Natural selection is a fundamental process driving evolution, where organisms better suited to their environment tend to survive and produce more offspring. This results in the gradual increase of advantageous heritable traits within a population over generations. It acts on the variations present within a group of organisms, allowing those with beneficial characteristics to pass them on.
Directional Selection
Directional selection occurs when individuals at one extreme of a phenotypic range have higher survival and reproductive rates than other individuals. This process leads to a shift in the average trait value of a population over time, moving it towards the favored extreme. The environment exerts pressure, favoring traits that become advantageous under new conditions.
Antibiotic resistance in bacteria is a common example. When a bacterial population is exposed to an antibiotic, most susceptible bacteria die. However, any bacteria possessing a random mutation that confers resistance can survive and reproduce. Resistant bacteria become more prevalent in the population, shifting the average susceptibility towards resistance.
Stabilizing Selection
Stabilizing selection favors intermediate variants in a population, reducing phenotypic variation. This type of selection acts against individuals at both extremes of the phenotypic range, promoting traits that are well-suited to a stable environment. It helps maintain the status quo for certain characteristics, ensuring that the most common form of a trait remains dominant.
Human birth weight illustrates stabilizing selection. Babies born with very low or very high birth weights tend to have higher mortality rates. Infants of intermediate birth weight generally experience the highest survival rates. This selective pressure maintains an average birth weight within the human population.
Disruptive Selection
Disruptive selection favors individuals at both extremes of the phenotypic range over intermediate phenotypes. This pattern can lead to the divergence of two distinct forms or morphs within a single population. It often occurs in heterogeneous environments where different niches or resources are available, each favoring a different extreme trait.
The beak size of the black-bellied seedcracker in Cameroon provides an example. These birds feed on small, soft seeds and large, hard seeds. Birds with very small beaks are efficient at cracking small seeds, or very large beaks are adept at handling large seeds. Individuals with intermediate beak sizes are less efficient at processing either type of seed, leading to selection against them and favoring the two extreme beak sizes.
How These Types Drive Evolution
These three distinct patterns of natural selection contribute significantly to evolution and biodiversity. Each type influences the genetic makeup of populations, dictating which traits become more common or less common over generations. Directional selection leads to significant changes in a population’s characteristics, while stabilizing selection acts to preserve existing adaptations. Disruptive selection, on the other hand, can be a driving force behind speciation, where a single population splits into two or more distinct species. These types of selection are not mutually exclusive and can occur in different populations or at different times, often in response to environmental shifts.