Natural selection is the process where organisms better adapted to their environment tend to survive and reproduce more effectively, passing on their advantageous traits to the next generation. Over time, this leads to the increase in frequency of beneficial traits within a population, shaping and adapting species to their environment.
Let's go through 3 types of natural selection with suitable examples.
1. Stabilizing Selection:
This type of selection favors the intermediate variants of a trait and reduces the extremes. It occurs when the environment is stable and tends to select against extreme phenotypes.
Example:
Birth weight in humans can be subject to stabilizing selection. Babies born with very low birth weight might struggle with health issues, while babies born with extremely high birth weight might face delivery complications. As a result, there's a preference for infants with average birth weight, leading to higher survival rates for those with weights closer to the average.
2. Directional Selection:
This type of selection occurs when one extreme phenotype is favored over the other, causing a shift in the population towards that extreme.
Example:
Peppered moths during the Industrial Revolution in England experienced directional selection. Before industrialization, most moths were light-colored, camouflaging them against light tree bark. With industrialization and pollution darkening the trees, the dark-colored moths had better camouflage, leading to a higher survival rate for the darker variant. As a result, the frequency of dark-colored moths increased dramatically.
3. Disruptive Selection:
This type of selection favors both extreme phenotypes over the intermediate phenotype, often driven by environmental heterogeneity.
Example:
Beak size in Darwin's finches is a classic example. On an island with two distinct seed types – small seeds and large seeds – finches with intermediate beak sizes might struggle to crack either seed type. Birds with smaller beaks can efficiently crack small seeds, while those with larger beaks can handle large seeds. This environmental pressure leads to the persistence of both small-beaked and large-beaked birds, reducing the number of medium-beaked individuals.
These examples illustrate how natural selection can act differently under various environmental conditions, leading to changes in the frequencies of traits within populations.
These examples illustrate how natural selection can act differently under various environmental conditions, leading to changes in the frequencies of traits within populations.