evolution simulation - rensselaer polytechnic...

Darwinian

Snails

Evolution

Simulation

Carcinus maenus, the green crab

Littorina obtusata, the flat periwinkle

http://www.arkive.org/common-shore-crab/carcinus-maenas/video-08.html

http://www.arkive.org/flat-periwinkle/littorina-obtusata/video-00.html

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

Shorter & thicker

Taller & thinner

Robin Seeley’s Experiment

•!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

7

14

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

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

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 4th Generation

No Inheritance Starting Population

No Inheritance

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 4th Generation Trial 3: No Selection Starting Population

Variation + Heredity + Differential =

Reproduction Natural

Selection

Part III: Darwin's Theory of Evolution


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

12th generation 9th generation

6th generation 3rd generation Starting Population

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

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.

Seeley’s two experiments

1. !Lab experiment

a)!Tethered 15 pairs of snails (one thick-shelled and one thin-

shelled) 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

Field Experiment Results

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.

•! 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


Distribution of shell thicknesses for snails living in crab-

free waters (West coastline).

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

crab-infested waters (East coastline).


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.

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?


•! 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

evolution simulation - rensselaer polytechnic...

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