Human Impact on Natural Selection and Evolution
- Human activities have become a major driver of selective pressures on species, altering their evolutionary paths.
- These pressures can cause evolutionary change within relatively short time spans compared to natural evolution.
Selective pressure
Selective pressure refers to any environmental factor (natural or human-made) that influences which traits are favorable for survival and reproduction in a population.
Over time, these human-induced pressures lead to evolutionary changes in species, sometimes in unexpected ways.
Climate Change and Evolutionary Pressure
- Climate change, mainly due to fossil fuel combustion, alters global temperature and rainfall patterns, forcing rapid adaptation.
- Organisms are now evolving under new environmental stressors, such as heat tolerance or altered breeding cycles.
- Polar bears are evolving smaller body sizes and altered hunting patterns due to reduced sea ice.
- Individuals able to travel farther or find land-based food sources are now more likely to survive and reproduce.
Rapid environmental changes mean not all species can adapt quickly enough, resulting in increased extinction rates.
Hunting, Poaching, and Artificial Selection in the Wild
- Human hunting directly changes trait frequencies in animal populations.
- Overharvesting large individuals (e.g., fish, elephants) selects for smaller body sizes or tuskless phenotypes.
Tuskless Elephants in Gorongosa, Mozambique
Cause: Intense Poaching During Civil War
- During the Mozambican Civil War (1977–1992), poachers killed elephants for their ivory to fund armed groups.
- Elephants with large tusks were more likely to be killed, leaving behind tuskless individuals.
- Over generations, the tuskless trait became more common, as tuskless females had a higher chance of survival and reproduction.
Outcome: Rapid Evolutionary Shift
- Before the war, tuskless elephants made up ~2% of the population.
- After intense poaching, 50% of females in Gorongosa National Park are now tuskless.
- This is an example of human-driven natural selection, where selective pressure from poaching changed the genetic makeup of the population.
Habitat Modification and Fragmentation
- Urbanization, agriculture, and deforestation fragment habitats, isolating species populations.
- Fragmented populations face genetic drift, limited gene flow, and novel selective pressures.
- Species that thrive in human-dominated areas evolve traits like:
- Tolerance to pollution
- Adaptation to noise and artificial light
- Altered feeding habits (e.g., scavenging human waste)
City-dwelling birds may evolve higher stress tolerance or altered singing frequencies to compete with urban noise.
NoteFragmentation reduces gene flow and creates isolated populations, a key driver of allopatric speciation.
Introduction of Invasive Species
- Humans transport non-native species globally, intentionally or accidentally.
- Invasive species create competitive or predatory pressures, forcing native species to evolve defensive mechanisms or face extinction.
Cane toads introduced in Australia have led to native predators evolving toxin resistance within a few decades.
