Natural Selection & Its Types - Practice Questions & MCQ

• Another mechanism for evolution is natural selection, which occurs when populations of organisms are subjected to the environment. 
• The fittest creatures are more likely to survive and pass their genes to their offspring, producing a population that is better adapted to the environment. 
• The genes of less-fit individuals are less likely to be passed on to the next generation.
• The important selective force in natural selection is the environment.
• There are several ways selection can affect population variation:
  • Stabilizing selection
  • Directional selection
  • Diversifying selection

Stabilizing Selection or Balancing Selection:

• It occurs when the environment selects against organisms of a population with extreme versions of a trait. 
• Stabilizing selection results in a decrease of a population‘s genetic variance.

Directional Selection or Progressive Selection:

• In this case, the environment selects for an extreme characteristic.
• The development of antibiotic-resistant bacteria in the modern era is an example of directional selection.
• Industrial melanism observed in England is a classical example of this type of selection.
• In a collection of moths made in the 1850s, i.e., before industrialization set in, it was observed that there were more white-winged moths on trees than dark-winged or melanized moths. 
• However, in the collection carried out from the same area, but after industrialization, i.e., in 1920, there were more dark-winged moths in the same area, i.e., the proportion was reversed.
• The explanation put forth for this observation was that ‘predators will spot a moth against a contrasting background’. 
• During the post-industrialization period, the tree trunks became dark due to industrial smoke and soots. 
• Under this condition the white-winged moth did not survive due to predators, dark-winged or melanized moth survived.
• Before industrialization set in, thick growth of almost white-colored lichen covered the trees - in that background the white-winged moth survived but the dark-colored moth was picked out by predators.
• Hence, moths that were able to camouflage themselves, i.e., hide in the background, survived.

Disruptive Selection or Diversifying Selection:

• Here, the environment favors extreme types in a population at the expense of intermediate forms, thereby splitting the population into two or more subpopulations. 
• In these cases, the intermediate phenotypes are often less fit than their extreme counterparts.