Natural Selection

Essential Question

How does the environment shape which traits become common?

Why are polar bears white, cacti spiny, and cheetahs fast? In each case, individuals with certain inherited traits left more offspring than others in their environment. In this lesson, you’ll break natural selection into its parts, use data to show how a population changes, and compare three patterns of selection.

Four Parts of Natural Selection

• Variation: individuals differ in traits
• Inheritance: some variation is passed to offspring
• Selection: some traits help individuals survive and reproduce more
• Time: helpful traits become more common over generations

Natural selection needs four things. First, individuals in a population vary in their traits. Second, at least some of that variation is inherited, so offspring resemble their parents. Third, in a given environment some traits help individuals survive and reproduce more than others, which is selection. Fourth, given enough time, the helpful traits become more common in the population. If any one of these is missing, natural selection cannot cause change. Variation first appears through mutation and is shuffled by sexual reproduction.

Variation in a Population

• Most traits vary across a population
• Variation comes from mutation and sexual reproduction
• Only inherited variation can be selected
• Variation can be shown in a histogram

The histogram shows sample data for beak depth in 100 finches. Most birds have a beak depth near the average, about 9.2 mm, and fewer birds have very shallow or very deep beaks. The beak depths are different because of variation in their genes, caused by mutations and by the mixing of alleles in sexual reproduction. Traits such as these, which are inherited, can be acted on by natural selection. Traits that result only from an animal’s experience, such as a scar, cannot.

Adaptations and Fitness

• An adaptation is an inherited trait that improves survival or reproduction
• Fitness: how many offspring an organism leaves
• A trait is only helpful in a certain environment

Type of adaptation	What it is	Example
Structural	A body part or body feature	Thick fur of an arctic fox; long beak of a hummingbird
Behavioral	An action	Geese migrating south for winter; birds building nests
Physiological	A process inside the body	Venom in a snake; antifreeze proteins in some fish

An adaptation is an inherited trait that increases an organism’s chance of surviving and reproducing in its environment. Adaptations can be structural, behavioral, or physiological. Biologists measure the success of a trait by fitness, which is the number of offspring an organism leaves that survive to reproduce. Fitness is not the same as strength or speed. A trait that is helpful in one environment may not be helpful in another, so an adaptation always depends on the environment.

Selection Changes a Population

• Individuals do not change; the population does
• Predators remove the most visible beetles
• The proportion of each trait changes
• Surviving beetles pass on their traits

The graph shows a sample population of beetles on dark brown soil before and after one season of bird predation. Before, 120 of the 200 beetles were green, which is 60%. Birds find green beetles more easily, and after predation only 48 green beetles and 64 brown beetles were left, so green beetles were 42.9% of the survivors. No beetle changed color. The population changed because brown beetles were more likely to survive and reproduce, and brown is inherited by their offspring. Over many generations, brown could become the most common color.

Three Patterns of Selection

• Directional: one extreme is favored
• Stabilizing: the middle is favored
• Disruptive: both extremes are favored

Natural selection can act on a trait in three main ways. In directional selection, individuals at one extreme survive better, so the average value of the trait shifts in that direction. In stabilizing selection, individuals near the middle survive best, so the variation shrinks and the average stays the same. For example, human babies of medium weight survive better than very small or very large babies. In disruptive selection, both extremes survive better than the middle, so the population can split into two groups.

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Worked Examples

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

Example 1: Finches in a Drought

A drought kills most of the soft seeds on an island, leaving mostly large, hard seeds. The table gives sample data on beak depth. (a) How much deeper is the average beak of the survivors? What percent change is that? (b) Explain why deep beaks helped. (c) Why does the average of the offspring show that beak depth is inherited?

Group	Number of birds	Average beak depth (mm)
Before the drought	400	9.2
Survivors of the drought	40	9.7
Offspring of the survivors	120	9.6

Show the solution

1. (a) Change: 9.7 mm − 9.2 mm = 0.5 mm deeper. Percent change = 0.5 ÷ 9.2 × 100 = 5.4%
2. (b) Why deep beaks helped: only birds with deep, strong beaks could crack the large, hard seeds that were left, so they survived and reproduced.
3. (c) Inheritance: the offspring average (9.6 mm) is much closer to their parents’ average (9.7 mm) than to the original population (9.2 mm), which shows that deep beaks are passed on.
4. Answer: (a) 0.5 mm deeper, a 5.4% increase. (b) Only deep beaks could crack the remaining seeds. (c) Offspring resemble the survivors.

Example 2: Snails on a Lawn

A population of 500 snails on a plain green lawn has 300 banded snails and 200 plain snails. Birds find the banded snails more easily. They eat 50% of the banded snails and 10% of the plain snails. What percent of the snail population was banded before and after the birds ate? Show your setup.

Type of snail	Number before	Percent eaten
Banded	300	50%
Plain	200	10%

Show the solution

1. Before: 300 ÷ 500 × 100 = 60% banded
2. Banded left: 300 − (0.50 × 300) = 300 − 150 = 150 snails
3. Plain left: 200 − (0.10 × 200) = 200 − 20 = 180 snails
4. After: total = 150 + 180 = 330 snails. 150 ÷ 330 × 100 = 45.5% banded
5. Answer: Banded snails were 60% before and 45.5% after.

Example 3: Which Type of Selection?

Graphs A, B, and C show a trait before (dashed curve) and after (solid curve) selection. Match each scenario to a graph and name the type of selection. (1) Newborn babies of medium weight survive better than very small or very large babies. (2) As predators begin to prefer small mice, only the larger mice survive. (3) Birds with very small or very large beaks eat the two kinds of seed on an island, while medium beaks are poor at both.

Show the solution

1. (1) Babies: graph B, stabilizing selection. The middle values survive best, so the curve gets narrower.
2. (2) Mice: graph A, directional selection. One extreme (larger size) survives best, so the whole curve shifts that way.
3. (3) Beaks: graph C, disruptive selection. Both extremes survive best, so the curve splits into two peaks.
4. Answer: (1) B, stabilizing. (2) A, directional. (3) C, disruptive.

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Practice

Read a histogram of variation, calculate how a population changes, identify types of selection, and classify adaptations. Try it on your own first, then check your work with the answer key.

• Natural Selection Practice Worksheet (PDF)
• Natural Selection Practice Answer Key (PDF)

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Lesson Slides

• 4.5 Natural Selection Slides (PDF)

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Big Ideas

• Variation, inheritance, selection, and time
• An adaptation is an inherited trait that improves fitness
• Individuals do not evolve; populations do
• Directional, stabilizing, and disruptive selection

Back to the essential question: the environment shapes which traits become common because individuals with helpful inherited traits survive and reproduce more. Over many generations, the proportion of those traits in a population increases, which is how populations become adapted to their environments. Next, you’ll describe this change using allele frequencies.

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