Selective Pressures and Allele Frequencies

Essential Question

How does a change in the environment change a population’s alleles?

Natural selection acts on traits, but evolution is recorded in a population’s genes. In this lesson, you’ll learn to describe a population by its allele frequencies, calculate how a selective pressure changes them, and see why bacteria can become resistant to antibiotics. You’ll also see that chance can change allele frequencies too.

The Gene Pool and Allele Frequency

• Gene pool: all the alleles in a population
• Allele frequency: the share of one allele in the gene pool
• Frequencies of all alleles of a gene add up to 1
• Evolution is a change in allele frequencies over generations

Each beetle has two alleles of a color gene, G (green, dominant) and g (brown, recessive). Together, the alleles of all beetles in the population make up the gene pool. The allele frequency is the fraction of the gene pool made up of one allele. In the diagram, 10 beetles have 20 alleles. A GG beetle has two G alleles, a Gg beetle has one G and one g, and a gg beetle has two g alleles, so there are 12 G alleles and 8 g alleles. The frequency of G is 12 ÷ 20 = 0.60, and the frequency of g is 8 ÷ 20 = 0.40. The two frequencies always add up to 1. In biology, evolution is a change in allele frequencies in a population over generations.

Selective Pressures

• A selective pressure is an environmental factor affecting survival or reproduction
• It favors some alleles over others
• Pressures can be living or nonliving
• The alleles that help in that environment increase

Selective pressure	Example	Alleles that increase
Predators	Birds eat beetles that are easy to see	Alleles for camouflage
Disease	A new disease kills most of a population	Alleles for resistance
Climate	Winters become colder	Alleles for thick fur or fat
Food supply	A drought leaves only hard seeds	Alleles for strong beaks

A selective pressure is any factor in the environment that affects how well individuals survive and reproduce. Predators, disease, climate, and the food supply are all selective pressures. When a pressure removes more individuals with one trait than another, the alleles for the helpful trait become a larger share of the gene pool in the next generation. If the pressure continues, the change continues. A change in the environment can therefore change which alleles are favored.

Allele Frequencies Change Over Time

• Selection acts on traits but changes allele frequencies
• Here, brown (gg) beetles are 50% less likely to survive
• The g allele becomes less common each generation
• The decline slows because g hides in Gg beetles

The graph is a model of a population where birds eat brown (gg) beetles, so gg beetles are 50% less likely to survive. The frequency of g falls from 0.40 to about 0.15 by generation 10. The decline slows over time. Gg beetles look green, so birds do not eat them, and every Gg beetle still carries a g allele. Selection can only act on alleles through the traits they produce, so a recessive allele is removed slowly. A change in allele frequency over time is what scientists mean by evolution.

Antibiotic Resistance

• Mutations create variation in bacteria
• An antibiotic kills susceptible bacteria
• Resistant survivors reproduce
• The population becomes resistant

Bacteria reproduce quickly, and a mutation can make one bacterium resistant to an antibiotic. In a population without the antibiotic, this resistant bacterium has no special advantage. When an antibiotic is used, it kills the susceptible bacteria, so the resistant bacterium survives and reproduces. Soon, almost all the bacteria in the population are resistant. The antibiotic did not cause the mutation. It was a selective pressure that favored the resistant bacteria. This is why antibiotics should be used only when needed and the full course should be finished.

Chance Also Changes Allele Frequencies

• Genetic drift: random change in allele frequencies
• Drift has the biggest effect in small populations
• Alleles can be lost or fixed by chance
• No allele has an advantage in drift

Not every change in allele frequency is caused by selection. In any generation, which alleles are passed on is partly a matter of chance, like flipping coins. This random change is called genetic drift. The graphs show a simulation with no selection at all. In a small population of 10 beetles, chance alone changes the frequency of G widely, and in two of the three runs, G was lost completely. In a large population of 500, the same chance events change the frequencies much less. Drift is strongest in small populations, such as the survivors of a disaster.

