Evolution describes how species change, driven by natural selection. For a population to evolve, some individuals must have traits that give them an advantage in survival and reproduction. Directional selection and sexual selection are two concepts that explain how this change can unfold over generations. While one focuses on survival and the other on mating, they are interconnected forces that shape the diversity of life.
Directional Selection: Favoring an Extreme
Directional selection is a pattern of evolution where one extreme version of a trait is favored over others. This preference causes the genetic makeup of a population to shift progressively in that single direction. A clear example is the evolution of the peppered moth (Biston betularia) in England during the Industrial Revolution. Before this period, most of these moths were light-colored with dark speckles, which provided camouflage against the lichen-covered trees where they rested.
The widespread burning of coal produced heavy soot that killed the light-colored lichens and blackened the tree bark. This environmental change made the light-colored moths stand out to predatory birds, while the rare, dark-colored moths were now better camouflaged. As a result, the dark moths survived and reproduced at a much higher rate.
This pressure from predators drove a rapid evolutionary change. In pre-industrial England, the dark form was rare, but by 1895 in Manchester, it was reported to make up 98% of the peppered moth population. This increase in the frequency of the dark-colored allele is a case of directional selection. The process reversed when clean air laws reduced pollution, allowing lichens to grow back and favoring the light-colored moths again.
The increasing resistance of bacteria to antibiotics is another example. As antibiotics are used, they eliminate the most susceptible bacteria, leaving behind those with mutations that allow them to survive. This pressure favors the most resistant variants, pushing the entire bacterial population toward greater resilience.
Sexual Selection: The Drive to Reproduce
Sexual selection is a mode of natural selection where the driving force is not survival, but the ability to secure a mate. This process can lead to the evolution of traits that may not help an organism survive and can even be detrimental. These traits are favored because they increase an individual’s reproductive success. Sexual selection operates through two mechanisms: competition between members of the same sex (intrasexual selection) and mate choice by the opposite sex (intersexual selection).
An example of intersexual selection is the elaborate tail of the male peacock. The long, iridescent train is energetically costly to grow and makes the male more conspicuous to predators. It evolved because peahens prefer to mate with males who have the most impressive tails. Experiments have shown that the offspring of males with more elaborate tails tend to be healthier, suggesting the tail is an honest signal of the male’s genetic quality.
Intrasexual selection is displayed in the behavior of male red deer during the mating season. Stags grow large antlers, which they use as weapons in contests with other males to gain control over a group of females. Winning these fights directly translates to mating opportunities, so there is pressure for males to evolve larger bodies and more formidable antlers.
The Overlap: When Mating Preference Drives Direction
The similarity between sexual and directional selection emerges because sexual selection is a process that often produces a directional pattern. The consistent preference of one sex for a particular trait in the other acts as a selective pressure. This pushes the evolution of that trait in a single direction, creating the same outcome as classic directional selection, even though the source of the pressure is different.
In this way, the collective preference of peahens or the competition among stags acts as the filtering mechanism, much like predators did for the peppered moths. The “environment” for the moth was the tree bark, while the “environment” for the peacock is the peahens’ preference. The pressure to outcompete rivals or attract mates shapes evolution just as powerfully as pressure from a physical environment, pushing traits in one direction.
Both selection types describe a process where certain traits are consistently favored, leading to a predictable change in a population. Whether the pressure comes from a predator or the preference of a potential mate, the outcome is a directed evolutionary shift. Sexual selection is a form of natural selection where the “fittest” individual is the one that most successfully reproduces, often driving evolution in a clear direction.