How can natural selection change a population?

Natural selection can change a population when heritable differences affect reproductive contributions in a particular environment. Heritable means that a difference can be passed to offspring. The change concerns the representation of traits across generations, not an individual deliberately changing its inherited type.

The fictional population begins with four striped and four plain organisms, so half are striped. Its simplified asexual inheritance rule copies each parent’s type to its offspring. This makes the connection between an existing inherited difference and the next generation explicit.

In the stated environment, each striped parent contributes two offspring and each plain parent one. Four striped parents therefore contribute eight striped offspring, while four plain parents contribute four plain offspring. The next generation contains twelve organisms, of which eight are striped: 8/12, or about 66.67%, compared with the original 50%.

The striped proportion rises because inherited types contribute unequally. No plain parent needs to become striped. The advantage is also conditional on the environment; equal contributions would leave the proportions unchanged in this model, while a reversed advantage could favor the other type.

These stipulated counts isolate the mechanism rather than forecast real births. Actual populations involve additional influences, including chance and changing conditions. The central implication is that differential reproduction can change population composition even when individual organisms retain their original type.