Docsity
Docsity

Prepare for your exams
Prepare for your exams

Study with the several resources on Docsity


Earn points to download
Earn points to download

Earn points by helping other students or get them with a premium plan


Guidelines and tips
Guidelines and tips

Evolutionary Mechanisms: Microevolution - Lecture Notes | BI 102, Lab Reports of Biology

Material Type: Lab; Class: General Biology II; Subject: Biology; University: Central Oregon Community College; Term: Unknown 1989;

Typology: Lab Reports

Pre 2010

Uploaded on 08/16/2009

koofers-user-htm
koofers-user-htm 🇺🇸

4

(1)

10 documents

1 / 4

Toggle sidebar

This page cannot be seen from the preview

Don't miss anything!

bg1
Evolutionary Mechanisms: Microevolution
Objectives
Identify the mechanisms that produce change in allele frequencies.
Test hypotheses about allele frequencies using simulations
Draw conclusions about how effective different evolutionary mechanisms can be.
You, as an individual, have a whole set of genes, your genome, which are segments of the DNA
molecules we call chromosomes. The genes code for building and maintaining you as a living
creature. Can you (or any individual) evolve in the biological sense? In other words, will your
alleles, the different versions of a particular gene, change over your lifetime?
You share genes with other humans and it is possible to determine allele frequencies for a
population, members of a species that share a particular area at a specified time. The sum total
of these genes in a population is called the gene pool. Population membership changes over
time. Think of some ways in which this could happen. If population membership changes,
would you expect that, under certain conditions, allele frequencies in this population could
change over time as well?
This is the topic of today’s lab—how allele frequencies can change over time or the mechanisms
of evolution. If allele frequencies are significantly changing over time, the population has
evolved. To observe evolution, we would need to look at several generations (perhaps 100’s or
1000’s) of a population. This would be difficult to achieve in a three-hour laboratory. Even
bacteria need approximately a half-hour to reproduce! Instead, you will use simulations
(simplified models of the real thing) to determine if and how evolution of population occurs. The
primary goal of this lab is to give you insight about how evolution can and does work.
Review of Mendelian Genetics
It is important to distinguish an individual’s appearance, or phenotype, from the alleles present in
that individual’s cells. Every individual carries two alleles of a given gene; the description of
these alleles is a genotype. Recessive alleles are those that are masked or hidden by another
allele. For example, the allele for blue eye color (recessive) is masked by an allele for a pigment
such as brown. Pigment is thus dominant over blue. By convention, recessive alleles are denoted
be lowercase letters and dominant alleles by uppercase letters. If “P” stands for pigment and “p”
stands for nonpigmented eye color, then blue-eyed individuals have the genotype “pp” since they
have two recessive alleles, while individuals with pigmented eyes are either “Pp” or “PP”.
We cannot identify the correct genotype for the pigmented phenotype unless we gain more
information about parents and/or offspring, since it takes only one dominant allele to mask a
recessive allele. Note that some genotypes involve identical pairs (“pp” and “PP”, called
homozygous (alike) and some involve a mixed pair (“Pp”, called heterozygous).
Most human traits, such as facial features, height, dexterity, etc. are controlled by numerous
alleles, and many of these have complex interactions among themselves and with the
environment. These traits can be very difficult to study. Some human physical and biochemical
traits are controlled through the inheritance of single genes with two or more alternate alleles, for
example traits such as blood type. We will examine eight such traits in this class.
Bi 102 Laboratory Microevolution 1
pf3
pf4

Partial preview of the text

Download Evolutionary Mechanisms: Microevolution - Lecture Notes | BI 102 and more Lab Reports Biology in PDF only on Docsity!

Evolutionary Mechanisms: Microevolution

Objectives

 Identify the mechanisms that produce change in allele frequencies.  Test hypotheses about allele frequencies using simulations  Draw conclusions about how effective different evolutionary mechanisms can be. You, as an individual, have a whole set of genes , your genome, which are segments of the DNA molecules we call chromosomes. The genes code for building and maintaining you as a living creature. Can you (or any individual) evolve in the biological sense? In other words, will your alleles, the different versions of a particular gene, change over your lifetime? You share genes with other humans and it is possible to determine allele frequencies for a population, members of a species that share a particular area at a specified time. The sum total of these genes in a population is called the gene pool. Population membership changes over time. Think of some ways in which this could happen. If population membership changes, would you expect that, under certain conditions, allele frequencies in this population could change over time as well? This is the topic of today’s lab—how allele frequencies can change over time or the mechanisms of evolution. If allele frequencies are significantly changing over time, the population has evolved. To observe evolution, we would need to look at several generations (perhaps 100’s or 1000’s) of a population. This would be difficult to achieve in a three-hour laboratory. Even bacteria need approximately a half-hour to reproduce! Instead, you will use simulations (simplified models of the real thing) to determine if and how evolution of population occurs. The primary goal of this lab is to give you insight about how evolution can and does work. Review of Mendelian Genetics It is important to distinguish an individual’s appearance, or phenotype , from the alleles present in that individual’s cells. Every individual carries two alleles of a given gene; the description of these alleles is a genotype. Recessive alleles are those that are masked or hidden by another allele. For example, the allele for blue eye color ( recessive ) is masked by an allele for a pigment such as brown. Pigment is thus dominant over blue. By convention, recessive alleles are denoted be lowercase letters and dominant alleles by uppercase letters. If “P” stands for pigment and “p” stands for nonpigmented eye color, then blue-eyed individuals have the genotype “pp” since they have two recessive alleles, while individuals with pigmented eyes are either “Pp” or “PP”. We cannot identify the correct genotype for the pigmented phenotype unless we gain more information about parents and/or offspring, since it takes only one dominant allele to mask a recessive allele. Note that some genotypes involve identical pairs (“pp” and “PP”, called homozygous (alike) and some involve a mixed pair (“Pp”, called heterozygous ). Most human traits, such as facial features, height, dexterity, etc. are controlled by numerous alleles, and many of these have complex interactions among themselves and with the environment. These traits can be very difficult to study. Some human physical and biochemical traits are controlled through the inheritance of single genes with two or more alternate alleles, for example traits such as blood type. We will examine eight such traits in this class.

