READ: Charles Darwin and Natural Selection

Natural Selection and Adaptation

The Theory of Evolution by Natural Selection means that the inherited traits of a population change over time through natural selection. Inherited traits are features that are passed from one generation to the next. For example, your eye color is an inherited trait (you inherited from your parents). Acquired traits are features such as strong muscles from working out.

Natural selection happens when some organisms have traits that make them better suited (they have better accommodation) to live in a certain environment than others. They are more likely to survive, reproduce and pass their traits on to future generations than those without the special traits. The process of natural selection helps us understand how organisms appear to be so well suited or adapted to their environments. Every plant and animal depends on its traits to survive. Survival may include getting food, building homes, and attracting mates. Most of these traits have been changed through natural selection so they allow a plant, animal, or bacteria to survive and reproduce relatively well in their environments. These traits are called adaptations. As environments have changed considerably over time, organisms must constantly adapt to those environments. It is the great diversity of species that increases the chance that at least some organisms adapt and survive any major changes in the environment.

Imagine how in winter dark fur makes a rabbit easy for fox to spot and catch in the snow. Natural selection suggests that white-fur is an advantageous trait that improves the chance that a rabbit will survive, reproduce and pass the trait of white fur on to future generations. Dark fur rabbits will become uncommon.

Polygenic Inheritance and Natural Selection

But natural selection leading to evolution does not just select for certain individuals, it selects for groups. More than one individual must adapt to the environment to maintain a population. Natural selection determines which groups of organisms survive, based on their traits, and which do not, that is, natural selection determines the differential survival of groups of organisms.

Although some traits are determined by a single gene, many are influenced by more than one gene (polygenic). The result of polygenic inheritance is a continuous spectrum of phenotypic values which often show a bell curve pattern of distribution.

Given this pattern of phenotypic variability, natural selection can take three forms. We will use the hypothetical color distribution in this figure to illustrate the three types of selection.Directional selection shifts the frequency curve away from the average by favoring individuals with an extreme form of the variation. The curve would still be bell-shaped, but it would have shifted to the left or right, in the direction of the lighter or darker alleles. Stabilizing selection selects for a group of phenotypically average individuals, with individuals with either extreme phenotype selected against. Disruptive selection selects for groups of individuals with extreme phenotypes, selecting against individuals with the average phenotype.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/