Disruptive Selection Definition
Disruptive selection is an evolutionary force that drives a population apart. The disruptive selection will cause organsisms with intermediate traits to reproduce less, and will allow those organisms with extreme traits to reproduce more. This causes the alleles for the extreme traits to increase in frequency. Over time, and with enough disruptive selection, a population can be completely divided. When this happens, the two populations can become diverse enough to form separate species. In the most basic sense, disruptive selection can act on a single gene, selecting between the different alleles present in a population. On a much broader level, disruptive selection can affect a variety of traits and drive a population to become reproductively isolated from the original population.
Disruptive selection, also called diversifying selection, is based on the variance of a trait in a population. A gene with only one allele would have no variance, and selection could not act on differences in the trait created by the gene. Most genes have many different alleles, which create a wide variety of functions. The result of these many alleles acting in a population and the other genes in affect lead to traits that have a distribution of different types, sizes, or patterns. If the trait has an almost infinite variety of different forms, it is continuous. If the trait exists in distinct entities, it is discrete. A continuous trait would be height, while a discrete trait might be eye color. Either way, most traits have a high level of variance, due to the interactions of various genes and alleles. Disruptive selection acts on the traits in the middle of the spectrum.
Disruptive selection is usually seen in high-density populations. In these populations resources become scarcer, and competition for the resources increases. This intraspecific competition can cause differences between organisms to have a more profound effect on each organism’s survival. Selective pressures that might not have factored into a low-density population can take effect, and the resulting disruptive selection can drive a population apart. In doing so, the populations are often pushed to different niches, lowering the competition between them. This leads to sympatric speciation, or speciation that occurs while populations occupy the same area.
Examples of Disruptive Selection
Finches on Santa Cruz Island
Darwin’s finches, or Galapagos finches, are a group of finches that inhabit the long chain of islands known as the Galapagos, famously visited by Charles Darwin. The birds have been rigorously studied, and various patterns of evolution have been seen in different populations on different islands. On Santa Cruz Island, disruptive selection was seen causing speciation in the population of finches that resides there. Due to forces of disruptive selection, intermediate beak sizes have been selected against for generations. The resulting population has almost no medium sized beaks. Beak size is important for more than just gathering food, and it has been found that beak size also changes the mating calls of the various finches. Researchers have found that the populations of birds, once a single population, have genetically diverged and are on the tipping point of being considered separate species.
Disruptive Selection in Plants
Famous biologist John Maynard Smith proposed disruptive selection as a method for plant speciation in the late 1960s. The idea is simple and have been applied to many examples since. Many plant traits, such as the color of pea pods, are controlled by individual genes. In a scenario where disruptive selection is affecting a population of plants, the most intermediate individuals are often the heterozygous individual, or those that contain different types of alleles for a gene. Homozygous individuals, on the other hand, have two of the same alleles for a trait. Whether the allele is functional or not, two of the same will produce a phenotype on the extreme end of the spectrum. These individuals will be protected during the disruptive selection, and reproduce more. Over time, the organisms may differ so much that they become reproductively isolated. Often the intermediates were serving the function of transferring genes between the two populations. Without them, in the presence of disruptive selection, speciation can occur.