Natural Selection

Natural Selection

Middle School

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Learning Objectives

• Describe how genetic variations help some individuals in a population survive and reproduce.

• Use data to explain how traits in a population change over many generations.

• Explain how natural selection can change how common a trait is over time.

• Explain the difference between natural selection and artificial selection.

• Identify the main sources of genetic variation in a population, including mutations.

Key Vocabulary

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Natural Selection

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

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Artificial Selection

This is the process where humans choose to breed specific organisms to get offspring with certain desired traits.

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Variation

Variation refers to the differences in heritable traits that exist among individual organisms within a population or species.

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Competition

Competition is the struggle that occurs between organisms when they both require the same limited resources for survival.

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Mutation

A mutation is a permanent alteration or change in the sequence of an organism's DNA or genetic material.

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Adaptation

An adaptation is a specific physical or behavioral trait that helps an organism to survive and reproduce successfully.

Key Vocabulary

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Distribution of Traits

This describes the frequency of how often different traits or characteristics appear within a population.

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Probability

Probability is the measure of the likelihood that a specific event or a particular outcome will occur.

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Predominance

Predominance refers to the gradual increase in the frequency of a particular trait within a population.

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Suppression

Suppression describes the steady decrease in the frequency of a certain trait found within a population.

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Darwin's Clue from Artificial Selection

• Charles Darwin proposed that evolution occurs through a natural process, or mechanism.

• He studied artificial selection, where humans breed organisms for specific desirable traits.

• For example, he bred fan-tailed pigeons from wild rock pigeons.

• This process is also used in animal husbandry to raise farm animals.

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Multiple Choice

What is artificial selection?

• The process where humans breed organisms for specific desirable traits.

• The natural process where the environment selects for advantageous traits.

• The way different species compete for the same resources.

• The random mutation of genes within a population.

Multiple Choice

What is the relationship between breeding fan-tailed pigeons and raising farm animals through animal husbandry?

• Both involve humans intentionally choosing which organisms will reproduce.

• Both are processes that happen randomly in nature.

• Both describe how wild pigeons are different from farm animals.

• Both are examples of how a single species can go extinct.

Multiple Choice

What conclusion could Darwin draw from observing artificial selection that helped him propose that evolution occurs by a natural process?

• That a similar selective process could also occur in nature.

• That all modern animals were created through artificial selection.

• That wild rock pigeons were the first animals to be domesticated.

• That natural processes could not produce changes as dramatic as artificial selection.

The Mechanism of Natural Selection

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Overproduction

• ​Species often produce a large number of offspring to ensure survival.

• ​​Many of these young will not live long enough to reproduce.

• ​This leads to a natural struggle for existence among the offspring.

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Variation

• ​Individuals within a species show a wide range of different traits.

• ​​These unique characteristics can be passed down to the next generation.

• ​Some variations may provide an advantage for survival in an environment.

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Competition

• ​Offspring must compete with each other for essential, limited resources.

• ​​Resources include things like food, water, sunlight, and living space.

• ​This competition determines which individuals are more likely to survive and reproduce.

Multiple Choice

Which statement best describes the mechanism of natural selection?

• A process where individuals change their traits to match the environment.

• A process where all offspring from a single species survive equally.

• A process where species choose to have a small number of offspring.

• A process where individuals with advantageous traits are more likely to survive, reproduce, and pass on those traits.

Multiple Choice

What is the relationship between variation and competition within a species?

• Competition causes all variations within a species to disappear over time.

• Variations can provide some individuals with an advantage in the struggle for limited resources.

• The number of offspring produced directly determines the number of variations.

• Competition for resources prevents any new variations from appearing in a species.

Multiple Choice

Imagine a population of plants that has a wide variety of traits. If the environment where these plants live becomes much drier over time, what is the most likely outcome for the population after many generations?

• All the plants will quickly learn to survive with less water.

• The population will likely have more individuals with traits suited for a drier climate.

• The plants will stop producing offspring until the climate becomes wet again.

• The amount of variation in the plant population will decrease to zero.

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Selection in Action & Environmental Change

Selection of Traits

• In a population, certain traits can help an organism survive and reproduce more effectively in its environment.

• Individuals with these advantageous traits are more likely to pass them on to the next generation.

• Over many generations, these helpful traits become much more common within the entire population.

Environmental Change

• A significant change in the environment can directly impact an organism's ability to survive.

• Traits that support survival in the new conditions become more widespread in the population over time.

• If environmental shifts are too extreme, populations may not adapt quickly enough and can face extinction.

Multiple Choice

What is the most likely long-term outcome for a trait that consistently helps an organism survive and reproduce in its environment?

• It will become more common within the population over many generations.

• It will cause the organism to move to a new environment.

• It will be acquired by all other organisms during their lifetime.

• It will disappear completely after a single generation.

Multiple Choice

What is the relationship between an environment and the traits of a population?

• It prevents any changes from occurring within a population.

• It causes individual organisms to develop new traits on demand.

