Natural selection is the fundamental process driving evolution, demonstrating that life constantly adjusts to environmental pressures. Individuals within a population possess natural variation in traits like size, color, or enzyme function. When the environment changes, individuals with advantageous traits for survival or reproduction leave more offspring. Since these traits are heritable, they become more common over successive generations, leading to a population better adapted to the new conditions. This process is actively shaping the biological world today, often accelerated by human activity.
Pathogens Adapting to Medical Treatments
The rapid evolution of microbes in response to modern medicine represents a clear example of natural selection in action. Antibiotics create intense selective pressure on bacterial populations, rapidly shifting the balance toward resistance. When a patient takes an antibiotic, the drug kills off the majority of susceptible bacteria, but any individual bacterium possessing a random genetic mutation that offers even slight protection will survive.
These surviving bacteria, which are now resistant, face no competition and can rapidly multiply, quickly repopulating the infection site with a resistant strain. Bacterial populations evolve quickly because they often have short generation times, sometimes doubling in number every 20 minutes. Resistance mechanisms include mutations that alter the cellular target of the drug, or the acquisition of resistance genes from other bacteria through horizontal gene transfer.
This evolutionary arms race is evident in the rise of multi-drug resistant organisms, such as Methicillin-resistant Staphylococcus aureus (MRSA), which has become a major public health crisis. The initial application of methicillin selected for S. aureus strains that possessed a gene allowing them to neutralize the drug. Similarly, the bacterium responsible for tuberculosis has developed strains that are resistant to multiple first-line and second-line drugs, making treatment exponentially more difficult and expensive. The presence of an antimicrobial acts as the environmental filter that drives the evolution of resistance.
Chemical Resistance in Agriculture
Agricultural practices that rely on widespread chemical application create a powerful selective force, leading to the swift evolution of resistance in pests and weeds. The repeated use of a single type of pesticide or herbicide acts as a massive culling event, eliminating all but the most genetically protected individuals. These few survivors often possess pre-existing mutations that allow them to detoxify the chemical or alter the chemical’s target site within their bodies.
Because many agricultural pests, such as insects and weeds, have high reproductive rates, a few resistant individuals can rapidly produce a population dominated by the resistant genotype. For instance, the widespread adoption of the herbicide glyphosate has driven the evolution of what are commonly called “superweeds.” Species like waterhemp (Amaranthus rudis) have developed glyphosate-resistant strains that require farmers to use multiple, older, or more potent herbicides to achieve control.
The problem is not limited to weeds; over 500 species of insects, mites, and spiders have developed some level of pesticide resistance worldwide. The Alabama leafworm, a moth pest of cotton, has developed resistance to multiple classes of insecticides in various regions. This continuous cycle of applying chemicals, which selects for resistance, forces the development of new control methods, creating significant economic costs and environmental challenges for sustainable food production.
Rapid Trait Changes Driven by Climate and Urbanization
Human modification of the environment, specifically through climate change and the expansion of cities, is creating novel selection pressures that drive rapid evolutionary change in plants and animals. Climate change, for example, alters the timing of seasonal events, causing organisms to evolve shifts in their life cycles, known as phenology. One example is the pitcher plant mosquito, where northern populations are evolving to delay their genetically programmed hibernation period in response to rising global temperatures and longer growing seasons.
Other environmental shifts include changes in body characteristics. In some coral populations, rising ocean temperatures cause bleaching, but researchers have observed a shift toward corals that host algae less sensitive to heat, suggesting a reproductive advantage for heat-tolerant individuals. This demonstrates that even subtle, temperature-driven stressors can select for new traits within a species’ existing genetic material.
In urban areas, the physical structures and noise pollution of cities act as distinct selective agents, favoring individuals with traits suited to this concrete environment. The Puerto Rican crested anole lizard, for instance, has evolved longer limbs and more specialized toe pads (lamellae) compared to its forest relatives. This physical change provides a better grip and faster movement on the smooth, flat surfaces of walls and windows, which are common in city habitats.
Urban noise, particularly from traffic, has also driven changes in animal communication. Some urban bird species, such as the common blackbird, have been observed singing at a higher pitch than their rural counterparts. This higher frequency allows their calls to cut through the low-frequency background noise of the city, improving their ability to attract mates and defend territory, demonstrating selection based purely on acoustic environment.