Natural Selection Mechanism

My AP Biology Thoughts

Episode #2

Welcome to My AP Biology Thoughts podcast, my name is Jacqueline and I am your host for Episode 2 called the Natural Selection Mechanism. Today we will be discussing the 5 components of the mechanism, and how they ultimately lead to evolution.

Segment 1: Introduction to Natural Selection

• Natural selection, as you probably already know, is the process in which organisms who are better adapted to their environments and have higher fitness pass on their traits to offspring. It is a driving force of evolution, which is the change in the genetic and allele frequencies in a species' gene pool over time. Charles Darwin and Alfred Wallace were the co-discoverers of the theory of Natural selection, although Darwin is most often credited as the sole contributor. Natural selection may be one concept, but it is a broad one, and it can be split into five major components: overpopulation, variation, competition, fitness, and reproduction. These five make up what is known as the natural selection mechanism.

Segment 2: More About Natural Selection

• As I explain the natural selection mechanism, I’ll be using the example of Canada geese. Let’s start with overpopulation. Overpopulation is the occurrence where a species’s population increases beyond its habitat’s carrying capacity. It is the rather simple first step of the mechanism, but it sets up a chain reaction of more complicated events. Let’s say the population of Canada geese living in a lake habitat has grown to the point where their aqueous plant food source has become limited and can no longer sustain all of them. Overpopulation has occurred.
• One side effect of overpopulation and the second mechanism of natural selection is variation. As more members of a species are born, the genetic and physical variance in that population will increase. This occurs often due to random mutations, which can introduce new traits into populations. It may also happen because of immigration of another population (of that same species) but with different genes into the area, known as gene flow. Basically, more members of a species means more variation of genotypes and phenotypes in the population. A wider array of different traits will be developed among them, some of which may convey advantages or disadvantages for the organism. Now let’s say that the Canada Geese population, which is experiencing an influx in birth rate (AKA overpopulation, the first part of the mechanism), is also more likely to have random mutations, which will lead to increased variation of traits. Short, medium, and long necks are all now traits prevalent in the species.
• Another side effect of overpopulation and the third mechanism of natural selection is competition. When there are too many organisms and limited resources, individuals of a species must fight the others around them for those necessities, or risk dying out. However, only the most adept will be able to survive and ultimately reproduce. This ability is known as fitness, and it is the 4th mechanism of natural selection. Being able to outcompete the competition and survive to reproductive age to pass on one’s genes is the prime signifier of greater biological fitness. These individuals often have advantageous genes or traits that give them a leg up against rivals and are more likely to be inherited by future generations. The Canada geese population has breached carrying capacity and now has too small of a plant food source for too large of a group. The geese begin to compete among themselves for the resource. The geese with the trait of longer necks have greater fitness because they are able to reach the plants easier and outcompete those with shorter necks, who die out before reproduction. The surviving long-neck geese reproduce and are officially deemed more fit.
• Reproduction is the final component of the...