Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Define natural selection and its mechanisms (VIST).
• Explain how natural selection leads to adaptation.
• Describe the three types of selection: directional, stabilizing, and disruptive.
• Analyse data representing selection pressures.
• Apply the concept to South African examples.

Lesson summary

This week, we delve into the heart of evolutionary theory: natural selection. Understanding natural selection is crucial not just for grasping Life Sciences, but also for comprehending the world around us. From the emergence of antibiotic-resistant bacteria in our hospitals to the adaptation of plant species to changing climates, natural selection is a driving force shaping life on Earth, including the unique biodiversity found in South Africa. It directly relates to issues of food security, public health, and conservation, making it highly relevant to our lives as South African citizens.

Lesson notes

What is Natural Selection? Natural selection is the process by which organisms that are better adapted to their environment tend to survive and reproduce more successfully than organisms that are less well adapted. This leads to the gradual change in the characteristics of a population over time. It's the "survival of the fittest," but "fittest" doesn't necessarily mean the strongest; it means the most well-suited to a particular environment.

The Four Key Components (VIST): To understand natural selection, it's essential to grasp these four crucial components: Variation: Individuals within a population exhibit variations in their traits. These variations can be physical (e.g., beak size in birds, coat color in mammals), physiological (e.g., resistance to toxins), or behavioral (e.g., mating rituals). Variation arises through genetic mutations (random changes in DNA) and sexual reproduction (which shuffles genes).

Inheritance: Many of these variations are heritable, meaning they can be passed down from parents to offspring. This heritability is crucial because natural selection can only act on traits that can be passed on.

Selection: The environment "selects" for certain traits. Individuals with traits that are advantageous in a particular environment are more likely to survive, reproduce, and pass on their genes to the next generation. This selection can be driven by various factors, such as predators, food availability, climate, and competition.

Time: Natural selection is a gradual process that occurs over many generations. The accumulation of small, advantageous changes over time can lead to significant evolutionary changes in a population.

Types of Selection: There are three main types of natural selection: Directional Selection: Favors individuals at one extreme of a phenotypic range. This results in a shift in the population's average phenotype towards that extreme.

Example:* Imagine a population of moths living in a forest with light-colored tree bark. If pollution darkens the tree bark, darker-colored moths will be better camouflaged and less likely to be eaten by birds. Over time, the population will shift towards a higher proportion of darker-colored moths. This happened during the Industrial Revolution in England with the peppered moth (Biston betularia).

Stabilizing Selection: Favors intermediate phenotypes and selects against extreme phenotypes. This reduces variation in the population.

Example:* Human birth weight. Babies that are too small are vulnerable to complications, while babies that are too large can cause complications during childbirth. Babies with intermediate birth weights tend to have the highest survival rates. Over time, stabilizing selection maintains a narrow range of birth weights in the population.

Disruptive Selection: Favors individuals at both extremes of a phenotypic range and selects against intermediate phenotypes. This can lead to the formation of two distinct subpopulations.

Example:* Imagine a population of birds living in an area with two types of seeds: large, hard seeds and small, soft seeds. Birds with large, strong beaks are better at cracking the large seeds, while birds with small, delicate beaks are better at handling the small seeds. Birds with intermediate beaks are not efficient at eating either type of seed. Over time, the population may split into two subpopulations: one with large beaks and one with small beaks. An example of this is African seedcrackers (Pyrenestes ostrinus) where beak size is either very large or very small.