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Introduction to
Natural Selection

Explore how heritable variation, differential survival, and reproduction drive the evolution of populations over time.

AP Biology

What Is Evolution?

In biology, evolution is the change in allele frequencies within a population over successive generations. It is not about individual organisms changing during their lifetimes: it is about populations shifting over time.

Natural selection is one of the primary mechanisms that drives evolution. Charles Darwin and Alfred Russel Wallace independently proposed this mechanism in the 19th century.

The Four Conditions for Natural Selection

1
Variation: Individuals in a population differ in their traits (phenotypes).
2
Heritability: Some of that variation is genetically inherited from parent to offspring.
3
Differential survival: Some variants survive better in a given environment than others.
4
Differential reproduction: Survivors pass their advantageous alleles to the next generation at higher rates.

Explain fitness as contribution to later generations.

One illustrative breeding-season ledger separates survival from offspring contribution. The beetle illustration and four-angle strip provide organism context only; they are not identified as synthetic adults A–D or used to infer their offspring.

Beetle context alongside an illustrative table of survival and offspring counts for four adults.
One illustrative breeding-season ledger separates survival from offspring contribution. The beetle illustration and four-angle strip provide organism context only; they are not identified as synthetic adults A–D or used to infer their offspring.

Visualizing Variation in a Population

Beak Depth (mm) Frequency Mean Phenotypic Variation

A population of Darwin's finches shows a bell-shaped distribution of beak depths. The mean represents the average, while the full spread shows the variation that natural selection can act upon.

The Core Mechanism of Natural Selection

Variation Trait differences Heritability Genetic basis Selection Pressure Environment filters traits Differential Reproduction Fit individuals leave more offspring Allele Frequency Shifts Population evolves over generations Cycle repeats each generation

Understanding Fitness

In evolutionary biology, fitness does not mean physical strength. It is a measure of an organism's reproductive success: how many viable offspring it contributes to the next generation relative to other individuals.

High Fitness
Organism survives to reproduce and passes on alleles at a higher rate than average.
Low Fitness
Organism fails to survive or reproduce effectively: its alleles decline in frequency.

Natural Selection vs. Other Evolutionary Mechanisms

Natural Selection Genetic Drift
Driver Environmental pressure Random chance events
Direction Non-random: favors advantageous traits Random: no consistent direction
Population size Operates in any size population Strongest in small populations
Outcome Adaptive evolution (better fit to environment) Non-adaptive change (may lose beneficial alleles)

Three Modes of Selection

Directional
Selection favors one extreme phenotype. The distribution shifts left or right over time.
Stabilizing
Selection favors the intermediate phenotype. Extremes are selected against, narrowing the distribution.
Disruptive
Selection favors both extremes over the intermediate. The distribution becomes bimodal.
Directional Stabilizing Disruptive

How to Identify Natural Selection in a Scenario

1
Identify the population. What species are we examining, and what is the trait in question?
2
Confirm variation exists. Do individuals differ in this trait? Is there a range of phenotypes?
3
Verify heritability. Is the variation at least partly due to genetics (not purely environmental)?
4
Identify the selection pressure. What environmental factor (predation, food scarcity, disease) is doing the filtering?
5
Predict the outcome. Which allele frequencies will increase? What will the population look like in future generations?

Example 1: Peppered Moths

Pre-Industrial Light bark Hidden Visible Light moths survive more Post-Industrial Dark soot Hidden Visible Dark moths survive more Pollution Selection pressure shifted: allele frequencies changed This is directional selection

Example 2: Antibiotic Resistance

Before Antibiotic 1 resistant (red) 4 susceptible (blue) Antibiotic After Antibiotic Resistant survives Reproduce Next Generation All resistant
Frequency of resistance allele: \(20\% \rightarrow 100\%\)

Example 3: Stabilizing Selection in Human Birth Weight

Birth Weight (kg) Survival % 1.5 2.5 3.5 4.5 5.5 Optimal survival Too light Too heavy

Stabilizing selection in human birth weight: babies that are too light or too heavy historically had lower survival rates. The optimal range around 3.5 kg had the highest infant survival, keeping the population mean stable over generations.

Adaptation vs. Acclimation

Adaptation
A heritable trait that increases fitness in a specific environment. Occurs over many generations via natural selection.
Acclimation
A reversible physiological adjustment within an organism's lifetime. Not heritable and does not involve allele frequency changes.

Common Mistakes

✕ "Organisms evolve on purpose"
Evolution has no goal or direction. Natural selection acts on existing variation: organisms do not "try" to evolve.
✕ "Survival of the fittest = strongest"
Fitness means reproductive success, not physical strength. A small, fast-breeding organism can be fitter than a large, slow one.
✕ "Individuals evolve"
Individuals do not evolve. Populations evolve as allele frequencies shift across generations.

Summary: Key Concepts

Concept Definition Key Detail
Natural Selection Differential survival and reproduction based on traits Non-random, leads to adaptation
Fitness Reproductive success relative to others Not physical strength
Directional One extreme favored Distribution shifts
Stabilizing Intermediate favored Distribution narrows
Disruptive Both extremes favored Distribution splits (bimodal)
Genetic Drift Random allele changes Strong in small populations

Question 1 of 5

Which of the following is required for natural selection to occur?

Question 2 of 5

A population of rabbits lives in a snowy environment. Over many generations, the proportion of white-furred rabbits increases while brown-furred rabbits decrease. This is an example of:

Question 3 of 5

When bacteria are exposed to an antibiotic, a few resistant individuals survive and reproduce. The antibiotic did not:

Question 4 of 5

In evolutionary biology, fitness is best defined as:

Question 5 of 5

Human birth weight exhibits stabilizing selection because: