Introduction to Natural Selection — AP Biology

AP Biology · Unit 7: Natural Selection · 14 min read

1. Darwin's Observations and Core Inferences ★★☆☆☆ ⏱ 4 min

**Variation**: All populations have genetic and phenotypic variation among individuals for most traits
**Overproduction**: All species produce more offspring than can survive to reproductive age in their environment
**Limited resources**: Environmental resources (food, space, mates) are finite, leading to competition for survival and reproduction
**Heritability**: Most phenotypic traits are passed from parent to offspring via genetic material

From these four observations, Darwin drew two core inferences that remain the foundation of evolutionary biology: (1) Individuals with traits that better match their environment (adaptations) are more likely to survive and reproduce, leaving more viable offspring than individuals with less suitable traits (this is called *differential reproductive success*). (2) Over multiple generations, favorable heritable traits accumulate in the population, leading to evolutionary change and the origin of new adaptations.

📐 Worked Example

A biologist studies a population of wild goats living in a mountain environment with increasingly cold winter temperatures. She records: (1) Goats vary in the thickness of their winter fur, (2) Fur thickness is passed from parent to offspring, (3) The environment can only support 40% of goat kids born each year through winter, (4) Goats with thicker fur are more likely to survive cold winters to reproduce. Match each observation to Darwin's framework and state the final inference.

Variation in fur thickness matches Darwin's first observation of phenotypic variation in natural populations
Fur thickness being heritable matches the fourth observation of heritable trait transmission
Only 40% of goat kids survive matches the second observation of overproduction (more offspring are produced than the environment can support)
Higher survival of goats with thicker fur confirms the first inference: differential reproductive success for individuals with environment-matched traits
Over multiple generations, average fur thickness in the population will increase, which is the second inference of evolutionary change by natural selection

2. Required Conditions for Natural Selection ★★☆☆☆ ⏱ 3 min

For natural selection to cause evolutionary change in a population, three non-negotiable conditions must be met: if any condition is missing, selection cannot change trait frequencies over time.

**Variation in the trait**: There must be differences in the trait between individuals in the population. If all individuals have the exact same trait, there is nothing for selection to act on.
**Heritability of the trait**: The trait must be passed from parent to offspring. If a trait that improves survival is not heritable, it cannot become more common in the next generation.
**Differential fitness linked to trait variation**: Different variants of the trait must lead to differences in reproductive success. If the trait has no effect on how many offspring an individual produces, selection cannot change its frequency over time.

📐 Worked Example

Which of the following scenarios allows natural selection to occur? Justify your answer. A) All sunflowers in a population have the same flower height, and taller flower height increases seed production. B) Flower height varies among sunflowers, height is heritable, and taller sunflowers produce more seeds than shorter sunflowers. C) Flower height varies among sunflowers, taller sunflowers produce more seeds, but height is entirely determined by how much water the sunflower receives during growth, not genetics.

Recall the three required conditions for natural selection: trait variation, heritability, and differential reproductive success linked to the trait
Evaluate Scenario A: All sunflowers have the same height, so the first condition (trait variation) is not met. Natural selection cannot occur here
Evaluate Scenario C: Flower height is not heritable, so the second condition is not met. Even though taller plants produce more seeds, their offspring will not inherit the tall trait, so no evolutionary change occurs
Evaluate Scenario B: All three conditions are satisfied: height varies, it is heritable, and variation in height correlates with variation in seed production (reproductive success). Natural selection can occur here

3. Biological Fitness and Common Misconceptions ★★★☆☆ ⏱ 4 min

There are three pervasive common misconceptions about natural selection that are regularly tested on the AP Biology exam:

*Misconception*: Natural selection gives organisms what they "need" to survive. **Correction**: Natural selection acts on pre-existing random variation; it does not generate new traits because a population needs them.
*Misconception*: Natural selection acts for the good of the species. **Correction**: Natural selection acts on individual fitness. Traits that increase an individual's fitness can spread even if they harm the overall population.
*Misconception*: Natural selection produces perfect traits. **Correction**: Selection can only act on existing variation, and is constrained by evolutionary trade-offs, so all adaptations are compromises.

4. Applied AP Style Worked Examples ★★★☆☆ ⏱ 3 min

📐 Worked Example

Which of the following best describes the process of natural selection leading to antibiotic resistance in a population of bacteria? A) Antibiotic exposure causes new mutations that make some bacteria resistant, and these bacteria survive better than non-resistant bacteria. B) Resistance mutations already exist in the population before antibiotic exposure, and resistant bacteria have higher reproductive success after exposure, leading to increased resistance frequency over time. C) All surviving bacteria develop resistance after exposure to low doses of antibiotics, and this acquired resistance is passed to their offspring. D) Natural selection works to improve the species, so over time all bacteria will become resistant to all antibiotics.

Recall that natural selection acts on pre-existing heritable variation, and mutations arise randomly before environmental pressure
Eliminate Option A: it incorrectly claims that antibiotic exposure causes resistance mutations
Eliminate Option C: it describes Lamarckian inheritance of acquired traits, which does not occur
Eliminate Option D: it repeats the misconception that selection acts "for the good of the species", ignoring fitness costs of resistance
Confirm Option B correctly describes the process of natural selection for antibiotic resistance. Correct answer: B

📐 Worked Example

In a population of wild clover, some plants produce a toxic compound called cyanide that deters herbivores from eating them, while other plants do not produce cyanide. Cyanide production is controlled by a single gene and is fully heritable. Researchers study the population in two different fields: Field 1 has very high densities of herbivores that eat clover, Field 2 has almost no herbivores. (a) Identify the three required conditions for natural selection to act on cyanide production. (b) Predict how the frequency of cyanide-producing plants will differ between fields after 10 generations. Justify. (c) Producing cyanide requires extra energy that could be used for seeds. Explain how this trade-off affects fitness in Field 2.

Part (a): The three required conditions are (1) Variation: the population has cyanide and non-cyanide plants, (2) Heritability: the trait is heritable as stated, (3) Differential reproductive success: cyanide variation changes offspring production due to herbivore damage
Part (b): Prediction: Frequency of cyanide-producing plants will be significantly higher in Field 1 than Field 2. Justification: In Field 1, high herbivore density means cyanide plants are less likely to be eaten, so they produce more offspring, increasing the trait frequency over generations. In Field 2, cyanide provides no benefit.
Part (c): In Field 2, non-cyanide plants use energy saved from not producing cyanide to make more seeds. This means non-cyanide plants have higher fitness than cyanide plants, so natural selection will act against cyanide production in Field 2, even though it is adaptive in Field 1.

Common Pitfalls

Why: Students confuse Lamarckian inheritance with Darwinian natural selection, incorrectly assuming variation arises in response to environmental need.

Why: Students mix up the level of selection and the level of evolution.

Why: The phrase "survival of the fittest" leads students to prioritize survival over reproductive output.

Why: Students confuse the random origin of mutations (variation) with the selection process itself.

Why: Students assume any trait must be selected for, ignoring neutral byproducts of other processes.

Why: Students use the terms interchangeably, but evolution is the outcome, natural selection is one mechanism of change.

Introduction to Natural Selection — AP Biology

1. Darwin's Observations and Core Inferences ★★☆☆☆ ⏱ 4 min

2. Required Conditions for Natural Selection ★★☆☆☆ ⏱ 3 min

3. Biological Fitness and Common Misconceptions ★★★☆☆ ⏱ 4 min

4. Applied AP Style Worked Examples ★★★☆☆ ⏱ 3 min

Common Pitfalls

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