Download 1.0.Darwinian Snails

Evolution
Simulation
Littorina obtusata, the flat periwinkle
http://www.arkive.org/flat-periwinkle/littorina-obtusata/video-00.html
Darwinian
Snails
http://www.arkive.org/common-shore-crab/carcinus-maenas/video-08.html
Carcinus maenus, the green crab
http://www.pbs.org/wgbh/evolution/educators/teachstuds/svideos.html
Range of Carcinus maenas. Blue areas are the native range; red areas are the
introduced or invasive range. Black dots represent single sightings that did
not lead to invasion, and green areas are the potential range of the species.
Appledore Island
•! One of seven islands forming Isles
of Shoals
•! About 7 mi off coast of Maine
Robin Seeley’s Experiment
14
Shorter & thicker
7
•!compared shells in two populations of
snails on Appledore Island
•!1871- before arrival of green crab
•!1980s - after arrival
•!1980s shells: thicker and narrower range
of variation
Taller & thinner
Snail Population Evolved
Based on his analysis, Seeley predicted that the
snail population had evolved
Evolution:
change in composition of population
•! Average shell thickness
•! Range of variation
•! Spire height
Part I:
Simulation Based on Seeley’s Experiment
A Model of Evolution by Natural Selection
Starting population
4th Generation
Part II: Requirements for Evolution
by Natural Selection
•! Examine three conditions which may affect
evolution by selection
•! Variation
•! Inheritance
•! Selection
•! See what effects each of these conditions has on
evolution of the snail population
Part II: Requirements for Evolution
by Natural Selection
Variation
Would you expect the snail population to
evolve if all the snails had the same shell
thickness at the start?
No Variation
Starting Population
No Variation
2nd Generation
No Variation
Part II: Requirements for Evolution
by Natural Selection
Inheritance
Would you expect the snail population to evolve
if shell thickness were not inheritable; i.e. if
there were no genetic basis for shell thickness?
Would the population evolve if each snail grew
its shell to a random thickness that had nothing
to do with its parent’s shell thickness?
No Inheritance
No Inheritance
Starting Population
No Inheritance
4th Generation
Part II: Requirements for Evolution
by Natural Selection
Selection
Would you expect the snail population to evolve if
predator crabs were especially large and could
crack snail shells no matter how thick they are?
These “super crabs” would eat snails randomly,
without any preference for thinner shells.
http://www.sml.cornell.edu/sml_mm_video_smlnew1.html
No Selection
Trial 1: No Selection Starting Population
Trial 1: No Selection 4th Generation
Trial 3: No Selection Starting Population
Trial 3: No Selection 4th Generation
Part III: Darwin's Theory of Evolution
by Natural Selection
Variation + Heredity + Differential = Natural
Selection
Reproduction
Under which conditions
will the snail population
evolve toward thicker
shells?
Part IV: What is the Source of Variation?
MUTATIONS
•!Mutation: an error during reproduction
•!Result: Mutant offspring do not resemble
parents. There is an equal probability that
shells may be thicker or thinner than parents
•!How can the population be driven towards
thicker shells with mutations than without
mutations?
Part IV: Source of Variation Among
Individuals: MUTATIONS
Starting Population
3rd generation
9th generation
12th generation
6th generation
Part V: What makes populations evolve?
a)! After they were born, did the individual snails ever change
their shell thickness? If the individuals didn’t change, how
was it possible for the population to change?
b) Did snails grow thicker shells because the snails needed
them in order to survive? If not, where did new
thicknesses come from?
c) Did the predators create a need for the snails to change - a
need to which the snails responded? Or did the predators
simply determine which snails survived to reproduce and
which didn’t?
Part VI: CHALLENGE - Evolution by
natural selection in flat periwinkles
Seeley’s two experiments
1.!Lab experiment
Average time to eat thin-shelled
snail - 42 sec.
a)! Only one of eight crabs was
able to eat thick-shelled
snail within 8 min.
b)! The other crabs (7/8) gave
up within 8 min.
