Artificial and Sexual Selection

Essential Question

Besides survival, what else can drive selection?

Survival is only part of the story. Traits also spread when humans choose which organisms breed, when animals choose mates, and when two species change each other. In this lesson, you’ll compare artificial selection, sexual selection, and natural selection, and see how coevolution links the evolution of different species.

Artificial Selection

• Artificial selection: humans choose which organisms breed
• Used for crops, livestock, and pets
• Works only on inherited variation
• Many forms can come from one species

In artificial selection, humans, not the environment, decide which organisms get to reproduce. Farmers and breeders choose parents with the traits they want, and the offspring inherit those traits. Kale, cabbage, Brussels sprouts, broccoli, and kohlrabi are all the same species, Brassica oleracea, bred from wild mustard. Each crop is the result of selecting for a different part of the plant. Dog breeds, dairy cows, and fruit are other results of artificial selection.

Artificial Selection Over Generations

• Breeders choose the highest-oil plants each generation
• Oil content rose from 4.5% to 21%
• Selection works on existing variation
• The rate of change can slow down

The graph shows sample data modeled on a long-term breeding experiment with corn. In each generation, breeders chose the plants whose kernels had the most oil and used only those plants as parents. The average oil content rose from 4.5% to 21% over 100 generations. The rise is fastest early on and slows later, as the variation that breeders can select from is used up. The experiment shows that selection can change a population greatly when there is inherited variation to work on.

Sexual Selection

• Sexual selection: traits that help get mates
• Males compete, or females choose
• Examples: peacock tails and deer antlers
• A trait may lower survival but raise mating

Sexual selection is selection for traits that help an organism get mates, such as bright colors, songs, and antlers. Females may choose males with showy traits, or males may fight for access to females. The graphs show sample data for a bird with a long tail. A longer tail attracts more mates, but it also makes the bird easier for predators to catch. Because fitness depends on both surviving and reproducing, selection favors the tail length with the best balance. Traits can spread through a population this way even if they lower survival.

Natural, Artificial, and Sexual Selection

• All three change which traits become common
• They differ in what does the selecting
• All need inherited variation

Type	What selects?	Trait favored	Example
Natural	The environment	Traits that improve survival and reproduction	Camouflaged beetles survive predators
Artificial	Humans	Traits that humans want	Broccoli bred from wild mustard
Sexual	Mates or rivals	Traits that help get mates	Peacock tails, deer antlers

The three types differ in what does the selecting, but they work the same way. In each, some individuals leave more offspring than others, and the traits they carry become more common. Sexual selection is often considered a form of natural selection, because a trait that helps in getting mates helps the organism reproduce.

Coevolution

• Coevolution: two species change each other over time
• Flowers and their pollinators
• Predators and their prey
• Each species is a selective pressure on the other

In coevolution, two species that interact act as selective pressures on each other, and each changes over time in response to the other. A hummingbird with a bill that matches the flower’s tube can reach the nectar and pollinate the flower. Flowers whose tubes match the bill get pollinated, and birds with bills that fit get fed. The graph shows sample data in which bill length closely matches tube length across six pairs of species. Predators and prey, and parasites and hosts, can also coevolve.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Breeding Corn for Oil

Use the graph of corn oil content. (a) How many times greater was the oil content at generation 100 than at generation 0? (b) Find the average increase in oil content per generation from generation 0 to 50, and from 50 to 100. (c) Explain why the rate slowed down.

Show the solution

1. (a) Ratio: 21% ÷ 4.5% = 4.7 times as much.
2. (b) Rates: from 0 to 50: (15 − 4.5) ÷ 50 = 0.21 percentage points per generation. From 50 to 100: (21 − 15) ÷ 50 = 0.12 percentage points per generation.
3. (c) Slowing: as breeders keep choosing the same extreme, there is less variation left to select. The population gets closer to a limit, so each generation gains less.
4. Answer: (a) 4.7 times. (b) 0.21 and 0.12 percentage points per generation. (c) Less variation is left to select.

Example 2: The Peacock’s Tail

Reproductive success = survival probability × average number of mates. Use the graphs to find the reproductive success for each tail length. Which tail length is favored by selection? Explain the trade-off.

Show the solution

1. Calculate each product: 100 cm: 0.90 × 0.8 = 0.72; 120 cm: 0.85 × 1.5 = 1.275; 140 cm: 0.75 × 2.4 = 1.8; 160 cm: 0.60 × 2.8 = 1.68; 180 cm: 0.35 × 3.0 = 1.05
2. Compare: the highest value is 1.8, at 140 cm.
3. Trade-off: longer tails attract more mates, but they lower survival. At 180 cm, birds get the most mates (3.0) but only 35% survive, so their reproductive success is lower than at 140 cm.
4. Answer: 140 cm (reproductive success 1.8). Longer tails gain mates but lose survival.

Example 3: Which Kind of Selection?

Name each example as natural selection, artificial selection, sexual selection, or coevolution. (a) A farmer saves seeds only from the tomato plants that resisted a disease. (b) Female frogs mate more often with males that have deeper calls. (c) Cheetahs that run faster catch more prey and have more cubs. (d) Flowers with deeper tubes and hummingbirds with longer bills each favor the other’s traits.

Example	Type of selection
(a) Tomato seeds
(b) Frog calls
(c) Fast cheetahs
(d) Flowers and hummingbirds

Show the solution

1. (a) Artificial selection: a human chooses which plants reproduce.
2. (b) Sexual selection: mates choose males with a certain trait.
3. (c) Natural selection: the environment (prey and predator) determines which cheetahs survive and reproduce.
4. (d) Coevolution: two interacting species act as selective pressures on each other.
5. Answer: (a) Artificial. (b) Sexual. (c) Natural. (d) Coevolution.

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Practice

Classify examples of selection, read graphs of selective breeding and mate choice, calculate reproductive success, and explain coevolution. Try it on your own first, then check your work with the answer key.

• Artificial and Sexual Selection Practice Worksheet (PDF)
• Artificial and Sexual Selection Practice Answer Key (PDF)

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Lesson Slides

• 4.7 Artificial and Sexual Selection Slides (PDF)

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Big Ideas

• Artificial selection: humans choose the parents
• Sexual selection favors traits that help get mates
• Fitness depends on survival and on reproduction
• Coevolution: interacting species select each other

Back to the essential question: traits can spread for reasons other than survival. In artificial selection, humans choose which organisms breed, in sexual selection mates or rivals do the selecting, and in coevolution two species act as selective pressures on each other. In every case, inherited traits that lead to more offspring become more common. Next, you’ll see how selection and isolation can split one species into two.

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