My Science Box

Teacher Background
Perhaps the most important idea in all of biology, or perhaps all of science, is the idea of evolution through natural selection. This idea by Charles Darwin provides the foundation of all of current scientific thinking in life science.

What is evolution? Quite simply, evolution is descent with modification. This includes both the idea that the frequency of a gene will change in a population over time as environmental conditions change and also the idea that new species descend from common ancestors over many generations. Ultimately, evolution can explain the vast diversity of life on this planet and the idea that all life on Earth shares a common ancestor.

Although there are many mechanisms for organisms to change over time, the most important of these is natural selection. It works in this way:

• There is variation in a population. Different individuals have different traits.
• There is heredity. Traits can be passed on from parent to offspring through our genes.
• There is competition (sometime referred to as differential survival and reproduction) so that some individuals survive and reproduce more than others.
• The end result is natural selection – the individuals with the traits that best fit the environment are most likely to survive, reproduce, and pass on their traits to the next generation. In this way, future generations, when viewed at the level of an entire population, will have more advantageous traits and fewer disadvantageous traits compared to their parents.

There are many other mechanisms for evolutionary change besides natural selection:

• Artificial selection is a common practice for humans to breed together plants or animals with the most advantageous traits (the sweetest tomato, the most loyal dog, the fastest horse, etc.). Thus, future generations will have more of these traits.
• Mutation can affect the distribution of traits by introducing a new trait to a population.
• Migration can rapidly change the overall distribution of traits in a population when a group of organisms with different traits enters a new area. For example, a group of small beaked finches is blown over to an island with primarily large beaked finches by a hurricane.
• Genetic drift can affect a population through a random chance event – like a hurricane or human activity – randomly destroying organisms with one trait but not another. For instance, when a new house is constructed in a neighborhood with both red and brown ground squirrels, it accidentally bulldozes the nest of the largest red ground squirrel family. Suddenly, brown ground squirrels predominate.

A final important term that is often misused by students is adaptation.  An adaptation is a trait that is very well suited to a given environment that has, through natural selection, increased in the population over many generations. Students often talk and think about adaptation as if an organism can try to adapt or is able to get what it needs. In neither case are they correct. An organism can’t get the genes it needs to survive by “trying”; it either has the genes or not. Similarly, natural selection doesn’t have a goal in mind and cannot give a creature what it “needs”; either the genes are there in the population or not.

In this activity, students come to understand natural selection, evolution and adaptation through modeling changes in 2 populations – a population of birds with different beak traits and a population of beans with different color traits.

Student Prerequisites
None required although an understanding of variation in traits is helpful (see Human Traits activity or Snail Variations project).