• It determines which traits are advantageous and more likely to be passed on.

• It ensures that all helpful traits are immediately passed to every offspring.

Multiple Choice

A specific environment experiences a sudden and extreme shift in climate. Based on the principles of selection, what is a likely consequence for a population that is not well-suited to the new conditions?

• The population will immediately develop traits to survive the new climate.

• The population will remain stable because traits do not change.

• The population may not adapt quickly enough and could face extinction.

• All individuals in the population will migrate to a new, more suitable environment.

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Probability in Natural Selection

• Natural selection is a process based on probability, not random chance.

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

• For example, fur color matching the current environment increases survival probability.

• This improves the chances of survival but does not guarantee it.

Multiple Choice

Which statement best describes the process of natural selection?

• It is a process where some traits increase the probability of survival.

• It is a process that happens completely by random chance.

• It is a process that guarantees survival for the strongest organisms.

• It is a process unrelated to an organism's traits.

Multiple Choice

What is the cause-and-effect relationship between a trait and survival in natural selection?

• A beneficial trait increases an organism's chances of living to reproduce.

• An organism's traits are determined by its chances of survival.

• All traits in a population have the same effect on survival.

• The environment has no relationship to an organism's traits.

Multiple Choice

A population of rabbits has fur that matches its grassy field environment. If a wildfire blackens the field, which statement best predicts what will happen?

• The rabbits with fur matching the environment are more likely to survive and pass on that trait.

• The predators will learn to hunt for other food sources instead.

• The color of a rabbit's fur has no effect on its individual survival.

• All of the rabbits will eventually be caught by predators.

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Modeling Population Changes

• Scientists use mathematical tools like graphs to explain natural selection.

• These models track how populations change over many generations.

• They clearly show how the distribution of traits changes over time.

• This change is a response to pressures in the environment.

Multiple Choice

What is the main reason scientists use mathematical models when studying populations?

• To track how traits in a population change over generations.

• To count the exact number of organisms in a single habitat.

• To change the environmental pressures affecting a species.

• To observe the daily behaviors of an individual animal.

Multiple Choice

What key relationship do scientific models show about how a population changes over time?

• How individual organisms choose their mates.

• The specific number of offspring each parent produces.

• How environmental pressures cause the distribution of traits to shift.

• The rate at which new species are formed.

Multiple Choice

Imagine an environment with dark-colored trees becomes lighter due to a decrease in pollution. What would a scientific model predict about a moth population in that area over many generations?

• The population of dark-colored moths would likely increase.

• The population of light-colored moths would likely increase.

• The moth population would develop entirely new colors.

• The total size of the moth population would stay the same.

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The Genetic Basis of Variation

Genes and Inheritance

• ​Darwin knew variations were passed on, but he didn't know they were passed on through genes.

• ​​Gregor Mendel's work showed that traits are inherited through units called genes.

• ​Genetic variations are the result of different combinations of genes from each parent.

Mutations

• ​A mutation is a change in an organism's genetic material, which can create new traits.

• ​​For a mutation to be passed to the next generation, it must occur in the sex cells.

• ​A mutation in a body cell will only affect the individual and is not passed on to offspring.

Multiple Choice

According to Gregor Mendel's work, what are the fundamental units through which traits are inherited from parents to offspring?

• Genes

• Mutations

• Body cells

• Variations

Multiple Choice

What is the key difference in the outcome of a mutation that occurs in a body cell versus one that occurs in a sex cell?

• A mutation in a body cell affects only the individual, while a mutation in a sex cell can be passed to offspring.

• A mutation in a sex cell affects only the individual, while a mutation in a body cell can be passed to offspring.

• Mutations in body cells create new traits, while mutations in sex cells do not.

• Mutations in sex cells are always harmful, while mutations in body cells are always beneficial.

Multiple Choice

A rabbit is born with a new fur color that was not seen in either of its parents. For this new trait to be passed on and become a variation in the rabbit population, what must be true?

• The new trait must be the result of a mutation in a parent's sex cell.

• The new trait must have been caused by a mutation in the rabbit's own body cells after it was born.

• The new trait must be from a combination of the parents' existing genes.

• The new trait must provide a disadvantage for the rabbit's survival.

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Common Misconceptions

Misconception	Correction
Individuals can choose to adapt to their environment.	Adaptation occurs in a population over many generations, not within an individual.
Natural selection is a random process.	Genetic mutations are random, but selection for traits by the environment is not.
Adaptation happens quickly.	Adaptation is a very slow process that occurs over many generations.
All mutations are harmful.	Mutations can be harmful, neutral, or beneficial, providing a survival advantage.

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Summary

• Natural selection is the main driver of evolution, favoring traits for survival.

• It depends on overproduction, heritable variation from mutations, and competition.

• Helpful traits lead to adaptation, which scientists track using mathematical models.

• If the environment changes too fast, species may not adapt and can face extinction.

Poll

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

• 1

• 2

• 3

• 4