4

BIOL 119, Fall 2012 Natural Selection

NATURAL SELECTION

Overview

This simulation of the process of natural selection will illustrate how a population of seeds can become adapted to an environment over a period of generations. An initial population with a large amount of variation will be exposed to a predator (you), and changes in the proportions of different prey phenotypes will be measured in three different environments.

Introduction

The purpose of this lab is to demonstrate the process of natural selection by simulating the effect of a selective predator feeding upon a variably colored prey population in several different environments. The change in frequency of different colored prey types in the population, as a result of the predator’s feeding efforts, will represent adaptation to the environment by the prey population.

Ecology is the study of the interactions of organisms with one another and with their physical environment. These interactions are also the driving mechanism of evolution via natural selection. In the strictest sense, biological evolution is defined as any change in gene frequency. An organism’s genes in large part determine the characteristics of that organism. Thus, the genotype of your cells determine many of your physical traits such as hair color or eye color. These traits together make up your phenotype, or the sum total of your physical characteristics. In many cases an individual’s physical characteristics determine its ability to survive and reproduce in a given environment. Natural selection is simply the process by which individuals with certain phenotypes survive better and reproduce more than individuals with other phenotypes within a particular environment.

Natural selection is only one of the forces that can change gene frequencies (what are some others?). However, most evolutionary biologists agree that natural selection is the major driving force of evolution. In this lab we will illustrate how a common ecological event - predation - can serve as an agent of natural selection. Predators usually can find or capture some prey individuals more easily than others. If the differences in prey susceptibility are traits which are determined by their genes, and therefore these traits are heritable (offspring inherit these traits when they get their genes from their parents), then the prey population may evolve in the direction of reduced susceptibility to capture.

The evolution of color patterns in the British peppered moth, Biston betularia, is a classic example of directional selection in response to predation (and changing environmental conditions). These moths, which rest on tree trunks during the day, occur in a light colored form with a sprinkling of dark spots and a dark, or melanic, form.

In the woodlands around Manchester, England, industrialization in the early part of this century killed the lichens which normally grew on trees and blackened the tree trunks with soot. As this happened, the previously rare dark phenotype came to dominate the woodlands and the light form was rarely seen. In other areas, further from the factories and smokestacks of Manchester, lichens abounded on the tree trunks and the light phenotype was common while the dark phenotype was rarely seen. In each habitat the common phenotype was better camouflaged.

Kettlewell (1955, 1961, 1965) knew that these moths were preyed upon by birds during the day, and he hypothesized that birds preyed selectively on whichever phenotype was less cryptic, causing the observed correspondence between moth color and habitat color. By releasing both phenotypes in each habitat and observing bird predation from a hiding place he was able to confirm this hypothesis. As further evidence, industrial pollution around Manchester has abated in recent years and the areas which had soot-darkened, lichen-free tree trunks are returning to their pre-industrial condition. In response to this environmental change, the light phenotype is once again replacing the dark phenotypes in such areas (Berry 1990, Clarke et al. 1990).

This type of selection, in which the distribution of phenotypes is shifted in one direction (i.e. light to dark, or dark to light) is called ‘directional selection’. Natural selection also can act to maintain the existing distribution of phenotypes in a population (stabilizing selection), or selection can favor extreme phenotypes at the expense of intermediate phenotypes (disruptive selection). The goal of this simulation is to learn how natural selection functions, and to determine which of these three types of selection is operating in each of the three artificial environments.

Methods

Creating the Population

You will use ten different seed types that vary in color and shape but are all approximately the same size to represent the various phenotypes in our population. This is meant to illustrate a population of a single species in which there are ten different color phenotypes. The initial population will consist of 10 individuals of each of the ten seed types (100 seeds in all). Each group of students will conduct the experiment in each of three habitat types: homogeneous white, homogeneous packing peanut, and patchy.

Natural Selection

Scatter the seed population within the chosen environment. We will imagine that the population was formerly adapted to gaudy surroundings; perhaps a tropical area with a great array of flowering plants. A climatic catastrophe, which the seeds have survived, has resulted in these three new habitat types. One of you will act as the predator (who also survived the catastrophe). The predator ‘enters’ the populated area and gathers seeds as rapidly as possible within the guidelines for predator behavior given below. The predator must always take the first seed it sees. Seeds are to be taken one at a time and must be dropped into an Erlenmeyer flask. This will ensure that visual contact with the foraging area is broken momentarily following each seed capture. Foraging will continue until the appropriate number of seeds has been taken. The exact number of seeds taken in each seed category must then be tabulated on the appropriate line of the data sheet in order to know which seeds remain in the population.

