Evolution
Biology
HS
A Selection Pressure
Conflicting Selection Pressures
Mutations
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.
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.
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.
This unit is motivated by the discovery questions:
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.
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.
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.
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.
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.
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.
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.
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.
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.
How do populations adjust to changing environments?
Differential survival and reproduction of individuals over time leads to changes in the population as a whole.
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.
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.
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.
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.
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.
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.
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.
How can mutations help a population adjust to a changing environment?
Mutations provide new traits for the population on which selection can act.
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.