What is the difference between natural selection and genetic drift

Natural selection and genetic drift are the two primary mechanisms that change allele frequencies in a population over time. Natural selection is a non‑random process: alleles that increase an organism’s survival or reproductive success become more common because their carriers leave more offspring. Genetic drift, by contrast, is a stochastic process; allele frequencies shift simply because individuals are sampled randomly from one generation to the next, a effect that is strongest when the population is small.

How Natural Selection Works

Consider a classic example of peppered moths in industrial England. Before the soot‑covered factories, the light‑colored form was common because it blended with lichen‑covered trees, reducing predation. When pollution darkened the bark, birds more easily spotted the light moths, and the dark morph, which carried a different allele, had higher fitness. Within a few decades the frequency of the dark allele rose from 5 % to over 90 %, illustrating how differential survival drives predictable changes.

Genetic Drift in Action

Imagine a small isolated pond with ten diploid fish, each carrying either allele A or allele a at a single locus. Initially the frequency of A is 0.6 (12 copies) and a is 0.4 (8 copies). If only five fish reproduce by chance, the next generation might receive 7 copies of A and 3 copies of a, shifting the frequency of A to 0.7. Repeating this random sampling over generations can eventually fix one allele (frequency = 1) or lose the other, even though neither allele affects fitness. This random walk is the essence of genetic drift.

Both mechanisms can operate simultaneously, but their relative impact depends on the ecological context. In large populations with strong fitness differences, selection overwhelms drift, leading to rapid adaptation. In tiny populations, drift can dominate, erasing advantageous alleles or fixing deleterious ones purely by chance. Recognizing which force is at work helps predict evolutionary trajectories and informs conservation strategies for endangered species.

The main ways they differ

• Cause
• Predictability
• Population‑size effect
• Typical time scale

How to decide which mechanism is likely

1. 1Assess population size
2. 2Look for fitness differences among phenotypes
3. 3Check for random fluctuations in allele frequencies
4. 4Consider environmental stability

A side‑by‑side comparison