Determine the number of seeds of each type that escaped predation by counting the number of seeds of each type that were caught. After counting them, place the captured seeds back in the stock of unused seeds (please keep the seed varieties separate). Record the number of seeds of each phenotype that were captured and that survived (see Sample Data Sheet).

Reproduction of Survivors

The reproductive process in our simulation will be asexual to avoid the necessity of simulating the genetic complexities of sexual reproduction (and to keep your minds focused on the goals of the lab!). Each survivor will produce offspring that are genetically and phenotypically identical to itself. Count out the appropriate number of seeds of each variety, scatter these offspring onto the habitat and start a new bout of predation. Make sure you start with 100 seeds each time!

Different Habitats

Perform the experiment in each of the three habitat types. A different student may be the predator in different habitats, but all predators should be able to distinguish the colors of all seed types and have good visual acuity.

Rates of Predation

Pick one habitat and perform the experiment with 2 different predation rates. Be sure to balance the rate of predation and the rate of reproduction so that the total number of seeds is 100 at the start of each predation session. For example, the predator might take 50 seeds, in which case each of the remaining seeds should produce a single identical offspring. If the predator takes 80 seeds, each remaining seed should produce 4 identical offspring.

Summarizing Your Results

For each experiment, graph the number of seeds of each type as a function of generation.

Alternative Experiment

Although natural selection is commonly referred to as ‘survival of the fittest’, the true measure of fitness is successful reproduction. An individual that survives to a ripe old age, but who does not leave any offspring, has a much lower fitness than an individual who dies young after having many offspring. This simulation deals only with differential survival and does not address differential reproduction. How might you design an experiment to explore the interaction between differential survival and differential reproduction? How much of an increase in reproduction would be necessary to balance a higher mortality rate? You don't need to actually do this experiment, but think through how you would!

Hand In

1. A data sheet and a set of graphs for each habitat type. The graphs should show the number of seeds of each phenotype that were present at the start generations 1 - 4.

2. Answer the following questions:

a. Which seed phenotypes were selected for or against in each habitat type?

b. What type of selection (directional, stabilizing, disruptive, other) is indicated by each of the graphs? Did the type of selection appear to remain constant for each habitat type?

c. How did the pattern of selection differ between the homogeneous and the patchy habitats?

d. Did the effect of natural selection differ as a function of the rate of predation?

e. What kinds of predators do you think would be more or less likely to influence the evolution of seed color?

3. Your alternative experimental design.

Literature Cited

Berry, R.J. 1990. Industrial melanism and peppered moths (Biston betularia (L.)). Biological Journal of the Linnean Society 39:301-322.

Clarke, C.A., F.M.M. Clarke, and H.C. Dawkins. 1990. Biston betularia (the peppered moth) in West Kirby, Wirral, 1959-1989: updating the decline in f. carbonaria. Biological Journal of the Linnean Society 39:323-326.

Kettlewell, H.B.D. 1955. Selection experiments on industrial melanism in the Lepidoptera. Heredity 10:287-301.

Kettlewell, H.B.D. 1961. The phenomenon of industrial melanism in Lepidoptera. Annual Review of Entomology 6:245-262.

Kettlewell, H.B.D. 1965. Insect survival and selection for pattern. Science 148:1290-1296.

Seed Type / BB / BP / GP / GN / KB / PB / SF / SP / SN / YC / Total
Generation 1 / 10 / 10 / 10 / 10 / 10 / 10 / 10 / 10 / 10 / 10 / 100
# captured
# surviving
# of offspring
Generation 2 / 100
# captured
# surviving
# offspring
Generation 3 / 100
# captured
# surviving
# offspring
Generation 4 / 100
# captured
# surviving
# offspring

Sample Data Sheet

Habitat Type: ______Rate of Predation: ______

Seed Types: BB: Black Bean PB: Pinto Bean

BP: Blackeyed Pea SF: Sunflower Seed

GP: Green Pea SP: Split Pea

GN: Great Northern Navy SN: Small Navy

KB: Kidney Bean YC: Yellow Corn