natural selection in brine shrimp -...

Natural Selection in Brine Shrimp 2013 Adapted from http://goo.gl/wwlBcl and Carolina’s Natural Selection for AP Biology handout

Natural Selection in Brine Shrimp OBJECTIVES: When you complete this activity, you should be able to

1. Evaluate evidence provided by data collected to qualitatively and quantitatively investigate natural selection's role in evolution.

2. Use mathematics to analyze data from a real or simulated population to predict what will happen to the population in the future.

3. Convert a data set from a table of numbers that reflect a change in the genetic makeup of a population over time and to use mathematics and conceptual understandings to investigate the cause(s) and effect(s) of this change.

4. Connect evolutionary changes in a population over time to a change in the environment.

KEY TERMS: adaptation, natural selection, evolution, fitness, differential reproductive success, mutations, phylogenetic tree, cladogram

Background Inherent in every population is genetic variation—the fact that all organisms contain a diversity of genes (and thus alleles) inherited from their parents is an important component of the theory of evolution by natural selection. This inherited variation in populations is what generates characteristics of a population that may become adaptations. An adaptation is an inherited trait that populations develop over generations which enable them to survive in their environment, thereby increasing their fitness. These adaptations enable populations to survive, reproduce, and pass on the genes for these favored traits to future generations. This process increases the frequency of the alleles for these traits in the population, causing evolution to occur over long periods of time. When a certain trait found in a population is favored in an environment, the process of natural selection will occur. In natural selection, the environment “selects” the characteristics of a population that are favorable and thus are passed on to future generations. Either organisms in a population possess a characteristic favored by the environment that allows them to survive, thrive and reproduce, or they do not. Organisms that are well-suited for the environment—the ones that are fit—reproduce with higher frequency than those that are not fit for the environment. This differential reproductive success is what drives the increase in favorable traits in a population, and thus, evolution. In this experiment, you will examine natural selection, using brine shrimp and saltwater solutions as model organisms and environments. Environmental conditions on Earth change--features such as temperature, moisture, amount of UV radiation and salinity fluctuate. These changes may be small-scale and short-term or large-scale and long-term and any of them may influence the survival of organisms. Scientific evidence points to our time as the period of the sixth mass extinction that has occurred on Earth. A relatively large number of species has become extinct since the last Ice Age and that extinction rate continues. The five other mass extinctions that occurred were likely due to large-scale changes that took place more rapidly than many populations’ ability to adapt to them (e.g., periods of global warming or global cooling). After each mass extinction, a period of speciation occurred. Numerous species evolved from the populations of organisms that survived the extinction. The species derived from populations that had inherited adaptations favored by the changed environmental conditions. The availability of unoccupied new niches helped spur the radiation of new species during each of these biological recoveries. A population’s ability to adapt to changing conditions begins with genetic variation among individuals. Genetic variations arise from mutations of the genome. Many mutations are harmful and decrease fitness of individuals in a population. Some mutations are beneficial and can increase the fitness of individuals, and some mutations are neutral and have no effect on the individual at all. Some individuals in populations possess traits that allow them to survive environmental conditions that others cannot tolerate. The genes that the survivors pass on to their


offspring include the genes coding for the beneficial trait. Over time, these genes (and traits) become more prevalent. There are many structural and physiological similarities among organisms. Shared characteristics often indicate common descent. Phylogenetic trees and cladograms illustrate relationships among groups of organisms and thus reflect evolutionary history. A cladogram shows grouping on the basis of shared adaptations, referred to as “shared derived characters,” which indicate probable relationships. For example, all organisms that belong to the class of crustaceans known as branchiopods (meaning “gill foot”) have gills on their appendages. Examine the cladogram of brachiopods shown below. The point at the base represents the common ancestor of all the organisms included on the cladogram. The organisms at the tips of the cladogram represent extant (living) species. The paths from the common ancestor to the living organisms represent the possible evolutionary history. Each horizontal line across the cladogram represents the adaptations shared by the group(s) above it on the diagram. For example, the bivalve carapace (shell) is shared by two genera, Limnadia and Lynceus. But only one genus, Lynceus, has a hinged carapace. The more shared derived characters two groups have in common, the more closely related they are considered to be. Hutchinsoniella is not a branchiopod and does not share derived characters with other organisms on the cladogram; it is presented as an “outgroup” on the cladogram, to help viewers orient the branchiopods in the larger tree of life.

Figure 1: Cladogram showing physical adaptations and possible evolutionary relationships of the branchiopods.

