Download GDriftlab

Week #12 Lab: Evolution by Genetic Drift
Introduction: You are hiking in the mountains & accidentally step on a group of beetles that
is on the trail. This species of beetle has genetically-determined body color & displays a lot of
genetic variation for the trait – a bunch of them are blue, some are yellow, & a few are green.
Just by chance, you accidently squash all of the green beetles with your boot & only blue and
yellow beetles remain. Due to that chance event, the beetle population now contains a greater
percentage of the alleles that code for blue & yellow body color, while the alleles for green
body color have been eliminated from the population. You just caused evolution by genetic
drift in the beetle population!
Genetic drift, like natural selection, can cause the allele frequencies for genes to change in
populations. That is, it can cause populations to evolve. But in natural selection, organisms
live or die (&/or reproduce or don’t) because of their genes. Natural selection is NOT a
random process. In genetic drift, however, organisms live or die (&/or reproduce or don’t)
regardless of their genes – it’s simply due to chance. Genetic drift IS a random process.
Genetic drift influences the evolution of all traits, in all populations, all of the time. Even if
natural selection is acting on a trait in a population, chance differences in survival &
reproduction (genetic drift) will always occur, & therefore will always effect the evolution of the
trait to some extent. How powerful a role genetic drift (chance) will play in the evolution of a
trait depends on the size of the population. In large populations, genetic drift tends to be a
relatively weak force of evolution: the loss of just a few alleles due to chance will barely
change the allele percentages of a very large group. But if/when populations are very small,
such as during population bottlenecks or founder events, genetic drift can be a very
powerful force of evolution: the loss of just a few alleles will profoundly change the allele
percentages of a very small group.
Both natural selection & genetic drift have caused human traits to evolve over time. While
much of the genetic variation in human traits around the world can be explained by natural
selection, much can also be explained by genetic drift. Both fossils & DNA provide strong
evidence that our species, Homo sapiens, evolved in Africa approximately 200,000 years ago.
As small groups of humans emigrated out of Africa & founded populations in other regions &
on other continents, these founder events created populations that were genetically different
from the original, source population.
A famous local example of founder effects is the unusually high percentage of geneticallydeaf people that existed on the island of Martha’s Vineyard for over 200 years. Martha’s
Vineyard was founded by a small group of immigrants from the county of Kent in southern
England in the late 1600’s into the early 1700’s. The first known deaf settler on record was
Jonathan Lambert, who immigrated to the Massachusetts Bay Colony in 1694 & settled on the
island. As his form of deafness was heritable, he passed it to his children, who passed it their
children, & so on, causing in an increase in the percentage of deaf people in the community.
The high numbers of deaf people resulted in the formation of a unique from of sign language
known as Martha’s Vineyard Sign Language (MVSL). Census data show that the incidence of
deaf islanders peaked at 1 in 155 islanders; whereas the incidence on the mainland was
roughly 1 in almost 6,000 people. When a deaf school in Connecticut opened in the 1800’s,
many deaf children left the island for school & ended up staying on the mainland to live. By
1
the 1950’s, MVSL had been merged with the newly-formed American Sign Language (ASL) &
the last native-born practitioner of MVSL died, along with the language. By the late 1900’s the
incidence of deafness & the impact of the founder event declined due to increased gene flow
as people continued to immigrate & emigrate to & from the island.
Using the same materials that you used in last week’s lab, follow the procedure below to
simulate genetic drift by a founder event in a moth population.
Learning Objectives: This lab is designed to help you to…
 Understand how genetic drift can act as a force of evolution.
 Distinguish between natural selection & genetic drift.
 Practice problem-solving skills by calculating allele percentages in a population.
 Analyze planetary issues (evolution by genetic drift) & their effect on life & work.
Laboratory Activity #1: Simulating Genetic Drift due to Founder Effects
To simulate the evolution of genetic drift, you will simulate a founder event in moths by having
a small group of moths emigrate from a large population to a new area to begin a new
population. You will then be asked to compare the allele percentages for the color gene from
the original population with that of the newly-founded population. For the simulation, assume
the following: 1) moth color is determined by one gene with two alleles, B (black) and b (white);
and 2) there is incomplete dominance between the two alleles, so moths of genotype BB =
black, Bb = grey, and bb = white.
