Evolution is the change in heritable characteristics within a biological population over successive generations. This process concerns shifts in the overall traits of a population across time, not changes in an individual organism during its lifetime. The outcomes of this process are observable and provide clear evidence that a population’s inherited makeup has been altered.
Changes in Observable Population Traits
A direct outcome demonstrating evolution is a measurable change in the physical or behavioral traits of a population over time. These heritable alterations are passed from parents to offspring and often reflect the population’s adaptation to environmental pressures.
The industrial melanism of the peppered moth (Biston betularia) in Britain provides a clear example. Before the Industrial Revolution, the light-colored form was common because it was camouflaged against lichen-covered trees, while a rare, dark-colored form was easily spotted by predators. As industrial pollution blackened tree trunks, the lighter moths became more visible, while the dark moths became better camouflaged.
This environmental shift created a selective pressure where dark moths were more likely to survive and reproduce, passing the gene for dark coloration to their offspring. Over several decades, the population’s composition changed. In some industrial areas, the dark form increased from being rare in 1848 to making up 98% of the population by 1895. This shift in the common coloration is an observable outcome of evolution by natural selection.
Evolutionary Outcome: Antibiotic Resistance
The rise of antibiotic resistance in bacteria is a modern outcome of evolution. This process shows how a population changes when faced with an environmental pressure like an antibiotic, resulting in a population that is no longer affected by a previously effective drug.
Within any large bacterial population, natural variation exists due to random mutations. A small number of bacteria may possess a gene that provides resistance to an antibiotic. When the antibiotic is introduced, it eliminates susceptible bacteria, but the few resistant individuals survive.
The resistant survivors reproduce, passing their resistance genes to their offspring through direct inheritance or horizontal gene transfer. With each generation, the proportion of resistant bacteria increases. Over time, the population becomes dominated by resistant individuals, rendering the antibiotic ineffective.
This shift is a result of natural selection. The antibiotic does not create resistance but instead selects for pre-existing resistance, allowing those individuals to thrive. The emergence of multi-drug-resistant strains like MRSA demonstrates how quickly populations can adapt under intense selective pressure.
Genetic Evidence of Evolutionary Change
While observable traits provide visual evidence, a documented change in a population’s genetic makeup is a more direct measure of evolution. Evolution, at its core, is a change in the frequency of alleles—different versions of a gene—within a population over generations. Tracking these frequencies provides a quantifiable measure of this change.
Scientists collect genetic samples from a population at different times and use DNA sequencing to calculate allele frequencies. A shift in these frequencies signals that the population has evolved. For example, with the peppered moths, the allele for dark coloration became more common in industrial areas as the allele for light coloration decreased, producing the observable change in color.
This genetic perspective unifies all other evidence of evolution. The development of antibiotic resistance is fundamentally the increasing frequency of resistance alleles in a bacterial gene pool. Likewise, changes in finch beak sizes are driven by shifts in the frequencies of genes that control beak shape. Therefore, the most precise outcome demonstrating that a population has evolved is the measured change in its allele frequencies.