Topic 7.1 Notes – Introduction to Natural Selection

1. What Natural Selection Is

Natural selection is the process by which individuals with certain heritable traits survive and reproduce more than others in a given environment.

A few foundations have to be clear in your head:

• Evolution = change in allele frequencies in a population over time.
• Individuals do not evolve during their lifetime.
• Selection acts on phenotypes (what you can see).
• Evolution happens at the level of genotypes (allele frequencies in the population).

Darwin’s key claims

Darwin proposed three big ideas:

• Species change over time.
• All species share common ancestors.
• Natural selection drives that change.

The mechanism is simple but powerful. Individuals in a population vary. Some of that variation affects survival and reproduction. Those individuals leave more offspring. Over generations, the alleles underlying those helpful traits become more common.

That shift in allele frequency is evolution.

On tests, they love giving you a scenario and asking whether evolution occurred. If allele frequencies did not change, evolution did not occur, even if individuals struggled or died.

2. The Four Conditions Required for Natural Selection

All four must be present for natural selection to occur.

a. Variation Exists

Individuals in a population are not identical.

Sources of variation:

• Mutations (new alleles)
• Crossing over during meiosis
• Independent assortment
• Random fertilization

No variation means nothing for selection to “choose” from.

b. Traits Are Heritable

The variation must be genetic.

• Traits must be passed from parents to offspring.
• Learned or acquired traits are not inherited.

If a trait increases survival but is not genetic, it will not increase in frequency over generations.

c. Overproduction and Competition

Populations produce more offspring than can survive.

Resources are limited:

• Food
• Space
• Mates
• Light, water, nutrients (in plants)

This creates competition, which sets the stage for selection.

d. Differential Survival and Reproduction

Some individuals leave more offspring than others because of their phenotype.

This is differential reproductive success.

Over time:

• Helpful alleles increase in frequency.
• Harmful alleles decrease.
• The population becomes better adapted.

That improved fit to the environment is called an adaptation.

3. Evolutionary Fitness

In evolution, fitness means reproductive success.

It is measured by:

• Number of offspring
• Survival of those offspring
• Whether those offspring reproduce

An organism with high fitness passes more copies of its alleles to the next generation.

Fitness is always relative to the environment.

• Thick fur increases fitness in cold climates.
• The same fur decreases fitness in hot climates.

If the environment changes, the “best” phenotype can change too. That’s something the AP exam often builds into multi-part questions.

4. Environmental Pressures That Drive Selection

The environment determines which traits are favorable.

a. Biotic Factors (living)

• Predators
• Competitors
• Parasites
• Pathogens
• Available prey or food

These interactions can cause rapid evolutionary change. For example, as predators get better at hunting, prey with better defenses survive more often. That back-and-forth can shift allele frequencies quickly.

b. Abiotic Factors (nonliving)

• Temperature
• Water availability
• pH
• Light intensity
• Soil composition
• Natural disasters

Abiotic pressures often show up in data-based questions where environmental conditions shift over time.

Environmental Stability and Change

Different genetic variations may be favored in different generations if conditions shift.

5. Patterns of Natural Selection

Natural selection changes the distribution of phenotypes in predictable ways.

Here’s the classic graph you need to recognize. The dashed curve shows the original normal distribution, and the solid curve shows the population after selection.

Directional, stabilizing, and disruptive selection patterns

a. Directional Selection

• One extreme phenotype is favored.
• The mean shifts left or right.
• On the graph, the entire curve moves toward one extreme.
• Example pattern: increasing resistance to a pesticide.

b. Stabilizing Selection

• Intermediate phenotype favored.
• Extremes selected against.
• Variation decreases.
• On the graph, the curve becomes narrower around the same mean.

c. Disruptive Selection

• Both extremes favored.
• Middle selected against.
• Variation increases.
• On the graph, the distribution splits into two peaks.
• Can contribute to speciation over time.

On exams, they often give you a graph of trait frequency before and after a drought or disease outbreak and ask which pattern it represents. Look at what happens to the mean and the spread.

6. How Natural Selection Affects Populations

Natural selection changes populations by altering allele frequencies.

It can:

• Increase beneficial alleles
• Decrease harmful alleles
• Reduce genetic variation (strong stabilizing selection)
• Maintain variation (changing environments)
• Increase diversity when different phenotypes are favored

One last idea that shows up constantly:

• Variation arises randomly (mutations).
• Selection is non-random. The environment consistently favors certain phenotypes.

If you can trace a path from genetic variation → phenotype differences → differential reproduction → allele frequency change, you understand this topic at the level the AP exam expects.