How can natural selection change trait frequencies in a population?

Think of a population as a group in which a trait has different heritable forms. If individuals with one form, on average, contribute more offspring to the next generation than individuals with another form, that inherited form may make up a larger share of the next generation. Natural selection describes this difference in reproductive contribution; it is not an individual choosing to change, or nature planning ahead. [S1]

For example, imagine a hypothetical population with light and dark coloring. Assume the coloring differences are heritable and, in the current environment, dark-colored individuals on average contribute more offspring. If offspring tend to inherit the coloring of their parents, dark coloring may become more frequent in later generations. The relevant advantage depends on the conditions; a change in the environment could favor a different form. [S1]

Here is a simplified, hypothetical illustration. The counts are placeholders to show how proportions could shift, not research results or a prediction. Assume the population starts with 100 individuals, and that the difference in reproductive success carries through to the next generation:

Generation 1: 30 dark + 70 light = 100
             dark 30% | light 70%
                         ↓
Different average reproductive contributions
                         ↓
Generation 2: 55 dark + 45 light = 100
             dark 55% | light 45%

Read it from top to bottom: dark coloring starts as the less common form, then represents a larger share of the illustrative next generation. The change happens across generations because of differences in who contributes offspring—not because an individual changes color in response to need.

To make a real diagram, count the trait forms in each generation using the same categories, then divide each count by that generation’s total. Label the numbers as hypothetical unless they come from actual observations. In this example, the diagram illustrates a shift in the frequency of a visible trait. It does not establish the underlying genetic details or show that natural selection was the only cause.

No. The outcome depends on conditions and on which heritable differences are associated with reproductive success. If the environment changes, the phenotype associated with greater success can change too. Natural selection also acts on variation already present; it does not guarantee that a population will become perfectly suited to its environment. [S1]

A change in a trait’s frequency alone is not enough to demonstrate that natural selection caused it. For the explanation to fit natural selection, you would need to consider whether the trait is heritable and whether individuals with different forms contributed differently to the next generation. Also, natural selection is one of multiple mechanisms that can change allele frequencies, so not every population change should automatically be attributed to selection. [S2]

Use this fill-in template:

In a population, individuals vary in [trait forms].
Assume the variation is heritable. In [environment or conditions],
individuals with [form A] contribute [more/fewer/about the same number]
of offspring on average than individuals with [form B].
Across generations, [form A] may therefore become [more/less/about as]
common. This is a population-level result, not an intentional change.
The outcome can depend on environmental conditions and other causes
of population change.

Keep the comparison specific: say which forms you are comparing, under what conditions, and whether you mean offspring contribution rather than simply survival. If you have no observations about inheritance or reproductive success, label those parts as assumptions instead of presenting them as established facts. A diagram can show the proposed pattern, but the numbers need evidence before they can be treated as real.