Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Define natural selection and its four key principles.
• Explain how natural selection leads to adaptation.
• Describe the different types of selection: stabilizing, directional, and disruptive.
• Explain how environmental and human factors influence natural selection.
• Apply these principles to examples from South Africa's biodiversity.

Lesson summary

Evolution by natural selection is the cornerstone of modern biology. It explains the diversity of life we see around us, from the smallest bacteria to the largest blue whale, and how all these organisms are related. In South Africa, understanding evolution is particularly crucial. It helps us appreciate the unique biodiversity of our country, understand the impact of human activities on ecosystems, and address health challenges like antibiotic resistance and the evolution of viruses.

Furthermore, understanding the science behind evolution helps combat misinformation and promote informed decision-making regarding issues like conservation and public health.

Lesson notes

What is Natural Selection? Natural selection is the process by which organisms with traits that better enable them to survive and reproduce in a particular environment tend to leave more offspring, thereby increasing the frequency of those advantageous traits in the population over time. It's often summarized as "survival of the fittest," but "survival of the fittest (best adapted) to their environment" is a more accurate description. It is NOT a random process; it's driven by environmental pressures. The Four Key Principles of Natural Selection: Variation: Individuals within a population exhibit variation in their traits. This variation arises from mutations (random changes in DNA) and sexual reproduction (which shuffles genes). Without variation, there's nothing for natural selection to act upon. Think of different colours of beetles in a population – some might be green, some brown, and some black.

Inheritance: Traits are heritable, meaning they can be passed down from parents to offspring through genes. Offspring tend to resemble their parents in terms of these traits. If the colour of beetles is determined by genes, then green beetles are more likely to have green offspring. Differential Survival and Reproduction (Selection): In a given environment, some individuals with certain traits are more likely to survive and reproduce than others. This is where the "selection" part comes in. Individuals with advantageous traits have a higher "fitness," meaning they contribute more offspring to the next generation. Imagine that birds find brown beetles easier to see and eat on green foliage. Green beetles will survive and reproduce at a higher rate.

Adaptation: Over time, the frequency of advantageous traits increases in the population, leading to adaptation. The population becomes better suited to its environment. After many generations, the beetle population may be predominantly green.

Types of Selection: Stabilizing Selection: Favors intermediate phenotypes (traits) and selects against extreme phenotypes. This reduces variation in the population.

Example: Birth weight in humans. Babies with very low or very high birth weights have a higher risk of complications and death. Babies with average birth weights tend to have the best survival rates.

Directional Selection: Favors one extreme phenotype over the other. This shifts the distribution of traits in one direction.

Example: Antibiotic resistance in bacteria. When bacteria are exposed to antibiotics, most are killed, but some may have mutations that make them resistant. These resistant bacteria survive and reproduce, leading to a population of bacteria that is predominantly resistant to the antibiotic. This is a huge concern in South Africa with high rates of TB and HIV co-infection leading to drug resistant strains.

Disruptive Selection: Favors both extreme phenotypes and selects against intermediate phenotypes. This can lead to the formation of two distinct subpopulations.

Example: African seedcracker finches. These birds have beaks of different sizes, adapted to cracking different types of seeds. Birds with intermediate-sized beaks are less efficient at cracking either type of seed and are therefore at a disadvantage.

This can lead to two distinct groups: birds with large beaks and birds with small beaks.

Worked example

Example 1: Peppered Moths in Industrial England

Before the Industrial Revolution, most peppered moths were light-colored, blending in with the lichen-covered trees. Dark-colored moths were rare.

However, as industrial pollution darkened the trees, the light-colored moths became more visible to predators, while the dark-colored moths were better camouflaged. As a result, the dark-colored moths survived and reproduced at a higher rate, leading to a shift in the population towards more dark-colored moths.

Variation: Light and dark-colored moths

Inheritance: Color is genetically determined

Selection: Predators preferentially preyed on light-colored moths on darkened trees

Adaptation: Increased frequency of dark-colored moths in the population

Example 2: Pesticide Resistance in Mosquitoes (Malaria Vector)

Malaria is a major health problem in South Africa. Mosquitoes that transmit malaria are often controlled with pesticides.

However, over time, mosquito populations have evolved resistance to these pesticides. Some mosquitoes have genes that allow them to break down the pesticide or reduce its effect. These resistant mosquitoes survive and reproduce, passing on their resistance genes to their offspring.

Variation: Some mosquitoes are resistant to the pesticide, others are not.

Inheritance: Resistance is genetically determined.

Selection: The pesticide kills susceptible mosquitoes, but resistant mosquitoes survive.

Adaptation: Increased frequency of resistant mosquitoes in the population, making the pesticide less effective. This is a big problem for malaria control in South Africa.