Natural selection represents a fundamental mechanism driving evolutionary change across all forms of life. This process explains how populations of organisms adapt over generations to their environments. Understanding natural selection provides insight into the immense diversity of species observed on Earth. It clarifies how organisms acquire traits that enhance their survival and reproduction within specific ecological contexts.
Variation Within Populations
Individuals within any given population exhibit a wide range of differing characteristics. This inherent variability is a prerequisite for natural selection to occur. The primary sources of this diversity are genetic mutations, which are random changes in an organism’s DNA sequence, and genetic recombination, the shuffling of genetic material during sexual reproduction. These processes continuously introduce new traits into a population. Without this foundational diversity, there would be no differences for environmental pressures to act upon.
Inheritance of Traits
Many of the varying traits observed within a population are heritable, meaning they can be passed from parents to their offspring. This genetic material carries the instructions for developing various characteristics. The ability to inherit traits ensures that advantageous variations, if they arise, can be successfully transmitted to subsequent generations, allowing them to persist and increase in frequency.
Overproduction of Offspring
Most organisms produce more offspring than their environment can support. This widespread biological phenomenon leads to intense competition among individuals for limited resources such as food, water, shelter, and mates. The disparity between the number of offspring produced and the availability of resources creates a continuous “struggle for existence.” Not all individuals will survive long enough to reproduce.
Differential Survival and Reproduction
Given the overproduction of offspring and the limited resources, individuals possessing traits that better suit their environment are more likely to survive and successfully reproduce. This concept is termed differential survival and reproduction. Organisms with advantageous traits have a higher “fitness,” which refers to their reproductive success and ability to pass on genes. Environmental pressures, such as predation, climate, or resource scarcity, act as selective forces, favoring certain traits over others. Over time, this selective pressure leads to an increase in the frequency of beneficial traits within the population, resulting in adaptation.
Natural Selection in Action
Natural selection principles are evident in various biological phenomena, illustrating how populations change.
Antibiotic Resistance
One example is antibiotic resistance in bacteria. Some bacteria possess mutations conferring resistance. These resistant bacteria reproduce rapidly. When antibiotics are introduced, susceptible bacteria are killed, but resistant ones survive. These survivors reproduce, passing on resistance genes. This differential survival leads to populations dominated by antibiotic-resistant strains, making infections harder to treat.
Peppered Moth
The peppered moth (Biston betularia) illustrates natural selection during England’s Industrial Revolution. Light-colored moths were common, camouflaged against lichen-covered trees. Dark variants existed due to mutations. As industrial pollution darkened tree trunks, light moths became more visible to predatory birds, while dark moths became better camouflaged.
Overproduction meant many moths would not survive, and birds preferentially preyed on visible light moths. Dark moths had higher survival and reproduction, passing their dark coloration to offspring. This led to a dramatic increase in dark peppered moths, a phenomenon known as industrial melanism. When pollution was reduced, the trend reversed, and light moths became prevalent again.
Darwin’s Finches
Darwin’s finches on the Galápagos Islands provide an example of natural selection shaping traits. These finches exhibit variations in beak size and shape. Diverse food sources on the islands lead to competition. Finches with beaks better suited for particular food types, such as large, hard seeds or small, soft seeds, have a survival advantage.
During droughts, small, soft seed availability decreases, favoring finches with larger, stronger beaks that can crack harder seeds. These large-beaked finches are more likely to survive and reproduce, passing their beak traits to offspring. Over generations, this differential survival and reproduction leads to populations with beak sizes adapted to prevailing food sources on each island.