Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Explain the role of variation in a population.
• Describe how environmental selection pressures work.
• Define adaptation and give South African examples.
• Explain how natural selection can lead to new species (speciation).
• Apply these principles to real-world scenarios like antibiotic resistance.

Lesson summary

Evolution by natural selection, the cornerstone of modern biology, explains the diversity of life we see around us and how organisms adapt to their environments. This week, we delve deeper into the mechanisms driving this process, focusing on variations, selection pressures, adaptation, and speciation. Understanding natural selection is crucial not just for academic success in Life Sciences but also for comprehending pressing real-world issues like antibiotic resistance in bacteria (a significant health challenge in South Africa), the evolution of pesticide-resistant pests affecting our agriculture, and the impacts of climate change on our unique biodiversity, from the fynbos to the Kruger...

Lesson notes

2. 1.

Variation: The Raw Material of Evolution Variation is the difference between individuals within a population. Without variation, there is nothing for natural selection to "select" from.

Variation arises from several sources: Mutation: Random changes in the DNA sequence. Mutations are the ultimate source of new alleles (different versions of a gene). Mutations can be harmful, neutral, or beneficial. A mutation providing resistance to a common disease in a specific community would be beneficial.

Sexual Reproduction: The shuffling of genes during meiosis (crossing over, independent assortment) and fertilization creates new combinations of alleles, leading to offspring with different traits than their parents.

Gene Flow: The movement of genes from one population to another. This can introduce new alleles into a population and increase variation. Imagine a group of Khoisan people migrating into a Zulu area, introducing new genetic variations related to, for example, skin pigmentation or disease resistance. 2.

2. Selection Pressures: Shaping Evolution Selection pressures are environmental factors that influence the survival and reproduction of individuals in a population. These pressures can be biotic (living things, like predators, competitors, or pathogens) or abiotic (non-living things, like temperature, rainfall, or sunlight).

Predation: Predators exert selection pressure on their prey, favoring individuals with traits that help them avoid being eaten (e.g., camouflage, speed, defenses). For example, the mottled coloration of many South African insects provides camouflage against predators like birds.

Competition: Individuals compete for resources like food, water, and mates. Those best adapted to acquire these resources are more likely to survive and reproduce. For example, different species of aloes competing for sunlight and nutrients in the same habitat in the Eastern Cape. The species best adapted to the specific soil and light conditions will thrive.

Climate: Changes in temperature, rainfall, or other climatic factors can select for individuals with traits that allow them to tolerate these changes. Consider the succulents found in the arid Namaqualand region. They have evolved specific adaptations (e.g., thick leaves, water storage) to survive in this harsh environment.

Disease: Pathogens (disease-causing organisms) exert selection pressure on their hosts, favoring individuals with immune systems that can resist infection. The high prevalence of HIV/AIDS in South Africa creates a strong selection pressure for individuals with genetic resistance to the virus. 2.

3. Adaptation: Fitting the Environment Adaptation is the process by which a population becomes better suited to its environment over time, through the inheritance of advantageous traits. An adaptation is a characteristic (structural, physiological, or behavioral) that increases an organism's survival and reproductive success in its environment.

Structural Adaptations: Physical features of an organism.

Examples include: The long necks of giraffes, allowing them to reach high into trees for food. The streamlined bodies of fish, reducing drag in water. The thick fur of animals living in cold climates.

Physiological Adaptations: Internal processes that allow an organism to survive.

Examples include: The ability of some plants to tolerate drought conditions. The production of venom by snakes. The ability of some bacteria to resist antibiotics.

Behavioral Adaptations: Actions that an organism takes to survive.

Examples include: Migration patterns of birds. Hunting strategies of predators. Courtship rituals of animals. 2.

4. Speciation: The Origin of New Species Speciation is the process by which new species arise from existing ones. Natural selection can play a key role in speciation, particularly when populations become reproductively isolated.

Geographic Isolation: A physical barrier (e.g., a mountain range, a river, an ocean) separates a population into two or more isolated groups. Over time, natural selection and genetic drift can cause these populations to diverge genetically and become reproductively isolated. This is particularly relevant in South Africa, where diverse landscapes like the Drakensberg mountains can act as barriers to gene flow, leading to the evolution of endemic species.

Example: Different populations of Aloe species on either side of a mountain range evolve different flowering times, preventing interbreeding.

Reproductive Isolation: Mechanisms that prevent individuals from different populations from interbreeding, even if they are not geographically isolated. These mechanisms can be prezygotic (preventing the formation of a zygote) or postzygotic (reducing the viability or fertility of hybrid offspring).

Example: Two populations of birds may evolve different mating songs, making them unattractive to members of the other population.