Evolution by natural selection is among the most empirically supported theories in all of science. It is also among the most commonly misunderstood — distorted into forms that its most important critics accept as easily as its most enthusiastic proponents.
Most of the misunderstandings share a common source: they project intentionality onto a process that has none. Evolution is not trying to do anything. There is no goal, no direction, no progress. Natural selection is not a force that acts for the benefit of the species, or in the interests of individual organisms, or toward increasing complexity. It is a consequence of simple arithmetic applied to heritable variation.
The Core Mechanism
Darwin's central insight was that variation, heritability, and differential reproductive success are sufficient — without any additional ingredient — to produce change over time. Given:
- Individuals in a population differ from one another
- Some of those differences are heritable (passed from parent to offspring)
- Some differences affect the probability of surviving and reproducing
Then, over generations, the heritable traits that increase reproductive success will become more common. This is natural selection — not a force, not a plan, but the arithmetic consequence of differential reproduction.
What supplies the variation? Darwin didn't know; the mechanism (genetic mutation, recombination, and horizontal gene transfer in some organisms) was worked out in the twentieth century with the synthesis of genetics and evolutionary biology.
What supply the differences in reproductive success? Fitness in Darwin's sense — the ability to survive long enough to reproduce, and to reproduce successfully — is entirely relative to the environment. A trait that is advantageous in one environment can be neutral or lethal in another. "Survival of the fittest" does not mean survival of the strongest, fastest, or most complex. It means survival of whatever happens to work in a particular environment at a particular time.
Natural selection doesn't produce the "best" organism. It produces the organism that reproduces most under current conditions.
What Natural Selection Cannot Do
Natural selection cannot produce outcomes that would be beneficial but provide no reproductive advantage. An adaptation to a disease that will kill you after your reproductive years is selection-irrelevant. Selection cannot anticipate future environments; it operates strictly on present fitness.
Natural selection also cannot act on variation that is not heritable. A giraffe that develops a longer neck from years of reaching for leaves will not pass the acquired trait to its offspring (Lamarckian evolution). Selection works on genetic variants, not on acquired characteristics.
Genetic Drift
A second major mechanism — distinct from natural selection — is genetic drift: random changes in the frequency of genetic variants due to chance sampling effects.
In any finite population, some alleles will increase or decrease in frequency simply because some individuals reproduce and others don't, for reasons that have nothing to do with fitness. A beneficial mutation can be lost because its carrier happened to be eaten before reproducing. A neutral or even slightly deleterious mutation can spread to fixation in a small population purely by chance.
Genetic drift is most powerful in small populations and least important in large ones. The history of life includes many bottleneck events — episodes when populations crashed — during which drift could have significant effects on genetic diversity. Many characteristics of species may reflect the accidents of their ancestral population sizes rather than the fine-tuning of selection.
The Common Misconceptions
"Evolution is just a theory." In everyday usage, "theory" means a guess or speculation. In science, a theory is an explanatory framework supported by extensive evidence and capable of generating testable predictions. Evolutionary theory is a theory in the scientific sense — one of the most well-supported frameworks in biology.
"Humans evolved from monkeys." This misunderstands the branching structure of phylogeny. Humans and other great apes share a common ancestor — a species that was neither modern human nor modern chimpanzee. Modern primates and humans are cousin lineages; humans did not descend from any currently living primate species.
"Evolution has a direction toward complexity." The history of life shows both increases and decreases in complexity. Many parasites have reduced their genomes substantially over evolutionary time. Complexity evolves when it is selectively advantageous; it is lost when maintaining it is costly.
"Organisms evolve during their lifetimes." Individual organisms do not evolve. Populations evolve over generations, as the frequencies of heritable variants change. A bacterium that develops antibiotic resistance during a course of treatment has undergone somatic mutation, but this matters evolutionarily only if the trait is heritable.
Understanding evolution accurately matters not just for biology but for medicine (antibiotic resistance is an evolutionary problem), agriculture, and the broad question of where human beings fit in the natural world.
¹ Charles Darwin — On the Origin of Species (1859/1964), Harvard University Press ² Richard Dawkins — The Selfish Gene (1976/2016), Oxford University Press ³ Ernst Mayr — What Evolution Is (2001), Basic Books
