Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Define evolution.
• Explain the four principles of natural selection.
• Discuss the role of mutation and sexual reproduction in creating variation.
• Apply natural selection to explain adaptation.
• Distinguish between microevolution and macroevolution.

Lesson summary

Evolution is the cornerstone of understanding the diversity of life on Earth. In South Africa, with its incredible biodiversity, understanding evolution is crucial for informed decision-making regarding conservation, agriculture, and public health. Natural selection, the driving force behind evolution, explains how populations change over time in response to their environment. Understanding natural selection is key to understanding everything from the emergence of antibiotic-resistant bacteria to the adaptation of indigenous plants to harsh climates. It's not just about dinosaurs; it's about the world around us today.

Lesson notes

What is Evolution? Evolution, at its core, is a change in the genetic makeup of a population over time. This change can be subtle or dramatic, and it occurs across generations. It's not about individual organisms changing during their lifetime (that's development or adaptation to immediate circumstances, but not evolution). The key is that the changes are heritable, meaning they can be passed down from parents to offspring.

Think about it like a recipe: evolution is about changes to the recipe (the genes), not just changes to the cake (the individual) depending on how it's baked.

Natural Selection: Darwin's Big Idea Charles Darwin and Alfred Russel Wallace independently proposed the theory of natural selection to explain how evolution works.

Their theory rests on four key principles: Variation: Individuals within a population are not identical; they exhibit variations in their traits. These variations can be physical (e.g., beak size in birds, coat color in mammals), physiological (e.g., ability to digest lactose), or behavioral (e.g., mating rituals). Without variation, there's nothing for natural selection to act upon.

Think of a classroom of students: not everyone is the same height, or has the same eye color. That's variation.

Inheritance: Many traits are heritable, meaning they can be passed from parents to offspring. This happens through genes. If a trait is not heritable, then natural selection cannot act on it to cause evolutionary change. If a parent is tall because they ate a lot of food, and that isn't related to their genes, their child might not be tall.

Competition: Populations produce more offspring than the environment can support. This leads to competition for resources like food, water, shelter, and mates. This competition is often referred to as "survival of the fittest," though "reproductive success of the fittest" is more accurate. It's not just about who lives the longest, but who leaves the most offspring. Consider a group of stray dogs competing for limited food resources in a township. Differential Survival and Reproduction (Fitness): Individuals with traits that are better suited to their environment are more likely to survive and reproduce, passing on those advantageous traits to their offspring. This is what we mean by "fitness". Fitness isn't about being the strongest or fastest; it's about being the best adapted to the specific environment. Imagine a drought in a region; plants with deeper roots will be better suited to survive and reproduce.

How Natural Selection Works: A Step-by-Step Example Let's consider an example relevant to South Africa: antibiotic resistance in bacteria.

Variation: Within a population of bacteria, some individuals might have genes that make them slightly more resistant to a particular antibiotic than others. This variation can arise through random mutations.

Inheritance: If the resistance is due to a genetic mutation, it can be passed on to the bacteria's offspring.

Competition: When an antibiotic is used, it kills most of the bacteria.

However, the bacteria with higher resistance are more likely to survive. Differential Survival and Reproduction (Fitness): The surviving bacteria with the resistance genes reproduce, passing on their resistance to their offspring. Over time, the proportion of resistant bacteria in the population increases. Eventually, the antibiotic becomes ineffective against the entire population.

Mutations and Sexual Reproduction: The Engines of Variation Mutations: Mutations are random changes in the DNA sequence. They are the ultimate source of new genetic variation. Most mutations are harmful or neutral, but occasionally, a mutation arises that is beneficial in a particular environment. It is important to note that mutations occur randomly; the environment doesn't cause specific mutations to arise. The environment only selects for the mutations that are already present.

Sexual Reproduction: Sexual reproduction combines genes from two parents, creating offspring with new combinations of traits. This process, involving meiosis (specifically crossing over) and random fertilization, generates a huge amount of genetic variation within a population. Microevolution vs.

Macroevolution Microevolution: Refers to small-scale changes in gene frequencies within a population over a few generations. Our antibiotic resistance example is a case of microevolution.

Macroevolution: Refers to large-scale evolutionary changes that occur over long periods of time, leading to the formation of new species and higher taxonomic groups. Macroevolution is essentially the accumulation of many microevolutionary changes over vast stretches of time. Guided Practice (With Solutions)

Question 1: Explain how natural selection could lead to the evolution of darker skin pigmentation in human populations living closer to the equator.

Solution: Variation: There is variation in skin pigmentation within human populations.