time points?

• The cactuses in 1970 had smaller spines than the cactuses in 2015.

• The cactuses in 1970 had larger spines than the cactuses in 2015.

• There were more cactuses in 1970 than there were in 2015.

• There was more variation in spine size in 1970 than there was in 2015

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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 above 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 baby blue jay could have been born with genes for a mutant thin-beak trait and lived for a little while, but it would have died before it had any offspring.

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

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