Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Define natural selection and its key components.
• Explain how environmental changes drive natural selection.
• Apply these principles to antibiotic and pesticide resistance.
• Analyse data related to natural selection.
• Evaluate the evidence supporting the theory.

Lesson summary

Evolution by natural selection is a cornerstone of modern biology, explaining the diversity of life we see around us. Understanding this process is not just about memorizing facts; it's about grasping how living things, including humans, have adapted and changed over immense periods of time. In South Africa, with its incredibly rich biodiversity, understanding evolution helps us appreciate and conserve our natural heritage. Moreover, understanding natural selection is crucial for addressing challenges like antibiotic resistance in bacteria and pesticide resistance in agricultural pests, issues that directly impact the health and well-being of South African communities.

Lesson notes

What is Natural Selection? Natural selection is the process by which populations of living organisms adapt and change. Individuals in a population are naturally variable, meaning that they are all different in some ways. This variation means that some individuals have traits better suited to the environment than others. Individuals with adaptive traits—traits that give them some advantage—are more likely to survive and reproduce. These individuals then pass the adaptive traits on to their offspring. Over time, these advantageous traits become more common in the population. Through this process of natural selection, a population will gradually adapt to its environment.

Key Components of Natural Selection: Variation: Individuals within a population exhibit variation in their traits (phenotype). This variation arises from genetic mutations and sexual reproduction. Without variation, there's nothing for natural selection to act upon.

Inheritance: Traits are heritable, meaning they can be passed down from parents to offspring through genes. This ensures that successful traits are passed on to the next generation.

Differential Survival and Reproduction: Individuals with certain traits are more likely to survive and reproduce than others in a specific environment. This is often referred to as "survival of the fittest," but it's more accurately "reproductive success of the fittest." Fitness refers to an organism's ability to survive and reproduce in its environment. It's not necessarily about being the strongest or fastest.

Adaptation: Over generations, the frequency of advantageous traits increases in the population, leading to adaptation to the environment. Adaptation is the process by which an organism becomes better suited to its environment.

Examples to Illustrate Natural Selection: Antibiotic Resistance in Bacteria: Antibiotics are used to kill bacteria.

However, in any population of bacteria, there may be some that have genes that make them resistant to the antibiotic. When antibiotics are used, most of the bacteria are killed, but the resistant bacteria survive and reproduce. Over time, the population of bacteria becomes resistant to the antibiotic.

Variation: Some bacteria have natural resistance to certain antibiotics due to random mutations in their DN

A. Inheritance: This resistance is coded for in the bacteria's DNA and passed on during binary fission (bacterial reproduction).

Differential Survival and Reproduction: When antibiotics are present, non-resistant bacteria die, while resistant bacteria survive and reproduce at a higher rate.

Adaptation: Over time, the population shifts to become predominantly antibiotic-resistant. Worked

Example: Let's say we start with 1000 bacteria, and 10 of them are naturally resistant to a particular antibiotic. We treat the bacteria with the antibiotic. 990 of the non-resistant bacteria die, leaving 10 resistant and 10 non-resistant bacteria. These remaining 20 bacteria reproduce, and because the environment is still under antibiotic pressure, the resistant ones have a higher reproduction rate. After a few generations, the resistant bacteria will significantly outnumber the non-resistant ones. This is evolution in action.

Pesticide Resistance in Insects: Similar to antibiotic resistance, pesticide resistance occurs when insects develop resistance to pesticides. Farmers spray pesticides to kill insects that damage crops.

However, some insects may have genes that make them resistant to the pesticide. When the pesticide is sprayed, most of the insects are killed, but the resistant insects survive and reproduce. Over time, the population of insects becomes resistant to the pesticide.

Variation: Some insects have mutations that make them less susceptible to the pesticide's effects.

Inheritance: These mutations are passed on to their offspring.

Differential Survival and Reproduction: Insects with resistance survive pesticide application and reproduce more successfully.

Adaptation: The insect population evolves to become resistant to the pesticide, requiring farmers to use stronger or different pesticides. This can lead to environmental problems. Worked

Example: Imagine a maize field infested with maize stalk borers. A farmer sprays pesticide X. Initially, 95% of the borers die.

However, the remaining 5% are naturally resistant (due to genetic variation). These survivors reproduce, passing on their resistance genes. After a few generations and repeated pesticide applications, the pesticide is far less effective because the borer population is now largely resistant. This forces the farmer to use more harmful pesticides, increasing costs and potentially harming beneficial insects. Industrial Melanism in Peppered Moths ( Biston betularia): This is a classic example. Before the Industrial Revolution in England, most peppered moths were light-colored, which helped them camouflage against lichen-covered trees.