Lesson Notes By Weeks and Term v5 - Grade 12

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Performance objectives

• Define natural selection and its four components.
• Explain how natural selection leads to adaptation using South African examples.
• Differentiate between natural and artificial selection.
• Explain the development of antibiotic resistance.
• Interpret data illustrating natural selection.

Lesson summary

Evolution by natural selection is a cornerstone of modern biology. Understanding this process is crucial for comprehending the diversity of life around us, from the smallest bacteria to the largest mammals. It explains how populations change over time in response to their environment. In South Africa, understanding evolution is especially important in the context of conservation efforts, disease management (e.g., antibiotic resistance in bacteria, HIV evolution), and agriculture (e.g., developing drought-resistant crops). The principles of natural selection help us understand how species adapt to the unique challenges presented by South Africa’s diverse ecosystems and changing climate.

Lesson notes

2.1 What is Natural Selection? Natural selection is the process by which populations of living organisms adapt and change. It’s driven by the differential survival and reproduction of individuals with different traits. In simpler terms, organisms with traits that give them an advantage in their environment are more likely to survive, reproduce, and pass on those advantageous traits to their offspring. Over many generations, this leads to changes in the genetic makeup of the population, resulting in adaptation. 2.2 The Four Key Components of Natural Selection: Variation: Individuals within a population exhibit variation in their traits. This variation arises from mutations (changes in DNA) and sexual reproduction (which shuffles genes). Without variation, there's nothing for selection to act upon. For example, within a population of Aloe ferox plants, some might have thicker leaves than others, or some might flower earlier in the season.

Inheritance: Many traits are heritable, meaning they can be passed down from parents to offspring. This is due to the transmission of genes from one generation to the next. If the trait for thicker leaves in Aloe ferox is genetically determined, then offspring of plants with thicker leaves are more likely to also have thicker leaves. Differential Survival and Reproduction (Selection): Individuals with certain traits are more likely to survive and reproduce than others because those traits provide an advantage in their specific environment. This is often referred to as "survival of the fittest," but "fittest" in this context means best suited to the environment. If thicker leaves help Aloe ferox plants conserve water in a drought, then plants with thicker leaves will be more likely to survive the drought and reproduce.

Adaptation: Over time, the frequency of advantageous traits increases in the population. This leads to adaptation, where the population becomes better suited to its environment. Over generations, the Aloe ferox population in a drought-prone area will have a higher proportion of plants with thicker leaves. 2.3 Worked Example 1: Peppered Moths in England (and a South African Analogy) While the classic example of peppered moths in England is well-known, we can draw a parallel to situations in South Africa. Before the industrial revolution, light-colored peppered moths were more common because they were better camouflaged against the lichen-covered trees. Dark-colored moths were rare because birds easily spotted and ate them. As industrial pollution darkened the tree bark, the dark-colored moths became better camouflaged, and the light-colored moths became more vulnerable. As a result, the dark-colored moths became more common.

South African Analogy: Imagine a population of insects that feed on a specific type of Acacia tree. Suppose some insects have a mutation that gives them slightly darker coloration. If the Acacia trees in a particular area are growing in soil rich in iron, causing their bark to be a darker reddish-brown, the darker insects will be better camouflaged from predators. Over time, natural selection will favor the darker insects, and they will become more common in that area. 2.4 Worked Example 2: Antibiotic Resistance in Bacteria Antibiotics are drugs designed to kill bacteria.

However, bacteria can evolve resistance to antibiotics through natural selection.

Variation: Within a population of bacteria, some individuals may have genes that make them slightly more resistant to a particular antibiotic. This resistance may arise through a random mutation.

Inheritance: These resistance genes can be passed on to future generations of bacteria through reproduction (binary fission) and horizontal gene transfer (e.g., conjugation).

Differential Survival and Reproduction: When an antibiotic is used, most bacteria are killed, but the resistant bacteria survive.

Adaptation: The resistant bacteria reproduce and become more common, leading to a population of bacteria that is resistant to the antibiotic.

Calculation example (Conceptual): Imagine a population of 1000 bacteria. Initially, 10 are resistant to an antibiotic. After exposure to the antibiotic, the 990 non-resistant bacteria are killed. The 10 resistant bacteria survive and reproduce, doubling their population every hour. After 24 hours, if resources are unlimited, the population of resistant bacteria would be 10 * 2 24 = 167,772,

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0. This dramatic increase in resistant bacteria highlights the power of natural selection. The overuse and misuse of antibiotics in South Africa, both in human medicine and in agriculture, has contributed to a growing problem of antibiotic resistance, posing a serious threat to public health. For example, Multi-Drug Resistant Tuberculosis (MDR-TB) is a major concern in South Africa. 2.5 Natural Selection vs.