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Worked Examples

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

Example 1: Calculating Allele Frequencies

A population of 100 beetles has 36 GG (green), 48 Gg (green), and 16 gg (brown). (a) Find the frequency of G and of g. (b) Birds eat half of the brown beetles. Find the new frequencies of G and g.

Genotype	Number of beetles
GG	36
Gg	48
gg	16

Show the solution

1. (a) Count the alleles: 100 beetles have 200 alleles. G alleles = (2 × 36) + 48 = 120. g alleles = 48 + (2 × 16) = 80.
2. Frequencies: G = 120 ÷ 200 = 0.60. g = 80 ÷ 200 = 0.40.
3. (b) After the birds: half of the 16 gg beetles are eaten, leaving 8. There are 36 + 48 + 8 = 92 beetles and 184 alleles. G alleles = (2 × 36) + 48 = 120. g alleles = 48 + (2 × 8) = 64.
4. New frequencies: G = 120 ÷ 184 = 0.652. g = 64 ÷ 184 = 0.348. (Check: 0.652 + 0.348 = 1.000.)
5. Answer: Before: G = 0.60, g = 0.40. After: G = 0.652, g = 0.348.

Example 2: Dark Moths in a Polluted Forest

Sample data for a moth population in a forest near factories are shown below. Soot darkened the tree bark over this time. (a) How many percentage points did the dark moths increase from 1850 to 1890? (b) In which 10-year period was the increase fastest? (c) Explain the change using natural selection.

Year	Dark moths (%)	Light moths (%)
1850	2	98
1860	12	88
1870	35	65
1880	70	30
1890	95	5

Show the solution

1. (a) Total increase: 95% − 2% = 93 percentage points.
2. (b) Fastest period: the changes in each decade were 10, 23, 35, and 25 percentage points. The largest, 35, was from 1870 to 1880.
3. (c) Explanation: color is inherited. On dark bark, birds could see light moths more easily and ate more of them, so dark moths survived and reproduced more. The alleles for dark color became more common in the gene pool.
4. Answer: (a) 93 percentage points. (b) 1870 to 1880. (c) Birds ate more light moths on dark bark.

Example 3: Selection or Drift?

The graphs show simulations in which no allele has any advantage. In the population of 10 beetles, allele G was lost in two runs and became the only allele in one run. In the population of 500, all three runs stayed between 0.27 and 0.62. (a) Could selection explain these results? (b) Why do the two populations behave so differently?

Show the solution

1. (a) Not selection: in the simulation, no allele helps survival, so the changes were caused by chance. This is genetic drift.
2. (b) Population size: in a small population, a few chance events can change the frequency a lot, and an allele can disappear. In a large population, chance events average out, so the frequencies change much less.
3. Answer: (a) No. The changes are due to genetic drift. (b) Chance matters more in small populations.

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Practice

Count alleles in a gene pool, calculate how a selective pressure changes allele frequencies, read a graph of change over generations, and explain antibiotic resistance and genetic drift. Try it on your own first, then check your work with the answer key.

• Selective Pressures and Allele Frequencies Practice Worksheet (PDF)
• Selective Pressures and Allele Frequencies Practice Answer Key (PDF)

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Lesson Slides

• 4.6 Selective Pressures and Allele Frequencies Slides (PDF)

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Big Ideas

• Allele frequency: the share of one allele in the gene pool
• Selective pressures favor some alleles over others
• Evolution is a change in allele frequencies
• Antibiotics select for resistant bacteria
• Genetic drift changes small populations by chance

Back to the essential question: a change in the environment creates a selective pressure that favors some alleles over others, so the frequency of those alleles increases in the population from one generation to the next. Antibiotic resistance is an example. Chance events, called genetic drift, can also change allele frequencies, especially in small populations. Next, you’ll look at two more kinds of selection: artificial selection by humans and sexual selection by mates.

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