Natural and Artificial Selection
Why do almost all the corn plants in an Iowa field have big, full ears, while wild grasses have tiny seeds? And why do some germs stop dying when a doctor prescribes the same medicine again and again? Both questions have the same kind of answer: some individuals in a population do better than others, and their traits become more common over time. By the end of this page you will be able to explain both.
Variation and inherited traits
Look at a bag of jelly beans or a crowd at a basketball game. No two individuals are exactly alike. This difference among members of the same species is called variation. Beetles can be green or brown. Finches can have longer or shorter beaks. Dogs can be calm or jumpy.
Some of this variation is heritable: it is passed from parents to offspring through genes. A heritable feature of an organism is called a trait. A trait that helps an organism survive and reproduce in its environment is an adaptation.
Not every difference is inherited. A tennis player with big arm muscles got them from practice. Her children will not be born with big arm muscles. Only inherited traits can be selected.
Define natural selection*. Then name the term for a feature of an organism that is passed from parents to offspring and can help it survive.
Define natural selection. Then name the term for a feature of an organism that is passed from parents to offspring and can help it survive.
Natural selection
Now put variation in an environment where resources are limited. Predators hunt, food runs out, and winters are cold. Individuals with helpful traits are more likely to survive and reproduce, so they pass those traits to more offspring.
Natural selection follows four steps:
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Variation: individuals in a population differ.
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Inheritance: some of the differences are heritable.
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Survival and reproduction: some individuals survive longer and have more offspring because of their traits.
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Change over generations: helpful traits become more common.
Natural selection does not mean individuals "try" to change. A single beetle cannot turn brown because it needs to. The population changes because of who survives and reproduces. Change takes many generations.
Here is a quick way to compare averages. Suppose a drought leaves only large, hard seeds for finches. Birds with deeper beaks crack them and survive. If the average beak depth rises from 8.5 mm to 9.6 mm, the change is $9.6 - 8.5 = 1.1\ \text{mm}$.
A population of beetles living on a Texas hillside has green and brown individuals. Birds can see green beetles more easily against the brown soil. Describe what will…
A population of beetles living on a Texas hillside has green and brown individuals. Birds can see green beetles more easily against the brown soil. Describe what will most likely happen to the proportion of green and brown beetles over many generations, and explain your reasoning using the terms variation, inheritance, and survival.
A population of finches on an island has beaks of different lengths. After a drought, only large, hard seeds remain. Before the drought the average beak depth was 8.5…
A population of finches on an island has beaks of different lengths. After a drought, only large, hard seeds remain. Before the drought the average beak depth was 8.5 mm. Two generations later it was 9.6 mm. (a) Calculate the change in average beak depth. (b) Explain why the average changed.
Artificial selection
People have changed living things for thousands of years without any genetic lab. In artificial selection (also called selective breeding), humans choose which individuals breed because they have traits people want.
Examples from the US: Iowa farmers who save seeds from the biggest corn ears, dairy cattle that produce more milk, and many dog breeds. Broccoli, cabbage, kale, and cauliflower all came from one wild mustard plant. People kept choosing different parts: leaves gave kale, buds gave Brussels sprouts, and flower clusters gave broccoli.
| Natural selection | Artificial selection | |
|---|---|---|
| Who selects? | The environment (predators, climate, food) | Humans |
| What gets selected? | Traits that help survival and reproduction | Traits people want |
| Goal | None; it just happens | A planned result |
| Benefits the organism? | Usually yes | Not always (some breeds have health problems) |
Farmers in Iowa breed only the corn plants that produce the largest ears and plant their seeds the next year. Is this natural selection or artificial selection? Explain…
Farmers in Iowa breed only the corn plants that produce the largest ears and plant their seeds the next year. Is this natural selection or artificial selection? Explain who or what is doing the selecting.
Name two differences between natural selection and artificial selection. Write your answer in two complete sentences.
Name two differences between natural selection and artificial selection. Write your answer in two complete sentences.
Aaliyah wants to breed dogs that are calm and good at learning commands. Describe the steps she could follow over several generations to change the traits of her dogs…
Aaliyah wants to breed dogs that are calm and good at learning commands. Describe the steps she could follow over several generations to change the traits of her dogs through artificial selection. Then explain why this process would be slower or faster than a dog trait changing in the wild.
Why variation matters
Selection needs variation. If every individual is the same, nothing can be favored and the population cannot change when conditions change.
A farm field of plants that are all genetically alike is risky. If one disease or pest can harm one plant, it can harm them all, because none has a different trait that resists it. A wild population with little variation faces the same problem: one new disease could wipe it out. A varied population is more likely to contain a few survivors.
Bacteria in a hospital are exposed to an antibiotic. Most die, but a few bacteria that have a trait for resistance survive and reproduce. After several rounds of…
Bacteria in a hospital are exposed to an antibiotic. Most die, but a few bacteria that have a trait for resistance survive and reproduce. After several rounds of treatment, nearly all bacteria in the population are resistant. Write a claim, evidence, and reasoning (CER) response explaining how this is an example of natural selection. Then explain why the antibiotic did not "cause" the resistance trait to appear.
Compare the wild mustard plant with the crops broccoli, cabbage, and kale, which were all developed from it. (a) Identify which type of selection produced these crops…
Compare the wild mustard plant with the crops broccoli, cabbage, and kale, which were all developed from it. (a) Identify which type of selection produced these crops and explain the process. (b) Describe one risk of having a farm field where all the plants are genetically very similar, using the idea of variation in a population. (c) Explain how a similar lack of variation could be a problem for a wild population facing a new disease.
Back to the start: corn ears are big because farmers kept choosing them (artificial selection). Germs resist medicine because the resistant ones survive and reproduce while others die (natural selection).
Summary
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Variation exists in every population, and some traits are heritable.
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An adaptation is a heritable trait that helps an organism survive and reproduce.
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Natural selection: the environment favors some traits, so those individuals leave more offspring and the traits become more common over generations.
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Artificial selection: humans choose which organisms breed, as with corn, dogs, and broccoli.
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Populations change, not individuals, and only inherited traits count.
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Variation helps a population survive new challenges such as disease.