Will a founder event (genetic drift) cause your moth population to evolve (change genetically &
phenotypically)? What do you think will happen to the percentages of the B & b alleles as a
small part of the population emigrates from the main population & establishes (founds) a new
one? State your hypothesis & prediction below.
Hypothesis:
Prediction:
2
Procedure:
1. Prepare your lab supplies. Using scissors, cut out the 45 black, 45 grey, & 45 white squares
from the sheets labelled “Moths” at the end of this lab. These colored squares will
represent the “moths” in your simulation. Next, cut out the 10 larger black & 10 larger white
squares from the sheet labelled “Gametes” at the end of this lab. Crumple these squares
into balls (or fold in half twice) with the colored side showing on the outside. These little,
colored balls will represent the different alleles for the color gene carried by the “gametes”
(eggs & sperm) produced by the “moths” in your simulation.
2. Set up your moth population. Randomly place 15 black, 15 grey, & 15 white moths (colored
squares) on the “mainland” habitat provided. The phenotypes & allele percentages of this
mainland moth population are already recorded for you in Table 1 below.
3. Simulate a founder event. Close your eyes & randomly pick four moths from the population.
Move these four moths from the “mainland” habitat to the “island” habitat provided, to
simulate them flying off to an uninhabited island (with no bird predators).
4. Collect the appropriate “gametes” (colored balls of paper) from the four founder island
moths based upon their gentoypes, & place the eight gametes in a container – this is the
“gene pool” on the island. Like last week, simulate sex & reproduction of the founders by
picking two gametes at a time from the gene pool & placing a baby moth of the
corresponding genotype/color on the island habitat. Continue this process until you have a
population of 45 new baby moths, but remember to return the gametes to the gene pool
container after each pick!
5. These moths are an annual species, so assume the original four founder moths die after all
that sex! Record the colors of just the baby moths in this newly-founded island population
in Table 1 below.
Results: Table 1 – Colors & allele frequencies of mainland vs. island moths
Time
(moth
generations)
# black
(BB)
moths
# grey
(Bb)
moths
# white
(bb)
moths
#B
alleles
#b
alleles
%B
alleles
%b
alleles
mainland
15
15
15
45
45
50
50
island
Data Analysis: Count up & record the #B & # b alleles in the island population. When counting
up the alleles, remember that each black moth carries two B alleles & each grey moth carries
one B allele; each white moth carries two b alleles & each grey moth carries one b allele.
Thus, the total number of alleles should equal 90 alleles (#B alleles + #b alleles = 90). If they
do not, you’ve done something wrong, & will need to go back & check your numbers before
calculating allele percentages!
3
Calculate the percentage of B & b alleles using the equations below. Remember, the total
number of alleles each generation is 90. Record the percentages in the last two columns of
Table 1.
% B alleles =
% b alleles =
# B alleles x 100
total # alleles
# b alleles x 100
total # alleles
Discussion:
1. Did the founding of an island population of moths cause evolution? That is, did the
phenotypes & allele percentages of the newly-founded island population differ from those of
the mainland population from which they came? Explain.
2. If over time, more & more moths from the mainland flew to the island & reproduced, would
this gene flow cause the island population to become more or less similar to that of the
mainland population? Explain.
4
White “Moths” - Cut out the 45 squares below. Each square represents one adult moth
that is homozygous for the white color allele (phenotype = white & genotype = bb).
Grey “Moths” - Cut out the 45 squares below. Each square represents one adult moth
that is heterozygous for the color gene (phenotype = grey & genotype = Bb).
5
Black “Moths” – Cut out the 45 squares below. Each square represents one adult moth
that is homozygous for the black color allele (phenotype = black & genotype = BB).
6
White “Gametes” – Cut out each of the 10 white squares below. Roll up each square
into a ball (or fold twice). Each will represent a haploid gamete (egg or sperm) carrying
a white (b) allele for the color gene.
Black “Gametes” – Cut out each of the 10 black squares below. Roll up each square
into a ball (or fold twice) with the black color on the outside. Each will represent a
haploid gamete (egg or sperm) carrying a black (B) allele for the color gene.
7
Mainland Habitat
8
Island Habitat
9