Natural selection changes the genetic makeup of a population over generations by altering allele frequencies based on differential survival and reproduction. When heritable variation exists and the environment favors certain traits, individuals with those traits leave more offspring. Over time, the favored alleles increase in frequency, and the population's average traits shift. This process is cumulative and gradual, leading to measurable evolutionary change in the population as a whole. Key ideas

• Variation and heritability: Individuals differ in traits, and at least some of these differences are heritable. This provides the raw material for selection to act on. In a population, natural selection can increase the frequency of advantageous alleles across generations.

• Differential reproduction: In each generation, more offspring survive and reproduce when they possess advantageous traits, while others with less favorable traits have fewer offspring. This differential reproductive success drives changes in allele frequencies.

• Population-level change (microevolution): Because selection acts on the population rather than on individuals, the cumulative effect is a shift in the population’s genetic composition over time, which is what scientists refer to as microevolution. Over longer timescales and with reproductive isolation, these changes can contribute to the emergence of new species (macroevolution).

• Outcomes of natural selection: Depending on the environmental context, natural selection can lead to increased adaptation, maintenance of existing adaptations, or, in changing environments, new adaptations. It can also interact with other evolutionary forces like genetic drift and gene flow to shape population dynamics.

Common misconceptions

• Individuals evolve: It is populations, not individuals, that evolve. An individual organism’s traits do not change across its lifetime in a way that will be inherited by offspring; evolution is about shifts in allele frequencies across generations in the population.

• Selection creates new traits from nothing: Natural selection acts on existing variation; it does not create new advantageous traits from scratch. New variations arise via mutation and recombination, and selection then increases or decreases their frequencies.

Illustrative example

• Suppose a moth population lives in a forest where bird predators are more likely to spot light-colored individuals. If a heritable darker color reduces predation and increases survival and reproduction, darker moths will leave more offspring. Over many generations, the darker coloration allele becomes more common, shifting the population’s phenotype distribution toward darkness. This is a straightforward demonstration of how natural selection can drive a population-wide change.

If you’d like, I can tailor this to a specific organism or environment you have in mind and walk through a step-by-step scenario showing how allele frequencies would change under given selective pressures.