BFE - Evolution and Natural Selection (Lesson)
Evolution and Natural Selection
Biodiversity on Earth has been created through the process of evolution, the variation in the genetic makeup of a population of organisms through generations. Genetic changes take place in organisms within a population over many generations, occurring randomly or through the process of natural selection. Natural selection results when genetic traits that strengthen an organism's chance of survival and reproduction are passed on from generation to generation, ultimately altering the genetic makeup of a population, creating a new species. In the process of natural selection, some organisms in a population are better suited to survive than others; therefore, organisms that survive are those that have a reproduction advantage and are more likely to pass on their genes. Across time, entire populations contain traits that increase the likelihood to adapt, survive and reproduce.
Natural selection is the concept that individuals in a population exhibit variation in their heritable traits, and those organisms with traits that are best suited for the environment tend to produce more offspring than those that are less well suited. You may have heard the phrase, "survival of the fittest" to describe natural selection, but this phrase is often misleading, often putting people in the mind of animals fighting to the death to survive. The truth is that relative fitness depends on several different factors, such as the ability to gather food, protective coloration, or environmental adaptations. Each of these examples influences natural selection by determining how efficiently an organism can reproduce in a particular environment. Natural selection can alter the frequency distribution of heritable traits in three distinct ways depending on the influence of environmental factors.
Stabilizing Selection favors the intermediate phenotypes. For example, if bees preferred pink petunias over red or white, then the pink ones would be more fit and their population would increase while the red and white ones would decrease.
Directional Selection favors one extreme or the other. In the graph below, it seems that darker squirrels would be less frequently eaten, as they could hide in the shadows of tree limbs, and the population of darker-colored squirrels would increase.
Disruptive Selection favors extreme phenotypes. In the case of the petunias, if bees preferred either red or white, but not pink, then the pink ones would not be pollinated and would decline in population.
The traits that lead to success are adaptive traits or adaptations. For these traits to be passed on to offspring, genes in an organism's DNA must code for that trait. During DNA replication, which occurs in an organism's cells millions of times throughout its life, errors can occur. These errors lead to accidental alterations in DNA, called mutations. Most mutations don't have an effect, but some can be favorable and some can be fatal. The positive mutations can lead to a better ability to survive and reproduce in an environment, leading to natural selection. These mutations can occur through mating when genetic material combines during sexual reproduction.
Switching Genes On and Off
Please watch the video using the link below then complete a self-assessment using the flashcards that follow.
The environment plays a key role in determining which traits are beneficial; thus, an organism's environment, as well as genetic makeup, affects natural selection and ultimately the evolution of a species.
Island Biogeography Experiment
Background Information: Biogeography refers to the past and present geographic distribution of plants and animals in the world. The term "island" in this instance does not literally refer to an island. An "island" could be a literal island, but it can also be any ecosystem that is different from the ecosystems around it, such as a wetland in the urban community or a small rainforest surrounded by farmland.
Once the island reaches equilibrium, it will support a finite number of species. The diversity of those species mainly depends on three things:
- Extinction: how many species die out over time
- Immigration: how many new species move to the island from neighboring islands or landmasses
- Emigration: how many species leave the island to settle elsewhere
Time, climate, and isolation will also affect species diversity. The distance of an island or other isolated habitat from other habitats is an important contributor to the levels of immigration and emigration. If an island is close to a large mainland, more animals will arrive and colonize the island; whereas an island that is isolated or many miles away from the mainland will have less diverse species.
Click here to access and complete the Virtual Island Lab. Links to an external site.
RESOURCES IN THIS MODULE ARE OPEN EDUCATIONAL RESOURCES (OER) OR CREATED BY GAVS UNLESS OTHERWISE NOTED. SOME IMAGES USED UNDER SUBSCRIPTION.