Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Explain the process of natural selection.
• Apply the concept to real-world examples.
• Differentiate between natural and artificial selection.
• Describe the role of environmental factors.
• Interpret data related to natural selection.

Lesson summary

This week, we delve into the heart of evolutionary biology: natural selection. Understanding natural selection is crucial because it explains the diversity of life we see around us, from the smallest bacteria to the largest elephants, and even the variations within our own human population. In South Africa, with its incredibly rich biodiversity, grasping the principles of natural selection helps us appreciate the delicate balance of ecosystems and the importance of conservation efforts. It also directly relates to issues like antibiotic resistance in bacteria, which is a growing concern in our healthcare system.

Lesson notes

2.1 The Foundation: Variation, Inheritance, and Competition Natural selection operates on the principles of variation, inheritance, and competition.

Variation: Within any population of organisms, there is inherent variation. This means that individuals are not identical; they differ in their traits (physical characteristics, behaviours, etc.). These variations arise through random mutations in DNA during replication and through sexual reproduction, which shuffles genes.

Inheritance: Many traits are heritable, meaning they can be passed down from parents to offspring. This happens through the transmission of genes. Only heritable traits can be subject to natural selection. Traits acquired during an organism's lifetime (like muscle mass gained through exercise) are not heritable and therefore not relevant to natural selection (although epigenetic changes can be passed down).

Competition: Resources in the environment are limited. Organisms compete for these resources (food, water, shelter, mates). Not all individuals will survive and reproduce. This competition is the driving force behind natural selection. 2.2 The Process of Natural Selection: Differential Survival and Reproduction Natural selection is the process where individuals with traits that are better suited to their environment are more likely to survive, reproduce, and pass on those advantageous traits to their offspring. This leads to a gradual change in the genetic makeup of the population over time. This is often described as "survival of the fittest," but "fittest" in this context means "best adapted to the current environment." Differential Survival: Individuals with advantageous traits are more likely to survive challenges in their environment (e.g., predators, disease, drought).

Differential Reproduction: Surviving is not enough. To pass on their genes, individuals must also reproduce. Individuals with advantageous traits are more likely to reproduce successfully. This means they are more likely to find mates, produce more offspring, and/or have offspring that survive to reproduce themselves. 2.3 Examples of Natural Selection Antibiotic Resistance in Bacteria: This is a very relevant example in South Africa, where rates of antibiotic resistance are high. Initially, a population of bacteria may be mostly susceptible to an antibiotic.

However, due to random mutation, some bacteria may have a gene that makes them resistant. When the antibiotic is used, the susceptible bacteria are killed, but the resistant bacteria survive and reproduce. Over time, the population becomes dominated by resistant bacteria.

Example Calculation: Imagine a bacterial population of 10,

0

0

0. Initially, 10 are resistant to a certain antibiotic. The antibiotic is applied, killing 9990 susceptible bacteria. The 10 resistant bacteria now reproduce rapidly, and their offspring are also resistant. After a few generations, the population could be almost entirely resistant. The selection pressure is the presence of the antibiotic.

Camouflage in Insects: In many ecosystems, insects that blend in with their surroundings are more likely to survive and avoid predation. For example, stick insects resemble twigs, and moths can have colour patterns that match the bark of trees. Individuals with better camouflage are more likely to survive and reproduce, passing on their camouflage genes to their offspring.

Drought Resistance in Plants: In arid regions of South Africa (like the Karoo), plants that are better able to tolerate drought conditions (e.g., by having deep roots, thick leaves, or the ability to store water) are more likely to survive and reproduce during periods of drought. Over time, populations of these plants will become increasingly drought-resistant. 2.4 Natural Selection vs.

Artificial Selection Natural Selection: The environment determines which traits are advantageous.

Artificial Selection: Humans select which traits are desirable and breed organisms with those traits. This is also called selective breeding.

Example:* South African farmers have selectively bred livestock (e.g., cattle, sheep, goats) for traits such as meat production, milk yield, or wool quality. Maize crops have also been selectively bred for higher yields and disease resistance. 2.5 The Role of Environmental Factors Environmental factors (climate, resources, predators, etc.) play a crucial role in driving natural selection. Changes in the environment can lead to changes in the selection pressures, favouring different traits.

Example:* If a region experiences a prolonged drought, plants with drought-resistant traits will be favoured. Conversely, if rainfall increases, plants with traits that are better suited to wetter conditions may become more common. Guided Practice (With Solutions)

Question 1: A population of peppered moths lives in an area with both light-coloured and dark-coloured tree bark.