Field Experiment Results
a)! Tethered 15 pairs of snails (one thick-shelled and one thinshelled) to seaweed with fishing line
b)! Placed 15 pairs at 3 different locations on Appledore Island
•!Gleason Point - crabs abundant
•!Sipp Bay - crabs rare
•!Timber Cove - crabs absent
c)! Observations
•! Checked each locations at 6, 9 and 16 days
•! Could determine if snails were eaten by crabs, died (other
cause) or escaped
•! If eaten, part of shell would still be attached to fishing line
Field Experiment Results
•! Red circles=thick shells
•! Blue circles= thin shells
Part VI-A: CHALLENGE
1. Review the requirements for evolution by natural
selection.
a) What evidence, if any, does Seeley have that the flat
periwinkles of Appledore island vary in the thickness of
their shells?
b) What evidence, if any, does Seeley have that snails with
thick shells are more likely to survive than snails with
thin shells? (Think carefully about this.)
c) What evidence, if any, does Seeley have that shell
thickness is heritable? (Again, think carefully about
this.)
Part VI-A: CHALLENGE
2. Seeley’s data provide direct evidence that two
out of three of Darwin’s conditions for evolution
by natural selection are true of the flat
periwinkle population on Appledore Island.
•! For the third condition, the evidence is indirect
at best.
•! For which of the three conditions is the evidence
you have seen the weakest? Explain.
Part VI-B: CHALLENGE - Designing Your Own Experiment
•! Do the snails from the crab-infested environment
have thicker shells, on average, than the snails from
the crab-free environment?
•! Do the snail populations meet all three requirements
for evolution by natural selection?
•! Variation
•! Inheritability of shell thickness
•! Some snails survive to reproduce more successfully than
others
•! Do the snails from the two environments differ
because one or both has evolved by natural selection,
or do they differ simply because snails can smell crabs
and grow thicker shells when they need them?
Part VI-B: CHALLENGE - Designing Your Own Experiment
Question 1: Do the snails from the crab-infested environment have thicker
shells, on average, than the snails from the crab-free environment?
Distribution of shell thicknesses for snails living in crabfree waters (West coastline).
Distribution of shell thicknesses for snails living in
crab-infested waters (East coastline).
Part VI-B: CHALLENGE - Designing Your Own Experiment
Question 2: Do the snail populations meet all three requirements
for evolution by natural selection?
•! For evolution by natural selection to occur, the snail population
must contain variation in shell thickness.
•! For evolution by natural selection to occur, the variation must be
at least partly inheritable.
•! For evolution by natural selection to occur, some snails must
survive to reproduce at higher rates than others.
Part VI-B: CHALLENGE - Designing Your Own Experiment
Question 3:
Do snails from the two environments differ because one
or both have evolved by natural selection, or do they
differ simply because snails can smell crabs and grow
thicker shells when they need them?
Part VI-B: CHALLENGE - Designing Your Own Experiment
•! For each question design experiments to test the hypothesis.
•! Use controls: e.g., same numbers and sizes of snails in each
experimental tank, same numbers of crabs + bands on claws, tanks
with and without crabs
•! Limit the snails of different thicknesses (e.g., use one or a few
thicknesses
•! Describe your experiments
•! Use images of experimental set-up
•! Make sure your histograms are numbered and labeled.
•! Keep them in sequence.
Is Shell Thickness Inheritable?
Plan for today
1.! Complete the Darwinian Snails packet and hand it in
along with the labeled histograms.
2. Take the quiz on Darwinian Snails packet.
3. Download Mitosis Demystified and complete the exercise.
4. Take the self-test; omit question 11.
5. Remove the adult flies (F1) from your vial.
Links to download Mitosis Demystified:
http://simbio.com/downloads/AppFilesFall2010/MM/
MitosisandMeiosisMac.dmg
http://simbio.com/downloads/AppFilesFall2010/MM/
MitosisandMeiosisWin.zip
Pre-Lab Assignment
1.! Review the life cycle of Drosophila
2.! Read the Laboratory Exercise: Analyzing Eye
Pigment Mutations in Wild and Mutant Strains of
Drosophila
3.! Read the material from Campbell, Concepts and
Connections.
4.! Complete Mitosis Demystified, take the diagnostic
test and post or send your score.
5.! Prepare for the quiz on Darwinian snails, cell cycle
and mitosis