Brine Shrimp Brine shrimp (Artemia sp.) are small crustaceans found in various saltwater lakes around the world, such as the Great Salt Lake. They inhabit other hypersaline bodies of water, and can tolerate a salinity level of up to 25%. Their development is easy to observe with a microscope. A unique reproductive adaptation makes them an interesting model for studies of natural selection. Under ideal environmental conditions, female brine shrimp produce eggs that hatch quickly into live young; however, when conditions become less conducive, the shrimp


instead produce cysts – encased embryos that cease development (enter diapauses) until conditions are again favorable. When the temperature or the dissolved oxygen level becomes too low or the salinity too high, each egg laid is covered in a hardened, brown chorion, which may keep the embryo viable for many years (in a dry, oxygen-free environment). The brine shrimp used in this activity have been stored in this dormant stage. Once the cysts are incubated in saltwater, the embryos quickly resume their development and hatch. After the cyst breaks open, the brine shrimp remains attached to the shell, surrounded by a hatching membrane. This stage is known as the umbrella stage. The hatching membrane remains attached to the cyst for a number of hours until the young brine shrimp, known as a nauplius, emerges. During the first larval stage, the nauplius subsists on yolk reserved until it molts. During the second stage, the nauplius begins to feed on algae. The nauplius progresses through approximately 15 molts before reaching adulthood in 2 to 3 weeks. Brine shrimp populations are greatly influenced by environmental factors such as salinity. Given the relatively short development time from cysts to nauplius (24-48 hours), the use of brine shrimp in this study is a fast and easy way to observe how some individuals of a population may be better adapted to develop and survive in different environmental conditions.

Figure 2: Photographs illustrating various stages in the life cycle of brine shrimp (Artemia sp.)


Figure 3: Diagram illustrating life cycle of Artemia sp.

MATERIALS NEEDED 5 small petri dishes Paintbrush Salt solutions of varying salinity (0-2%) Double-sided tape Sharpie Graduated cylinder Stereomicroscope Brine shrimp (Artemia sp.) eggs PROCEDURE: DAY ONE

1. Obtain 5 petri dishes. Using the sharpie, label the bottom of each dish with the appropriate concentration of salt solution that will be placed into it.

2. Place a 2 cm piece of double-sided tape in the bottom of each dish. 3. Lightly touch the paintbrush to the side of the container holding the brine shrimp eggs. Your goal is to

collect only approximately 20 eggs on the brush. DO NOT cover the tip of the brush in eggs. Using a greater number of eggs complicates the counting sessions.

4. Carefully transfer the eggs to the first petri dish by dabbing the paintbrush to the tape inside it. 5. Repeat steps 3 and 4 for the remaining petri dishes. 6. Using a stereomicroscope, count the number of eggs in each Petri dish. Record the values for each dish in

the appropriate location in the data table under “0 hours.” 7. Once all your eggs have been counted and recorded, pour 10 mL of the appropriate saline solution in each

dish and cover the dish. 8. Place the filled dishes in an undisturbed location at room temperature for 24 hours. You will need to

return the next day to observe the eggs. You or someone in your group should allot about 15-20 minutes for this.


DAY TWO: 24 hours later 1. Using a stereomicroscope, examine each petri dish and count the number of swimming brine shrimp.

Record the number of swimming shrimp in the data table for the appropriate time period. 2. Count the number of dead or partially hatched shrimp and record this number in the data table for the

appropriate time period. 3. Count the number of unhatched eggs and record this number in the data table for the appropriate time

period. 4. Repeat steps 1-3 for each of the petri dishes.

DAY THREE: 48 hours later

1. Using a stereomicroscope, examine each petri dish and count the number of swimming brine shrimp. Record the number of swimming shrimp in the data table for the appropriate time period.

2. Count the number of dead or partially hatched shrimp and record this number in the data table for the appropriate time period.

3. Count the number of unhatched eggs and record this number in the data table for the appropriate time period.

4. Repeat steps 1-3 for each of the petri dishes. 5. When you have collected all your data, dispose of the eggs/hatchlings according to your teacher’s

instructions. PREDICTION Prior to collecting your remaining data, predict which saline solution will yield the greatest hatching viability. Be sure to justify your prediction. Write your prediction in the space below.


DATA: Record the data you collect for this lab on these data tables. TABLE 1 0 Hours After 24 Hours After 48 Hours

Dish % NaCl # eggs # eggs # dead or partially hatched

# swimming # eggs # dead or partially hatched

# swimming

1 0

2 0.5

3 1

4 1.5

5 2

Now you will calculate the hatching viability percentage. To do this, you will perform the following calculation:

Hatching viability =

Multiply this value to get a percentage. Enter the values in the table below. TABLE 2

Dish % NaCl # eggs Hatching viability %

1 0

2 0.5

3 1

4 1.5

5 2

CONCLUSION AND DISCUSSION For this lab, you will produce and present a mini-poster with your group to your peers and teacher. Your teacher will give you specific guidelines about the production of the mini-poster. Here are things you need to be sure to discuss with your group so that you can talk about them during your poster presentation.

Did your results support your prediction? How do the results of your experiment demonstrate principles of natural selection? What were the limitations of your experiment? How could you change the experiment to reduce the impact

of these limitations? What other independent variables could be tested to demonstrate natural selection in brine shrimp?

natural selection in brine shrimp -...

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