4. Natural Selection LAB Online PDF

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1 Biology 120 Lab

Name: ________________________________

4. NATURAL SELECTION LEARNING OUTCOMES: Upon successful completion of this lab, students will be able to 1. 2. 3. 4. 5.

determine the difference between a character and a trait understand that evolution occurs through the process of natural selection identify the steps through which natural selection operates identify the factors that influence the process of natural selection identify the assumptions and restrictions regarding natural selection and how those assumptions change in natural settings

INTRODUCTION: A population of organisms has enormous capacity for growth and could potentially cover the earth quickly under optimal conditions of no predation and unlimited resources. In nature, however, organisms are subject to predation and limited resources. A high reproductive capacity combined with limited available resources and predation results in many more individuals being born than can survive to reproduce. Individuals exhibit heritable variations, and some of these variations will be more advantageous in enabling individuals to survive and reproduce. Thus, individuals possessing favorable variations will be “naturally selected” simply because they are better able to survive and reproduce in a given environment than individuals without those particular traits. A trait represents a variation in a character. Pelage (hair) color in mice is a character possessed by individuals, while brown pelage and gray pelage are two different traits for that character. Traits represent the outward expression (phenotype) of the DNA (genotype) possessed by an individual. Individuals with favorable traits will leave more offspring in the next generation, and those offspring will inherit those traits. Possession of those traits in that particular environment will give those offspring an advantage as well, enabling them to survive to reproduce, and pass on those traits to their offspring. Over extended periods of time, the genetic composition of the population will be such that most individuals will be adapted to their environment. Natural selection does not create adaptations in the sense that a trait arises because there is a particular need for it. For example, a giraffe’s neck did not become long because it wanted to reach upper vegetation to feed. In a population of ancestral forms, there was genetic variation in neck length. Individuals with longer necks were able to reach higher vegetation that was inaccessible to other individuals. With an increased food supply, those individuals were better able to survive and reproduce. Their offspring would possess the trait of a longer neck, and thus, would also have a greater probability of surviving and reproducing. Over extended periods of

2 time, neck length became increasingly longer. Individuals that had very long necks had an advantage in that particular environment, and were adapted to that environment. Charles Darwin was one of the first individuals to notice that some individuals with particular traits were more prevalent in particular environments than other individuals. He formulated his theory of evolution through natural selection based on his observations that

1: there is variation within all populations, and 2: these individuals compete for resources (struggle for existence).

From this, he hypothesized that evolution through natural selection can occur in five basic steps: 1. 2. 3. 4.

All populations of sexually reproducing individuals show variation in particular traits. These populations produce an excess of offspring. Excess offspring compete for limited resources. Those individuals possessing traits that give them a competitive advantage in the environment will be able to secure more resources, and survive to reproduce. That is, the most “fit” individuals will survive. 5. Traits possessed by those “fit” individuals are heritable, and passed on to the offspring in the next generation. NATURAL SELECTION SIMULATION: Go to the following website: https://askabiologist.asu.edu/peppered-moths-game/index.html Click on "How to Play" and read the “instructions. Click on the “Play Game”

Follow arrows on the screen that says “Choose your forest” Start with the light forest. You are the predator (big black bird). Run simulation for the full 60 seconds, during which you will move the bird and eat moths. At the bottom of the screen you will see the “dark” and “light” percentages. Record the percentage of dark and light moths in the population. Repeat the above experiment using the “dark forest” background. QUESTION 1: Complete the table below. percentage of dark moths light colored forest dark colored forest

percentage of light moths

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QUESTION 2. What is the character in the moth population on which natural selection acted? What are the traits in the population?

QUESTION 3: Which color moth was best adapted to the pre-industrial forest background? Least adapted in to this background? How did this change with changing habitat?

QUESTION 4: In this simulation, if a moth does not get eaten, it may reproduce. Do all individuals get to breed in natural settings? Will some individuals leave more offspring than others?

QUESTION 5: Reproduction in moths is sexual. Do you think it is possible for a dark moth and a light moth to reproduce? How would this impact the traits of the offspring?

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