Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Define natural selection and its role in evolution.
• Describe the four key principles of natural selection.
• Provide South African examples of natural selection.
• Explain how the environment influences natural selection.
• Analyse data to draw conclusions about evolution.

Lesson summary

Evolution by natural selection is the cornerstone of modern biology. It explains how life on Earth has diversified over millions of years, leading to the incredible array of organisms we see today, including ourselves. Understanding this process is crucial for comprehending the interconnectedness of life, addressing challenges like antibiotic resistance in bacteria, and even developing sustainable agricultural practices. For South African learners, understanding natural selection is particularly relevant in the context of our rich biodiversity, the impacts of climate change on our ecosystems, and the importance of preserving our natural heritage.

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, thus increasing the frequency of those advantageous traits in future generations. This process is the primary mechanism of evolution. It's not a random process; it's driven by the interaction between organisms and their environment.

The Four Principles of Natural Selection: Variation: Individuals within a population exhibit variation in their traits. This variation can be physical (e.g., beak size in birds, coat color in mammals) or behavioural (e.g., foraging strategies). 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.

Example: Think about a population of impala in the Kruger National Park. Some impala might be slightly faster than others, have better eyesight, or be more resistant to certain parasites.

Inheritance: Traits are inherited from parents to offspring. This means that offspring tend to resemble their parents in terms of these traits. Genetic information is passed down through DNA. If traits weren't heritable, natural selection wouldn't lead to long-term evolutionary change.

Example: The faster impala are more likely to have offspring that are also faster, due to the genes they inherit. Survival and Reproduction (Differential Survival and Reproduction): In any given environment, some individuals are more likely to survive and reproduce than others. This is because their traits make them better suited to that environment. This isn't just about surviving; it's about reproductive success – producing more offspring. This is often referred to as "fitness" – how well an organism survives and reproduces in its environment.

Example: The faster impala are more likely to escape predators like cheetahs and lions, and therefore more likely to survive and reproduce, passing on their "fast genes." Impala more resistant to parasites are also more likely to survive and reproduce.

Time (Change over Time): Over many generations, the frequency of advantageous traits increases in the population, while the frequency of disadvantageous traits decreases. This leads to a gradual change in the characteristics of the population – evolution. Natural selection is not an instant process; it takes time for these changes to become significant.

Example: Over generations, the impala population becomes, on average, faster and more resistant to parasites. This is because the genes that contribute to these traits become more common.