Evolution Teacher Guide






Grade Level


Activity Name(s)

A Selection Pressure

Conflicting Selection Pressures


Being Prepared

These activities are computer simulations, so having students as close to 1 to1 with a computer is ideal. Plan significantly in advance of htis activity to ensure you have access to computers (state testing requisitions, other teacher use, etc.)

Laptops that require power cords should have power strips placed in such a way that walkways are kept clear.

Getting Started

The controls for the simulations are not always intuitive to new users. However, they always have very descriptive assistance provided in the text preceding the simulations. Encourage frustrated students to review the written instructions before attempting to interact with the model further.

There is no equipment required beyond a computer with sufficient power to run the simulations.

Suggested Timeline

The three activities together do not fit into either a 45-50 minute period or a 90 minute block. The three activities could all be delivered separately as about 45 minutes of an hour period for each activity. The other option is to combine the first two to fill most of a block period, and finishing the third the next day. It would not be recommended to attempt all three activities within a single block period.

Thinking about the Discovery Questions

This unit is motivated by the discovery questions:

  • How do populations adjust to changing environments?
  • What happens in a population if selection pressures are in conflict with each other?
  • How can mutations help a population adjust to a changing environment?

Populations change over time, both for the better and for the worse. When there are more offspring produced than can survive and reproduce, the successful individuals are most likely not chosen at random. In many cases, the most successful individuals depend on their interactions with the environment. What may be the most successful characteristics in one place, or most fit characteristics, may be very unfit in a different environment. Evolutionary biologists are working to describe the patterns we observe in nature regarding the fitness of different characteristics to make better predictions about how populations will change in the future.


The most commonly held misconception by the layperson (students and adults alike) is that there is a best trait. Typically evolution is viewed as always being selection and that selection means the best individuals get to live. In reality this misses a startling trend in evolution, that the trait we would view as "best" frequently is not the most fit. Students should be assisted in seeing that evolution is concerned with reproduction by whatever means possible. This simple idea can lead to some counterintuitive results, so focus on the data throughout the unit.

Learning Objectives

  • NGSS
    • Performance Expectations
      • HS-LS2-2 Use mathematical representations to support and revise explanations based on evidence about factors affecting biodiversity and populations in ecosystems of different scales.
      • HS-LS2-6 Evaluate the claims, evidence, and reasoning that the complex interactions in ecosystems maintain relatively consistent numbers and types of organisms in stable conditions, but changing conditions may results in a new ecosystem.
      • HS-LS4-3 Apply concepts of statistics and probability to support explanations that organisms with an advantageous heritable trait tend to increase in proportion to organisms lacking this trait.
      • HS-LS4-4 Construct an explanation based on evidence for how natural selection leads to adaptation of populations.
    • Disciplinary Core Ideas
      • HS-LS2: Ecosystems: Interactions, Energy, and Dynamics
        • LS2.C: Ecosystem Dynamics, Functioning, and Resiliance
          • A complex set of interactions within an ecosystem can keep its numbers and types of organisms relatively constant over long periods of time under stable conditions. If a modest biological or physical disturbance to an ecosystem occurs, it may return to its more or less original status (i.e., the ecosystem is resilient), as opposed to becoming a very different ecosystem. Extreme fluctuations in conditions or the size of any population, however, can challenge the functioning of ecosystems in terms of resources and habitat availability. (HS-LS2-2),(HS-LS2-6)
      • HS-LS4: Biological Evolution: Unity and Diversity
        • LS4.B: Natural Selection
          • Natural selection occurs only if there is both (1) variation in the genetic information between organisms in a population and (2) variation in the expression of that genetic information—that is, trait variation—that leads to differences in performance among individuals. (HS-LS4-2),(HS-LS4-3)
          • The traits that positively affect survival are more likely to be reproduced, and thus are more common in the population. (HS-LS4-3)
        • LS4.C: Adaptation
          • Natural selection leads to adaptation, that is, to a population dominated by organisms that are anatomically, behaviorally, and physiologically well suited to survive and reproduce in a specific environment. That is, the differential survival and reproduction of organisms in a population that have an advantageous heritable trait leads to an increase in the proportion of individuals in future generations that have the trait and to a decrease in the proportion of individuals that do not. (HS-LS4-3),(HS-LS4-4)
          • Adaptation also means that the distribution of traits in a population can change when conditions change. (HS-LS4-3)
    • Practices
      • Developing and using models
        • Evaluate limitations of a model for a proposed object or tool.
        • Use and/or develop a model of simple systems with uncertain and less predictable factors.
        • Develop and/or use a model to predict and/or describe phenomena.
        • Develop and/or use a model to generate data to test ideas about phenomena in natural or designed systems, including those representing inputs and outputs, and those at unobservable scales.
      • Analyzing and interpreting data
        • Apply concepts of statistics and probability (including mean, median, mode, and variability) to analyze and characterize data, using digital tools when feasible.
      • Using mathematics and computational thinking
        • Use mathematical representations to describe and/or support scientific conclusions and design solutions.
      • Constructing explanations and designing solutions
        • Construct a scientific explanation based on valid and reliable evidence obtained from sources (including the students’ own experiments) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
      • Engaging in argument from evidence
    • Cross Cutting Concepts
      • Patterns
        • Students recognize that macroscopic patterns are related to the nature of microscopic and atomic-level structure. They identify patterns in rates of change and other numerical relationships that provide information about natural and human designed systems. They use patterns to identify cause and effect relationships, and use graphs and charts to identify patterns in data.
      • Cause and effect
        • Students classify relationships as causal or correlational, and recognize that correlation does not necessarily imply causation. They use cause and effect relationships to predict phenomena in natural or designed systems. They also understand that phenomena may have more than one cause, and some cause and effect relationships in systems can only be described using probability.
      • Scale, proportion, and quantity
        • Students observe time, space, and energy phenomena at various scales using models to study systems that are too large or too small. They understand phenomena observed at one scale may not be observable at another scale, and the function of natural and designed systems may change with scale. They use proportional relationships (e.g., speed as the ratio of distance traveled to time taken) to gather information about the magnitude of properties and processes. They represent scientific relationships through the use of algebraic expressions and equations.
      • Stability and change
        • Students explain stability and change in natural or designed systems by examining changes over time, and considering forces at different scales, including the atomic scale. Students learn changes in one part of a system might cause large changes in another part, systems in dynamic equilibrium are stable due to a balance of feedback mechanisms, and stability might be disturbed by either sudden events or gradual changes that accumulate over time.
  • NSES
    • NSES Life Science – Biological Evolution
      • Species evolve over time. Evolution is the consequence of the interactions of (1) the potential for a species to increase its numbers, (2) the genetic variability of offspring due to mutation and recombination of genes, (3) a finite supply of the resources required for life, and (4) the ensuing selection by the environment of those offspring better able to survive and leave offspring.