  1. For each of the following traits, determine your phenotype and note it in Table 1. Record your genotype for each of the traits. Remember that if you have a recessive characteristic you must have both recessive alleles, but if you have a dominant phenotype you have no way of knowing if you carry a recessive allele (unless you have children). In this case, use a dash (–) to represent the unknown second allele.
  2. When you have determined your phenotypes for those listed in Table 1, record your results on the board. Fill in the table with the class results, and then determine the percentage of individuals who have each trait. To calculate percentages you will need the total number of students in your section. Letter Characteristic Your Phenotype Your Genotype Lab Totals Number Dom Rec Percentage Dom Rec

D Dimpled chin or

non-dimpled chin

E Free earlobes or

attached earlobes

W Widow’s peak or

straight hairline

T Taster of PTC or

nontaster

F Left thumb on top

Right thumb on top

B Bent little finger or

finger not bent

N Normal thumb or

Hitchhiker’s thumb

R Tongue roller or

nonroller

Population Simulations with Beans

To determine if a population is evolving it is necessary to keep track of allele frequencies in the population over time. Alleles, for sexually reproducing organisms, of course, are inherited as a pair, each associated with a homologous chromosome. One allele is inherited from the father (via sperm) and the other from the mother (via the ovum or egg). Therefore, each individual in the population will contribute two alleles. To simplify things, you will only consider two versions of a gene. Let’s say we have a dominant allele, A , and a recessive allele, a. We need to know the frequency of each in the initial population so that it is possible to track any changes in frequency over time. Let’s say the frequency of A is represented by the letter p. And q represents the frequency of a. Because, for this example, there are only A’s and a’s in the population and the sum of all frequencies should equal 100% (or 1.00), then what is the relationship of p and q to the total frequency of alleles in the population? Write the equation showing the relationship between the total frequency (i.e. 1.00) and the frequencies (p and q) of A and a in the population (see Question 1 on the lab assignment sheet to turn in).

Repeat this procedure again using only the beans in the new cup, then adjust the number of red and white beans in the cup based on your results. Continue to draw new population combinations until you have data for ten generations or have only one color of bean left. Record your genotype and allele frequencies for the tenth generation on the board.

Computer Simulations

The problem with the bean simulations is they can take up time to model even a few generations. Now we will model microevolutionary mechanisms, but using a computer program instead of beans. By using the computer, it is possible to collect simulated data over 100’s of generations rather than only a few. The computer program we will be using is called Ecobeaker. The chapter beginning on page L-155 in the Software guide entitled “Islands and Natural Selection” is the one that we will use for this computer simulation. After the instructor gives you an introduction to the program, follow the instructions below to conduct the models.

What happens when there is no selection?

  1. Go to the parameters box and change the number of beak length loci setting to one (instead of 10). We will ignore the beak depth for these models so set the initial deep beak freq to zero. Also note that there are only three lengths possible here: long, medium and short because there is only one gene with two alleles controlling length (the heterozygous combination expresses as the medium length). To start the simulation, set the carrying capacity to 1000 (this sets the population size).
  2. Click the start button and let the model run for about 400 generations. Keep track of how variable the average beak length is (upper graph with green dots) by noting the extremes (longest and shortest average beak lengths. The shortest possible is 2 and the longest possible is 10). After recording the range of average values from the upper graph, repeat the simulation four more times and record the range values each time.
  3. Answer the questions about this simulation on the lab report.
  4. Now try changing the population size. This is done by altering the carrying capacity setting for the model. Try setting the carrying capacity to 500 instead of 1000. Repeat the simulation five times to get a better sense of the pattern you observe because some randomness is built into the model. Compare the extreme high and low values that you see for this simulation compared with the first. Now set the carrying capacity to 250. Again run it four more times and note your extreme high and low values. Do this for carry capacity of 100 and finally 75 as well. On the report, report the carrying capacity, the range (low and high) and if the population seemed to go to one phenotype.

Selection Coefficient Simulations

Change the following parameters: Best beak length: 10 Length selection coefficient: 0. Carrying capacity: 500 Num beak length loci: 10 Now run the simulation for 300 generations and then answer question 9. Now tinker with the selection coefficient by changing its values (can range from 0-1). Remember to repeat each simulation five times to determine a pattern.