Darwin’s theory explains how populations change when organisms compete for scarce resources. Individuals with heritable traits that help them survive and reproduce in that context leave more offspring, shifting trait frequencies over generations.

Darwin’s core claim: competition shapes who leaves offspring

Darwin proposed that natural selection follows logically from two observations: organisms tend to produce more offspring than can survive, and environments cannot support unlimited population growth. The result is competition, and competition produces unequal outcomes.

Competition is not limited to direct fighting. It includes any situation in which limited resources prevent all individuals from surviving and reproducing equally.

Why competition is unavoidable in nature

In most populations:

• More individuals are born than the environment can sustain to adulthood.

• Resources fluctuate and are finite (even when populations are stable).

• Individuals differ in their ability to obtain resources or avoid hazards.

Darwin emphasized that competition can occur:

This figure illustrates interspecific competition using classic Paramecium growth curves: each species grows well alone, but one species excludes the other when both rely on the same limiting resource. It visually connects “competition for limited resources” to unequal survival/reproductive outcomes at the population level. Source

• Within a species (e.g., individuals competing for mates or nesting sites)

• Between species (e.g., different species consuming the same food source)

Differential survival: not all individuals are equally likely to persist

Once competition exists, individuals with certain traits will be more likely to survive to reproductive age. This is differential survival, meaning survival is not random with respect to phenotype when the environment “filters” individuals.

What makes survival “differential” in Darwin’s sense

For Darwin’s argument to hold, differences in survival must be linked to traits that affect performance in the competitive setting, such as:

• Ability to capture or process food

• Speed, camouflage, or defenses that reduce predation

• Tolerance to stressors that limit access to resources (e.g., drought reducing plant growth)

• Behaviours that increase access to shelter or mates

Darwin did not need every trait to matter; he needed only that some traits influence survival and that the effect is consistent enough to change which individuals contribute offspring.

Inheritance of favourable traits: survival differences become evolutionary change

Competition and differential survival alone do not guarantee evolution. Darwin’s crucial extension was inheritance: if individuals with advantageous traits survive and reproduce more, and if those traits are passed to offspring, then favourable traits become more common over generations.

A trait can be favourable only in context: “favourable” means it increases an organism’s chances of surviving and reproducing under the current competitive conditions. When more offspring inherit that trait, the population’s characteristics shift.

This plot shows how genotype frequencies ($p^2$, $2pq$, $q^2$) depend on allele frequencies ($p$ and $q$) in a population. It’s a useful bridge from “traits are heritable” to the quantitative idea that changing allele frequencies changes the population’s genetic and phenotypic makeup over generations. Source

How inheritance connects individuals to population change

Natural populations contain differences among individuals. Under competition:

• Individuals with advantageous heritable traits tend to leave more surviving offspring.

• Offspring tend to resemble parents for those heritable traits.

• Over many generations, the proportion of individuals with those traits increases.

This is the logic behind Darwin’s claim that competition for limited resources leads to differential survival and inheritance of favourable traits. The environment does not create the traits on demand; it sorts among existing heritable differences by affecting who reproduces.

Key clarifications for AP Biology

“Survival of the fittest” is about outcomes, not intent

Darwin’s theory does not imply purpose or striving toward perfection.

• Selection does not “try” to improve a species.

• Advantage is measured by reproductive contribution, but Darwin’s mechanism begins with competition and survival differences that ultimately affect reproduction.

Competition can be indirect and subtle

Outcomes can be driven by small differences in efficiency or timing, such as:

• Earlier access to food or breeding sites

• Slightly better resource use leading to higher survival during scarcity

• Reduced energy costs that allow more investment in offspring

Favourable traits can have costs

A trait may improve success under competition while also imposing trade-offs (e.g., energy use, increased visibility), so “favourable” depends on whether benefits outweigh costs in that environment.