Discussion: Setting the Stage

  • Imagine a dark cave in which a population of bugs live. There are jet black bugs and bright green bugs in that population. There are also predators that hunt the bugs in the cave. If you revisit that same population fifty years later, how do you think that population will look?

    The bright green bugs will likely be gone because they will be spotted by predators faster than the black bugs.

  • Continue thinking about the bug population. What if it costs the bug a large amount of energy to produce the black color, whereas the bright green color is energetically cheap. What effect will those conditions have on the population after a long period of time?

    The population will probably now contain both color morphs, because some green bugs will exist due to the lower energy cost.

  • Within our bug population, consider a new mutation. Imagine one bug is born a navy blue color. This color is genetically similar to the bright green color, so it is energetically inexpensive for the bug. What do you predict will happen in the generations following the birth of a bug with this mutation?

    The blue bugs will likely replace the green bugs, because they are not as camouflaged as the black bugs, but they are better hidden than the bright green bugs at the same energy cost.

Discussion: Formative Questions

  • As time passes, what is happening to the population size?

    The population should be either relatively stable or slowly growing. The important thing to remember is that the characteristics of the population can be changing, even while the size of the population may not be changing.

  • Why does the quality of teeth go down over time when the effects of the sheep teeth is turned off?

    There is still the downward selection pressure from the rancher removing sheep with the best teeth.

  • What causes a mutation to occur?

    They are random and occur due only to chance. The blue sheep are chosen to randomly appear every so often.

Discussion: Wrapping Up

  • What are the most important things a researcher must consider when making a prediction about a population changing over time?

    The characteristics of the environment that impact the fitness of the individuals in the population are most important. How the environment will change over time is also something that should be considered.

  • What kind of a role do spontaneous and unpredictable events play in the evolution of populations over large periods of time?

    Mutations and rare environmental events provide the disruptive events to maintain diversity in nature.

Additional Background

Populations do not exist in a vacuum, and even the lack of any influence from the environment will have an effect on the population over time. Selection is an important mechanism at work within populations, pushing the characteristics of the population over time to contain individuals that are best able to reproduce. Tiny changes in physical forms over the course of a few generations will be magnified over time scales far beyond anything we can easily comprehend. Geologic time, on the scale of billions of years, combines the small generational changes into the widely differing survival and reproduction strategies that we see in nature today.

