Why Evolution Has More Than One Driving Force
Evolution is often associated with natural selection, but it is not the only mechanism that changes allele frequencies in populations. Genetic drift also plays a major role, especially in small populations. Understanding the differences between these two processes is essential for IB Biology students studying evolution, population genetics, and biodiversity.
Natural selection is a non-random process in which individuals with advantageous traits are more likely to survive and reproduce. These traits increase in frequency over generations because they improve fitness. Selection is driven by environmental pressures such as predators, food availability, climate, and competition. Because the environment favors certain alleles, evolution through natural selection tends to be predictable and directional.
In contrast, genetic drift is completely random. It describes unpredictable changes in allele frequencies that occur by chance, not because the alleles provide a fitness advantage. Drift is most powerful in small populations, where chance events—such as random mating outcomes or accidents—can dramatically shift allele frequencies. Unlike natural selection, drift does not consistently favor beneficial alleles; harmful alleles can increase, and beneficial ones can disappear simply due to luck.
Two special forms of drift highlight this randomness:
- Bottleneck effect: A large population is drastically reduced by a random event, leaving behind an unrepresentative genetic sample.
- Founder effect: A small group establishes a new population, bringing only a small fraction of the original gene pool.
Both effects decrease genetic variation and make populations more vulnerable to environmental changes.
Another major difference is that natural selection increases adaptation, while genetic drift does not. Selection produces traits that better suit organisms to their environment. Drift simply alters allele frequencies with no regard for fitness.
Despite their differences, both mechanisms shape evolution. In large populations, natural selection has a stronger influence. In small populations, genetic drift can dominate, causing rapid and unpredictable evolutionary change.
In summary, natural selection is adaptive and driven by environmental pressures, while genetic drift is random and driven by chance events. Both influence evolution, but in fundamentally different ways.
