Natural selection represents a fundamental process in evolution, where organisms possessing traits better suited to their environment tend to survive and reproduce more successfully. This differential survival and reproduction lead to changes in the frequency of specific traits within a population across generations.
The Nature of Polygenic Traits
Polygenic traits are characteristics influenced by the cumulative effect of multiple genes. This means many genes contribute to the overall expression of the trait. Consequently, these traits typically exhibit continuous variation within a population, showing a wide range of possible phenotypes rather than distinct categories. For example, human height, skin color, and weight are classic examples of polygenic traits, displaying a smooth gradient of phenotypes across individuals.
Beyond genetic contributions, environmental factors can also significantly influence the expression of polygenic traits. The interplay between an individual’s genetic makeup and their environment shapes the final observable characteristic. This contrasts with Mendelian traits, which are usually determined by a single gene and show discrete, easily categorizable variations, such as the presence or absence of a specific genetic disorder.
Modes of Selection on Polygenic Traits
Natural selection can operate on polygenic traits in several distinct ways, each impacting the distribution of the trait within a population differently. These modes describe how selective pressures favor certain phenotypes over others, leading to evolutionary changes. The primary modes are directional, stabilizing, and disruptive selection, each having unique effects on the phenotypic distribution curve.
Directional Selection
Directional selection favors individuals at one extreme end of the phenotypic spectrum for a polygenic trait, shifting the average value of the trait in the population towards that favored extreme over successive generations. The distribution curve of the trait, which typically resembles a bell shape, will therefore move along the phenotypic range. This process leads to adaptation to new environmental conditions or continued improvement in response to ongoing selective pressures.
Stabilizing Selection
Stabilizing selection, in contrast, favors intermediate phenotypes and acts against individuals at both extreme ends of the phenotypic range. This mode of selection reduces variation within a population, as individuals with extreme traits are less likely to survive and reproduce. The mean value of the polygenic trait typically remains relatively unchanged under stabilizing selection. The distribution curve becomes narrower and taller around the mean.
Disruptive Selection
Disruptive selection, also known as diversifying selection, favors individuals at both extreme ends of the phenotypic range over intermediate phenotypes. This mode of selection can lead to an increase in phenotypic variation within a population and may eventually contribute to the formation of new species. The effect of disruptive selection on the distribution curve is the creation of two distinct peaks, with a dip in the middle where the intermediate phenotypes were once common. This bimodal pattern reflects the increasing divergence of the population into two distinct phenotypic groups. Over time, these groups may become so different that they no longer interbreed, leading to speciation.
Observing Selection in Action
Real-world observations provide clear illustrations of these modes of selection acting on polygenic traits. The patterns of change in traits like body size or coloration often reflect the influence of specific selective forces.
Directional selection is evident in the evolution of antibiotic resistance in bacteria. As antibiotics are increasingly used, bacteria with higher resistance are favored, leading to a shift in the bacterial population towards greater overall resistance. Similarly, the increasing height in human populations over recent centuries, influenced by improved nutrition and healthcare, shows a directional shift in this polygenic trait.
Stabilizing selection is well-illustrated by human birth weight. Babies with intermediate birth weights tend to have higher survival rates, while those who are either very small or very large face increased health risks. This consistent selection against extremes maintains the average birth weight within a relatively narrow range, reducing variation in this polygenic trait.
Disruptive selection can be observed in certain bird populations, such as African finches, where beak size is a polygenic trait. In environments with two distinct food sources—small, soft seeds and large, hard seeds—birds with intermediate beak sizes are less efficient at cracking either type. Consequently, birds with either very small beaks (for small seeds) or very large beaks (for large seeds) are favored, leading to two distinct beak size distributions within the same population.