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What is Natural Selection?

This lesson is available for grade 6 in Saskatchewan and Manitoba, grade 7 in British Columbia and Nova Scotia, grade 9 in Alberta, and grade 12 in Saskatchewan, Manitoba, Ontario, Newfoundland and Labrador, New Brunswick, and Quebec (Cégép).

Here are some vocabulary terms to help with this lesson.  

Bacteria (bak-teer-ee-uh)

Microscopic single celled microorganisms. Bacteria are found almost everywhere on Earth and are vital to the health of the planet. Many kinds of bacteria are helpful to human health. Some bacteria cause diseases. 

Mutation (myoo-tey-shuhn) 

The changing of the structure of a gene, resulting in a variant form that may be transmitted to subsequent generations.

Variability (vair-ee-uh-bil-uh-tee) 

Differences, lack of a fixed pattern, likelihood to change.

Antibiotics (an-tee-by-ot-ik)

Medicine to treat infections caused by bacteria.

Antibiotic resistance (an-tee-by-ot-ik   ri-zist-uhns)

When bacteria change and are no longer killed by the medicine designed to kill them.

Gene (jeen)

The basic unit of heredity, or passing information from parent to offspring. Genes carry the information that determines your features, or traits. 

Petri Dish (pee-tree dish) 

Petri dishes are mainly used for biological experiments. They enable biologists to grow microscopic organisms in a controlled environment. 

Cell Culture (sell kul-chr)

Cell culture is when cells are grown in a controlled environment; sometimes single-cell bacteria are grown, and sometimes cells taken from an animal or plant are grown outside the living organism in a controlled environment. This lets microbiologists perform specific studies. 

Natural Selection

Darwin introduced the term ‘natural selection’ to explain how evolution occurs. Natural selection is also sometimes referred to as ‘survival of the fittest’. Both concepts explain that in nature, the individuals who survive and have offspring will have their genetic material passed on to the next generation.

The traits passed down from one generation to the next are traits that allow individuals to survive the best in their environment. Within a population of animals, each pair of animals who produce offspring will have some variability (differences) in their offspring. There could be differences in the offspring that occur from mutations or mistakes in the genetic code. The mutations can cause the individual to have an advantage or disadvantage. If the mutation causes the individual to have an advantage, and that individual has offspring, the traits are passed on to other generations. These small changes over time help animals to adapt to their environment. 

Let’s explain how this works.

Say we are following a banana bread recipe.

The recipe has a list of ingredients and it explains what to do. 

Sometimes even if you follow the recipe, you make small mistakes. Maybe you left it baking for a little longer. Maybe you forgot to add an ingredient. Maybe you put in too much flour.

This means that each time the recipe varies (a bit different) from the first banana bread, it’s slightly different. 

Like a recipe for banana bread, bacteria also have instructions or a “recipe” that tells them how to make copies of themselves. This “recipe” is called the genome. The genome is the bacteria's genetic material. Just like the banana bread recipe, bacteria can change when they replicate, which can give them advantages like resistance to antibiotics.

Watch this video to see an example of natural selection and how variability and mutation are linked to survival:

Natural Selection Animation


Looking Closely at Natural Selection in Bacteria

In many animals, it can be challenging to observe evolution through natural selection because the lifespan of the animal is similar to the lifespan of a human - so we don’t notice the changes. However, if we look at organisms with shorter life spans, it’s easy to see changes that happen over time. One way we can observe natural selection happening is with bacteria. Bacteria are single-celled organisms that live less than a day and replicate or make copies of themselves to grow in number. 

Here is a typical bacterial cell and all its parts. 

Bacteria have short lifespans and high rates of mutation, or random mistakes when they copy their genetic code. Sometimes the mutations give the bacteria a benefit. For example, bacteria with a mutation can have a difference that allows them to survive when there is something threatening to kill them.  Antibiotics are medicines designed to kill harmful bacteria. When bacteria are exposed to antibiotics, some may experience a random mutation that can allow the bacteria to survive. When bacteria can survive in the presence of an antibiotic, it is called antibiotic resistance. This video shows how bacteria can develop mutations that help them survive. The bacteria grow quickly at the ends of the petri-dish where there are no antibiotics. When the bacteria come in contact with the antibiotic, they develop mutations that allow them to survive. By watching the bacteria in the video, we can watch natural selection happening.

The Evolution of Bacteria on a “Mega-Plate” Petri Dish (Kishony Lab)


What does it mean when bacteria develop resistance to antibiotics? Can we develop new antibiotics to ensure we have them when we need them? Scientists are researching different types of antibiotics so that we have new treatments when we need them. University students are also contributing to this search by looking for new possible antibiotics from bacteria found in soil samples from around the world. 

In the next video, Dr. Jo Handelsman, Director of the Wisconsin Institute for Discovery, shares how the Tiny Earth project is addressing a global health crisis while training the scientific workforce of the future. As part of this project, college students around the world are becoming an important part of the solution.

Tiny Earth: Studentsourcing Antibiotic Discovery


However, it takes a long time to develop new antibiotics and the bacteria can change and develop resistance even to new antibiotics. To ensure the preservation of antibiotics now and into the future, we have to be careful to only use antibiotics when they are prescribed by health care professionals. By preserving antibiotics, bacteria don’t have a chance to develop resistance. This saves antibiotics so that they work when we need them to. Without making a change, we face a world where we may not have antibiotics. 


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  Feb 13, 2024