Predictions regarding the appearance of lineages in the distant past and the future must be made using the evidence available today. Biologists seek to quantify the traits they see in nature to allow for mathematical models to produce predictions with ever-increasing confidence. As they attempt to peer further into the future, they rely on patterns exhibited in the past using fossil evidence as well as behaviors, genetic information, developmental, and geographic information available for extant species.


A Selection Pressure

  1. Combine your results with other groups. What can you conclude about the effect of a scarcity of grass on the changing characteristics of teeth? Be prepared to share your results with the rest of the class.

    A scarcity of grass provides a benefit to sheep that can more efficiently consume the grass.

  2. People use the word "adaptation" in many ways. In biology, an adaptation is a trait that allows an organism to survive and reproduce. In this situation, the individual sheep is not able to change its teeth to better survive; the sheep with better teeth is more likely to survive and reproduce. As a result, over many generations, the sheep population has better teeth than the starting population had. Give another example of a trait that does not change for an individual, but can change for a population over many generations.

    Answers vary, but focus on the trait not being something that can change for an individual. The only change is the average trait across a population over time.

  3. Give an example of a feature that changes during the life of the individual but is not passed on to future generations.

    Answers will vary.

  4. Think about these two theories about how giraffes acquired long necks. Theory 1: As they grew up, giraffes stretched out their necks to reach higher in trees for leaves. The babies inherited their parents' longer necks. This was repeated every generation until all giraffes had longer necks. Theory 2: There were different neck lengths in the giraffe population, due to natural variations and mutations. The giraffes with longer necks were more successful getting food because they could reach higher into the trees. They were more likely to reproduce, and their babies inherited their longer neck feature. Gradually there were more giraffes with long necks than short necks. Over many generations, the average neck length of giraffes increased. Which theory do you think is more correct and why?

    Hypothesis 2, because the first option would not be heritable. The same way someone cannot work out their whole life to make their baby very muscular.

  5. How do populations adjust to changing environments?

    Differential survival and reproduction of individuals over time leads to changes in the population as a whole.

Conflicting Selection Pressures

  1. What can you conclude about situations with conflicting selection pressures? Be prepared to share your conclusions with the class and compare your results to those of other groups.

    Conflicting selection pressures will push the equilibrium toward a middle position, compared to single or aligned selection pressures that favor an extreme trait.

  2. Moose may have evolved to become larger because they could better protect themselves from predators. What other factors might have caused this?

    Answers vary, but may include sexual selection, warmth, parasites and disease, etc.

  3. If these factors existed, why didn't moose just keep getting bigger and bigger? Think of some possible reasons.

    Conflicting selection pressures such as joint weakness due to excessive size, metabolic problems with increased size, lack of genetic raw material for increased growth.

  4. Male peacocks have very large tails, and they can hardly fly because their tails are so big! How could you explain the development of such large tails?

    The selection pressure present for large tails overpowered the pressure to maintain the capacity for flight.

  5. What happens in a population if selection pressures are in conflict with each other?

    The population will find an intermediate state the balances the opposing pressures.


  1. Think of some other examples of mutations, besides getting more energy from grass, which might be favored to become more common in the sheep population in this computer model.

    Answers vary, but may include mutations that assist in energy gathering.

  2. Suppose the environment changes. For example, think of some small mutations that might help the sheep population survive in these new circumstances.

    Answers vary, but could include adaptations that increase energy gathering or water conservation.

  3. How can mutations help a population adjust to a changing environment?

    Mutations provide new traits for the population on which selection can act.

Further Investigation

  • Selection and mutation are only two of several mechanisms that can cause change in a population over time. There are three others that have been defined by famous evolutionary scientists Hardy and Weinberg: Genetic Drift, Gene Flow, and Nonrandom Mating. Choose a new mechanism for change within populations and find an example to illustrate the phenomenon.

    Genetic Drift - change in a population due only to chance. No natural population 100% free of genetic drift, and very small populations experience very large amounts of drift.

    Gene Flow - change in a population due to migration between multiple populations. Organisms that have only partial mobility are good examples, such as tidal pool populations or prairie lizards in wooded areas broken by grassland.

    Nonrandom mating - this is NOT sexual selection. This is change due to inbreeding or outbreeding. The california condor is one famous example, while human inbreeding is another example such as the old English or Spanish royal family.