Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Define evolution and natural selection.
• Describe Darwin's observations that led to his theory.
• Distinguish between adaptation and variation.
• Explain how environmental changes drive natural selection.

Lesson summary

Evolution by natural selection is the cornerstone of modern biology. It explains the diversity of life on Earth and how organisms change over time in response to their environment. Understanding this process is crucial for comprehending many aspects of biology, from the development of antibiotic resistance in bacteria (a major public health concern in South Africa, given our high rates of TB and HIV) to the impact of climate change on our ecosystems and biodiversity. In South Africa, with our rich biodiversity and unique ecosystems like the fynbos, understanding evolutionary processes is essential for informed conservation efforts.

Lesson notes

2.1 What is Evolution? Evolution, in its broadest biological sense, is the change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes that are passed on from parent to offspring during reproduction. Evolution isn't about individual organisms changing during their lifetime; it's about the population changing over generations. 2.2 What is Natural Selection? Natural selection is a mechanism of evolution. It's the process where organisms better adapted to their environment tend to survive and reproduce more successfully, passing on their advantageous traits to the next generation. This leads to a gradual change in the genetic makeup of the population over time. 2.3 The Four Principles of Natural Selection: Natural selection operates through four key principles: Variation: Individuals within a population exhibit variation in their traits. This variation can be physical (size, color, shape), physiological (disease resistance, metabolic efficiency), or behavioral (mating rituals, foraging strategies). Without variation, there's nothing for natural selection to act upon.

Example: In a population of impala in Kruger National Park, some impala may be faster runners than others.

Inheritance: Traits are heritable, meaning they can be passed from parents to offspring through genes. This allows advantageous traits to be passed on to subsequent generations.

Example: The faster running ability of impala is partly determined by their genes, which they inherit from their parents. Differential Survival and Reproduction (Struggle for Existence): Organisms produce more offspring than the environment can support. This leads to competition for resources (food, water, shelter, mates) and a struggle for survival. Individuals with advantageous traits are more likely to survive and reproduce.

Example: Due to limited grazing areas and the presence of predators like lions and leopards, not all impala will survive to adulthood and reproduce. The faster impala are more likely to escape predators and survive.

Adaptation: Over time, the frequency of advantageous traits increases in the population. This leads to adaptation – the process by which organisms become better suited to their environment.

Example: Over many generations, the impala population will become increasingly composed of faster runners, as these are the individuals most likely to survive and reproduce, passing on their genes for speed. 2.4 Darwin's Observations and Inferences: Charles Darwin, during his voyage on the HMS Beagle, made key observations that led him to formulate his theory of evolution by natural selection: Observation 1: Geographic Distribution of Species: He observed that closely related species often lived in different geographic locations, suggesting they had diverged from a common ancestor. For example, the different species of finches on the Galapagos Islands.

Relevance to SA: The distinct flora of the Cape Floral Kingdom demonstrates this principle. Many plant species are unique to specific regions within the fynbos, suggesting they evolved in isolation and adapted to slightly different environmental conditions.

Observation 2: Fossils: He studied fossils of extinct animals that resembled living organisms, suggesting a connection between past and present life forms.

Relevance to SA: The fossil record in the Cradle of Humankind provides crucial evidence for human evolution.

Observation 3: Adaptation: He observed that organisms were well-suited to their environments, with specific adaptations that helped them survive and reproduce.

Relevance to SA: The succulent plants in arid regions of South Africa have adaptations such as thick, fleshy leaves to store water, allowing them to survive in dry conditions. Darwin inferred from these observations that: Species change over time. All species share a common ancestor. Natural selection is the mechanism driving evolutionary change. 2.5 Examples of Natural Selection in Action: Antibiotic Resistance in Bacteria: Overuse of antibiotics has led to the evolution of antibiotic-resistant bacteria. Bacteria with mutations that confer resistance to antibiotics are more likely to survive and reproduce when exposed to these drugs. This is a serious problem in South Africa, where antibiotic resistance is contributing to the spread of drug-resistant tuberculosis.

Example: A population of Staphylococcus aureus bacteria is exposed to an antibiotic. Most bacteria are killed, but a few individuals possess a mutation that allows them to resist the antibiotic's effects. These resistant bacteria survive and reproduce, passing on the resistance gene to their offspring. Over time, the entire population becomes resistant to the antibiotic.

Industrial Melanism in Peppered Moths: During the Industrial Revolution in England, the bark of trees became darkened by soot.