Natural selection is a mechanism for evolution. Simply put, the individuals best suited to a certain environment thrive and reproduce, while other, less adapted individuals die without reproducing. No environment can support an unlimited population, so individuals within the population have to compete with each other for resources and mates. Eventually, unfavorable traits can be bred out entirely. As the individuals in the population express more advantageous traits that make them more successful than their peers, these special individuals out survive and out mate their peers.
These are the basic principles of natural selection. While some of these were already being discussed at the time, most of the idea of natural selection is credited to Charles Darwin.
- Variation and Heredity: All individuals within a species are unique, with their own special variations in body and behavior. Some of these variations will be inherited by their offspring.
- Overproduction: More offspring will be produced than can survive.
- Limited Resources: No environment has unlimited carrying capacity. Survival and reproduction is determined by competition for limited resources, including habitat, mates, and food.
- Fitness: Some individuals have genetic variations that give them distinct advantages within their environment. These individuals will survive and pass on their traits better than those without such favorable traits.
- Genetic Composition: As a species moves forward by generations, each new generation will have a higher percentage of individuals with advantageous traits than the last.
Types of Natural Selection
There are five types of natural selection.
- Stabilizing Selection: This results in a decrease in a population's genetic variance and occurs when natural selection favors an average phenotype rather than variations.
- Diversifying Selection: This increases the genetic variation in a population and occurs when natural selection favors two or more extreme phenotypes that each have specific advantages.
- Directional Selection: This occurs when a population's genetic variance shifts toward a new phenotype and is caused by exposure to environmental changes.
- Kin Selection: This occurs when natural selection favors a trait that benefits related members of a group.
- Sexual Selection: Natural selection favors traits that maximize the ability of an individual to compete for mates and produce offspring. In the case of traits that help attract mates, like vibrant coloring, this can actually endanger the individual. While it does help attract mates, it also makes the individual easier to find by predators.
Natural Selection Script
1.Zoom: Top scene, left side
Hot Spot: Genes – Jeans
Information Mode Script: In this CoursePic, we examine natural selection and specifically focus on directional selection. Here, we see several people competing to be “King of the Hill” by climbing to the top wearing jeans, which reminds us of genes. The person on top, the winner, wears a crown to declare him the winner. He sports a pair of wings, suggesting this is how he could fly to the top of the hill.
Story Mode Script: The race is on. Levo’s Company is sponsoring a “King of the Hill” contest. Whoever climbs the famous Levo’s Hill will win a lifetime supply of jeans. The only hitch is that all contestants must be wearing jeans while they climb. Everyone has come out wearing their favorite pair of jeans and competing to reach the top. His wife meets him at the top and they embrace to celebrate the victory.
2. Zoom: Top scene, middle
Hot Spot: Genes - Jeans
Information Mode Script: The middle drawing depicts the passing on of genetic traits from generation to generation. Here, we see this shown with the contest winner’s newborn having wings and wearing purple genes, just like her father.
Story Mode Script: The King of the Hill and his wife proudly welcome a baby girl. Because of the father’s his superior skill of mountain climbing, some of his physical traits are passed on to their daughter. She inherited his wings as well as his special jeans, or genes.
3. Zoom: Top scene, right side
Hot Spot: Genes - Jeans
Information Mode Script: In directional selection, we see a steady trend toward the more dominant traits being more prevalent in the population. Thus, we see here that years later the competitors in the hill-climbing contest are all sporting wings and have similar genetic make-up as depicted by the purple jeans.
Story Mode Script: This next image is years into the future when the King of the Hill’s daughter is all grown up. She followed in her father’s athletic footsteps and has won Levo’s Jeans’ latest contest. These days, however, it seems everyone has better skills and here we see that all her competitors with wings and similar jeans (or genes).
4.Zoom: Bottom scene
Hot Spot: Directional – Director’s Chair
Information Mode Script: When characteristics that fall at one end of the phenotypic spectrum are favored over others, we refer to this as directional selection. Here, we see this depicted as a person sitting in a director’s chair orchestrating the events before him. His name, Cecil D. Rectional is meant to link the memory of Cecil B. DeMille and the words directional and director. The bugs wearing gas masks in the movie are not killed by the deadly spray. They mate, start a family, and pass their traits to survive on to the next generation.
Story Mode Script: Cecil D. Rectional is directing his first major production. Because he’s on a low budget, his actors are all bugs and the mythical alien he’s arranged to attack is just a giant bug spray canister. He directs two of his actors to put on a mask and then shouts “action.” All the unprotected bugs fall to their deaths. As it happens, the two actors that survive the scene fall in love and start a family. Their offspring inherit the trait of wearing gas masks and, as you may have guessed, are able to withstand any pollution to their air.