Natural Selection

Natural Selection

Middle School

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Learning Objectives

• Explain how natural selection causes certain traits to become more common in a population.

• Use proportions to explain how genetic variation helps a species survive.

• Use graphs to show how natural selection changes the distribution of traits over time.

• Explain how natural selection leads to adaptation in response to environmental changes over time.

Key Vocabulary

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Natural Selection

The process where organisms with favorable traits are more likely to survive and successfully reproduce.

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Fitness

An organism's ability to survive in its environment and pass its genes on to its offspring.

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Genetic Variation

The differences in genetic material and traits that exist among individuals within the same population.

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Adaptation

The process through which a species gradually becomes better suited to survive in its environment.

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Trait Distribution

The pattern showing how frequently different traits or characteristics appear within a specific population.

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Speciation

The evolutionary process by which populations evolve to become distinct, new biological species over time.

Key Vocabulary

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Allopatric Speciation

This is the formation of a new species when a population is divided by a geographic barrier.

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Sympatric Speciation

This is the evolution of a new species from a single ancestor while living in the same habitat.

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Population

A population is all the members of a single species that live together in a specific area.

What is Natural Selection?

• Natural selection begins with pre-existing genetic variation in a population.

• Advantageous traits increase an organism's probability of surviving and reproducing.

• Fitness is the ability to survive and pass on genes to the next generation.

Multiple Choice

What is the definition of 'fitness' in the context of natural selection?

• The physical strength of an individual organism.

• The number of different traits in a population.

• The ability to survive and pass on genes to the next generation.

• The way an organism changes its environment to suit its needs.

Multiple Choice

According to the principles of natural selection, what is the primary role of an advantageous trait?

• They guarantee that the organism will live a very long life.

• They decrease the genetic variation within the population.

• They increase its probability of surviving and reproducing.

• They are developed by an organism based on its needs.

Multiple Choice

If a sudden environmental change occurred, what would most likely happen to a population that lacks pre-existing genetic variation?

• The population would be less likely to survive because no individuals would have traits suited for the new conditions.

• The organisms would quickly develop new advantageous traits to survive.

• The population would remain stable because all individuals are the same.

• The organisms would reproduce more rapidly to increase their numbers.

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How Trait Distribution Changes

• Natural selection changes the distribution of traits in a population over many generations.

• ​Alleles for advantageous traits become more common; others become less common.

• On dark rocks, darker mice are more likely to survive and reproduce.

• This entire process is called adaptation, a key way species change.

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Multiple Choice

What is the main result of natural selection over many generations?

• The distribution of traits in a population changes.

• Individual animals change their traits to fit the environment.

• All alleles in a population become equally common.

• The environment changes to better suit the population.

Multiple Choice

Why does the allele for a specific trait become more common within a population over time?

• The trait is advantageous, helping individuals survive and reproduce.

• The allele for the trait is stronger than other alleles.

• The trait is less common and therefore more unique.

• The allele was recently introduced to the population.

Multiple Choice

A population of mice with both light and dark fur lives on a landscape of dark volcanic rocks. What would be the most likely long-term outcome if a volcano erupted and covered the dark rocks with light-colored ash?

• Alleles for lighter fur would become more common in the population.

• The mice would all learn to hide from predators more effectively.

• Alleles for darker fur would continue to be more common.

• The population of mice would not change at all.

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Using Math to Track Traits

• We use graphs and charts to track how traits change over generations.

• A bar graph can show changes in a moth population's colors.

• In a dark forest, the number of dark-colored moths would likely increase.

• Proportions help explain why some traits become more common than others.

Multiple Choice

What is the primary purpose of using graphs and charts when studying traits in a population?

• To track how those traits change over time.

• To determine the original color of an animal.

• To prove that all traits are equally common.

• To create a list of every animal in a habitat.

Multiple Choice

What is the relationship between tracking traits over time and the theory of natural selection?

• Tracking traits provides mathematical evidence to support the theory.

• Tracking traits disproves the theory of natural selection.

• The theory of natural selection is a type of graph.

• Natural selection only applies to traits that cannot be tracked.

Multiple Choice

A graph shows that the proportion of dark-colored moths in a forest has increased from 20% to 80% over 50 years. What is the best explanation for this change?

• The dark-colored moths had a trait that gave them a survival advantage.

• The light-colored moths decided to move to a new forest.

• The total number of moths in the forest decreased by half.

• Graphs are unable to accurately show changes in animal color.

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Speciation: The Rise of New Species

Allopatric Speciation

• A physical barrier, like a river, divides a single population into two isolated groups.

• The separated groups evolve independently, accumulating different genetic traits over very long periods of time.

• Eventually, the groups become so different that they can no longer interbreed, forming new species.

Sympatric Speciation

• New species form from a population living in the same geographic area without physical separation.

• Reproductive barriers, such as a new courtship song, emerge and divide the population into groups.

• These groups stop interbreeding with each other, leading to the evolution of distinct new species.

Multiple Choice

Which process describes the formation of a new species due to a physical barrier dividing a population?

• Allopatric speciation

• Sympatric speciation

• Genetic evolution

• Reproductive barriers

Multiple Choice

What is the key difference between allopatric and sympatric speciation?

• The time it takes for new species to form

• Whether or not a physical barrier is present

• The type of animals involved in the process

• Whether the new species can interbreed

Multiple Choice

A population of insects lives on the same plant. Some insects begin to prefer eating flowers, while others prefer eating leaves. Over time, the two groups stop interbreeding. Which is the best explanation for what happened?

• Allopatric speciation, because the two groups evolved independently

• Sympatric speciation, because a reproductive barrier emerged in the same area

• Allopatric speciation, because a new species was formed

• Sympatric speciation, because the birds developed different genetic traits

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Common Misconceptions About Evolution

Misconception	Correction
Individuals can evolve during their lifetime.	Evolution occurs in populations over many generations, not in one individual.
Natural selection creates 'perfect' organisms.	It favors traits that are advantageous in the current environment.
Natural selection has a guaranteed outcome.	It is a probabilistic process and does not guarantee a certain outcome.

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Summary

• Natural selection acts on genetic variation, making advantageous traits more common over time.

• We can use mathematical models to track probable changes in trait frequencies.

• Speciation is the process where new species arise from an ancestral species.

• Evolution occurs in populations, not individuals, and does not aim for perfection.

Poll

On a scale of 1-4, how confident are you about explaining how natural selection leads to changes in populations over time?

• 1

• 2

• 3

• 4