Download Butterfly evolution lab TA guide

Butterfly evolution lab – TA guide
Background: the students will have just learned about the events that led Darwin/Wallace to propose
that organisms change over time (descent with modification), and that a particular mechanism, natural
selection, could explain why organisms are so well suited to their environment. In the second week of
lectures, we will be connecting this historical phenotype-centered view with genetics and redefine
evolution as a change in allele frequencies across generations. Please note that this lab does not focus
on changes in allele frequencies at all, since it deals with quantitative traits and thus evolutionary
change is defined as a change in mean phenotype across generations. This of course does not mean
that there aren’t genes controlling these traits, but that we need to think about their effects in a
statistical way, producing a relationship between parent and offspring traits. This will be tough for the
students to grasp since they are used to just thinking about Mendelian traits.
They will be learning the general terms highlighted in the lab text in lecture as well as lab. I have
provided a glossary of terms for you that you can give to your students if you like. Please look these
over and make sure your presentations use the same terminology (these are the terms used in the
textbook, but not everyone uses these terms in precisely this way so check yourself in advance). The
details of how to quantify variation, selection and parent-offspring resemblance will only be presented
here so please make a point to define and talk about them as you go through lab. Some of these may be
new to you so spend some time making sure you are up to speed before you walk in the door.
Students ALWAYS think they know all about evolution and don’t need to learn about it again. This is
unfortunate, because in fact students have a lot of misconceptions about how evolution by natural
selection works and tend to do poorly on this material as a result. The goal of this lab is to have them
confront their misconceptions head-on and begin to replace them with precise information. I have
provided you with a misconceptions handout listing some of the issues we have seen in past classes,
emphasizing four that you can most directly challenge as you go through the lab exercises.
Where to Start: Start with a big overview of where they will be going this semester . The overarching
theme is biodiversity - describing and classifying the diversity of living things, where diversity comes
from and why, and the diversity of ways in which organisms deal with challenges imposed by their
environment. Some fun slides of wacky diverse organisms and how they seem to “fit” the places they
live in might be fun way to introduce this broad theme. Do not yet have them take out their lab
handout. Then ask the students themselves for an explanation of why organisms seem to be so well
suited to their environments. Someone will undoubtedly say evolution by natural selection. Ask them
to describe how this works, and outline what they say on the board with NO CORRECTIONS OR
SUGGESTIONS FROM YOU, especially if they volunteer something not quite right. The whole idea is that
their misconceptions should be up there in writing, ready to be tested with the lab exercises. Although I
know it is the usual procedure for labs, please do not do a slide presentation of the “correct” view of
evolution by natural selection at the beginning (you can do one at the very end to reinforce what they
have learned if you like). Once you have their view of the process on the board, you can introduce the
lab exercise, explain that they will be testing their ideas of how the process works with a natural
population of butterflies, and have them take out the lab text.
Points for discussion: As you finish each part of the lab, return to their statements and evaluate as a
group whether they are accurate - e.g., is it really correct to talk about “a mutant” in the case of the
butterflies? How can we better describe the phenotypes present in this population? Do fit individuals
always do well or does natural selection describe how phenotypes do on average? Is natural selection
an absolute property (if you “pass on your genes” you are fit) or should it be defined relative to others
(everyone might do ok, but some reproduce more than others)? Please try to get them to talk so you
can help them to articulate what they are learning in quantitative, relative, terms (rather than the
absolute, anecdotal, individual-focused style they are comfortable with).
Hopefully by the end you will have addressed all the misconceptions they proposed at the
beginning, and they should now be able to describe the process of evolution by natural selection with
the new terms and ideas you have learned about. A wrap-up Powerpoint could be used here to
reinforce the concepts if you like. Finally, you might have time to show them the cases of butterfly
species from the natural history collection, and get them talking about how the microevolutionary
process within a species you investigated in this lab might be able to explain the phenotypic diversity
among species we see in nature.
What to Expect: The students will be dealing with two computer programs (ImageJ and Excel). They
are going to need a lot of one-on-one mentoring on how to use them. Don’t expect them to be able to
do even basic tasks in Excel like copy and paste, let alone using formulas or creating and editing graphs.
Be prepared to demonstrate how to do all these tasks with your computer on the big screen! In terms
of the experimental results, expect that both selection differentials and heritabilities will be low, such
that the response to selection in a single generation will be tiny. This is actually good, as it allows you to
emphasize to the students that a major component to the process of adaptation is TIME.
Key goals:
1) Understand evolution by natural selection as a quantitative process with distinct components
2) Learn to describe and calculate population-level measures (distribution, mean, etc.)
3) Become familiar with using Excel for data entry, calculations and graphics
A helpful figure for you to conceptualize the relationships among R, S and h2 (this example is from cattle
breeding, and the equation is actually called the Breeder’s Equation). The dots are average values for all
pairs of parents and offspring, with the population mean at the origin and the line indicating the slope of
the relationship (h2). If you imagine you only selected a subset of the parents to contribute to the next
generation, such that the mean of the selected individuals equaled S, then you would expect the mean
of their offspring to be R.
h2
Useful background references for you in preparation for the lab:
1) Chapter 9 of Freeman and Herron, Evolutionary Analysis. A great overview of quantitative
genetics that includes all of the analyses done here.
2) A good example of using heritability to assess evolutionary potential in the face of climate
change: Doyle, C. M. et al. 2011. Heritability of heat tolerance in a small livebearing fish,
Heterandria formosa. Ecotoxicology 20:535-542.
3) An example of measuring the strength and direction of selection in a natural field population:
Johnson, D. W. & Hixon, M. A. 2011. Sexual and lifetime selection on body size in a marine fish:
the importance of life-history tradeoffs. Journal of Evolutionary Biology 24:1653-1663.
4) For an example of how phenotypic distributions can vary among populations in mean, standard
deviation and even shape (unimodal versus bimodal): Verdú, J. R. 2011. Chill tolerance
variability within and among populations in the dung beetle Canthus humectus hidalgoensis
along an altitudinal gradient in the Mexican semi-arid high plateau. Journal of Arid
Environments 75:119-124.