Natural Selection

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 helps 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 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.

• 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

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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?

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

• 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.

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The population of blue jays shown here 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.

• 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.

• 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.

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Scientists measured the spot size of all the guppies (small fish) that lived in a river 40 years ago. They measured the spot size of the guppy population again last year. When was there more variation in the spot size of guppies in the river?

• There was more variation last year.

• There was more variation 40 years ago.

• There was the same amount of variation in spot size 40 years ago and last year.

• These bar graphs do not show the amount of variation in the population.

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A population of lobsters includes ones with a bright blue shell. If this shell is not an adaptive trait, why is it still in the population?

• The blue shell allows it to blend into its environment better.

• The blue shell causes predators to be scared of it so it doesn't get eaten.

• The blue shell is neither a good or bad trait, so it doesn't affect the number of blue shells in the population.

• The blue shell is a frequent mutation and most get eaten before reproducing.

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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.

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True or false:

Variation in a population refers to the number of organisms that live to reproduce.

• True

• False

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True or false:

A distribution in a population is the number individuals that have each trait.

• True

• False

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Draw a star on the histogram with a lot of variation, then draw a triangle on the histogram with mostly yellow and a little variation, finally draw a smiley face on the histogram with only one color.

How do genes determine an individual's traits?

• Genes directly lead to traits.

• Genes are random and don't lead to traits.

• Genes give organisms the ability to change their traits

• Genes are instructions for making protein molecules and protein molecules determine traits.

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