Evolution through natural selection: A simple simulation

Transcription

1 Evolution through natural selection: A simple simulation Objectives: This is an observational lab, wherein you will do simulations that help to understand the basic principals of evolution through natural selection. When you finish this lab, you should: 1. Understand how prey phenotype, predation and the environment interact to determine prey fitness (natural selection) 2. Understand how differences among prey in fitness produces, across generations, differences in mean phenotype (evolution) I. Background: By the mid 1800s, the term "evolution" was already in use to describe observed changes in heritable phenotype across generations, but natural historians of the time disagreed about the cause (forces) of these changes. Darwin and Wallace's great breakthrough was to recognize that evolution could be explained by individual differences in reproductive success (number of offspring). Darwin's term for this was "natural selection," parallel to the already accepted term of "artificial selection" used to describe breeding programs for domesticated plants and animals. For populations to evolve, they must possess two attributes: a. there must be variation in phenotype among individuals b. the variation among individuals is heritable (due to genetic differences - variation) If there is no variation in a particular characteristic, such as number of legs, then no matter how favorable it might be to have six or eight legs, that trait cannot evolve. Similarly, a change in a trait can only evolve if the variation is genetically determined. Tattoos are not genetically determined and so the frequency of tattooing cannot change through biological evolution. Note that any observed phenotype can be tested for evolutionary change; quantitative (number of legs) and qualitative (color) traits can both evolve. If there is no correlation of phenotype to number of offspring, any evolutionary changes will be random. We call this genetic drift. By adding a third criterion to our list, evolution becomes non-random: c. individuals with a particular phenotype leave more offspring We are now proposing that selection favors some phenotypes over others. In natural selection, the environment presents challenges that some individuals overcome more easily than others as a result of their traits. Individuals with particular phenotypes may leave more offspring than others for a variety of reasons: they are more efficient at gathering resources and growing, they survive longer, they have more opportunities to mate, or they have healthier offspring. This is differential fitness, meaning that some individuals produce more offspring than others. Over successive generations, the phenotypes of individuals with higher fitness will become more common in the population. Differential fitness, favoring one phenotype over another, will produce a change in the average phenotype of the population; this means evolution has occurred. Importantly, note that selection occurs among individuals in one generation, while evolution occurs in a population across generations. Darwin sorted selective forces into two categories. Natural selection is imposed by the environment on the individual. Sexual selection is imposed strictly by the behaviors and conditions surrounding mating. In this lab, we are going to focus on natural selection as the mechanism for evolutionary change in a population of beans of three colors.

2 Bean Population Part 1 You will simulate natural selection in a bean population and test the following hypothesis. Hypothesis: Organisms (beans) that blend into their environment will possess greater fitness than organisms that are more visible. Procedure Part 1: 1) Choose one student to be record keeper. 2) Count out 20 of each color of bean and mix them together into a container. 3) The record keeper distributes the beans on the cloth while everyone else looks away. 4) Each "predator" has 20 seconds to capture beans with a set of tweezers and transfer them to a container. 5) At the end of 20 seconds, all predation must stop. Beans in process of being captured "escape". 6) Count up the total number of captured beans in each color, and subtract from the original 20. The remainders are the ones that survived, and they are allowed to produce 2 offspring each. Record your totals in Table 1. 7) Add the surviving beans along with their offspring to the cloth and repeat the procedure for two more generations. 8) After each generation, perform the same calculations and record your totals in Table 2 and 3. Sample calculation: For generation 0: 20 white beans Number captured by predators: 12 white beans Number of survivors: = 8 white beans Number of offspring: 8 survivors x 2 offspring each = 16. For generation 1, add 24 white beans: 16 white beans (offspring) + 8 white beans (surviving). Predictions: (RECORD your answers on a separate page.) Considering your selective environment, what differences in fitness do you expect between individual beans of different colors? Considering your lab section as a whole, two groups have plain white "environments" and three groups have patterned environments. What differences in selection and evolutionary change do you expect to find with these environmental differences?

3 Data sheet Part 1: TABLE 1 Generation Captured 1 Surviving 1 Offspring 1 Generation 1 = TABLE 2 Generation 1 (last number from Table 1) Captured 2 Surviving 2 Offspring 2 Generation 2 = TABLE 3 Generation 2 (last number from Table 2) Captured 3 Surviving 3 Offspring 3 Generation 3 = Evaluation Part 1: (RECORD your answers on a separate page.) What did you observe for the bean population as a whole? For each type of colored bean? Do your results support the hypothesis or not? Explain what has happened in your own words. Compare your results to one other group with the same environment. Did you see the same pattern of evolutionary change? Why or why not? Make a list of ALL factors you can think of that could influence the evolution of the bean population. (Discuss as a class.)

4 Bean Population Part 2 Choose one of the factors (variables) that could alter evolution of your bean population. Using your results from Part 1 AND this variable, formulate a new hypothesis about bean variation. Then modify the procedure from Part 1 to test your hypothesis. Lastly, make predictions about evolutionary change of the bean population. Part 2 - Hypothesis: (RECORD your answer on a separate page.) Part 2 - Modifications to Procedure: (Indicate your changes on a separate page.) Part 2 - Prediction(s): (RECORD your answers on a separate page.) Do you expect to see differential fitness for beans of different colors? Why or why not? Procedure Part 2: Repeat your experiment with modifications and record your answers in the Data Tables for Part 2. Data sheet Part 2: TABLE 1 Generation Captured 1 Surviving 1 Offspring 1 Generation 1 = TABLE 2 Generation 1 (last number from Table 1) Captured 2 Surviving 2 Offspring 2 Generation 2 =

5 TABLE 3 Generation 2 (last number from Table 2) Captured 3 Surviving 3 Offspring 3 Generation 3 = Evaluation: (RECORD your answers on a separate page.) What did you observe about fitness for the bean population as a whole? For each type of colored bean? Do your results support the hypothesis or not? Explain what has happened in your own words. In the real world, explain how your variable may lead to differential fitness in a population. If it helps, use a concrete example to describe the connection.