Biol 1441- Principles of Biology II

Microevolution
Simulating Natural Selection

Objectives

 1. Observe how natural selection can change gene frequencies over time using a simulation exercise involving cryptic coloration.

2.  Examine natural selection as a mechanism causing change or microevolution in a population.

3.  Explore the Hardy-Weinberg Principle as a tool to test that natural selection is occurring.

4.  Discuss factors that affect microevolution such as gene flow, genetic or random drift, and mutation.

Students need

 1. calculator with statistical functions
2. graph paper

To become familiar with doing the calculations for allele frequencies by completing the practice worksheet 

 

Reading

  Introduction to Hardy-Weinberg Equilibrium
  More on Hardy Weinberg
  When Hardy-Weinberg fails
Homework   Natural Selection and Hardy-Weinberg Principle 
Format

Natural Selection for Cryptic Coloration
The color of an organism can be an adaptation to escaping predation. Cryptic coloration is a form of passive defense in which the potential prey species is colored so as to blend into its surrounding habitat, making its discovery by a predator difficult. Aposematic coloration is found in organisms capable of producing defensive chemicals, and the organisms bright colors (usually red or yellow) serve as a warning to potential predators that have come to associate the color with nasty chemicals.

A potential prey species may also gain an advantage through mimicry of another species, often a distasteful species with aposematic coloration. One example of mimicry is in the seeds of weeds plants of agricultural fields. The seeds of some weeds have adapted to closely resemble a crop plant’s harvested seeds. (F. Gould, 1991, The evolutionary potential of crop pests, American Scientist 79: 496-507.

How could this occur through Natural Selection?

In this exercise you will examine natural selection for cryptic coloration in a population of bean seeds. This population contains two different color alleles: A is an allele for red color and a is an allele for white. The alleles are co-dominant, so there are three morphs in the population: red (AA), mottle (Aa), and white (aa). The frequency of A in the population is Hardy-Weinberg's, p and the frequency of a in the population is q.  The habitat for these beans is a tub of smaller red beans. You will act as a visual predator on the population of large beans by removing as many large beans as you can in a 20 sec time interval.  Then you will act as a night predator with 1min to hunt.

What would be the "control" for this experiment?

Procedure.
1.   Begin by making a population that has allele frequencies of p=0.5 for the A allele and q=0.5 for the a allele. Use the Hardy-Weinburg formula 

(p + q)2 = p2 + 2pq + q2 = 1.0

to calculate the proportion of each genotype that will be going into the habitat, and add a total of 60 beans to the habitat (round up for fractions of beans). Check with the lab instructor to make sure you have your numbers correct before you begin, then add your bean population to its habitat, put the lid on, and mix it up. This will be your starting population, before selection.

2.  You will act as a predator by finding and consuming (its just a simulation, so you don’t really have to eat them) as many of the large beans as you can in 30 seconds. A different student should act as the predator for each generation. This 30 second period represents one generation for the prey. At the end of time period, use a sieve to collect the surviving beans, count them, and record the data in the table. The table is designed to lead you through each calculation of allele frequencies to the next generation.

3. An example worksheet of the tables for two generations follows. Complete the calculations of the missing data.   

Use this link to familiarize yourself with the worksheet you need for lab 
 practice worksheet ]

4. Graph the results of A (p) and/or a (q) allele frequency for both types of predation. Put data from both experiments (clearly labeled) in the same figure.

How would you calculate the rate of allele change?
What would be the units?
How would the data look if only genetic drift was occurring?