Presentation on theme: "What Can Evolution REALLY Do? How Microbes Can Help Us Find the Answer May 10, 2008 Ralph Seelke, U. Wisconsin-Superior."— Presentation transcript:
What Can Evolution REALLY Do? How Microbes Can Help Us Find the Answer May 10, 2008 Ralph Seelke, U. Wisconsin-Superior
Where We’re Going Confessions of an experimentalist who loves making (bacterial) mutants What evolution has been able to accomplish What it has NOT been able to accomplish Some conclusions
Two Warnings 1) Do not expect to be overwhelmed. 2) Some of this will get complicated!
Four Take-Homes Lessons 1)We really CAN test evolution 2)Two is an evolution stopper 3)Even doing one thing at a time can be a problem for evolution- a gene can “evolve to become unevolveable” 4)God can even use dull, plodding people for his glory
Experimental Evolution? ??? Source: Ronald Pine,
We Can Do This “Absurd” Experiment With Microbes! For Evolution to Occur You Need A LARGE Population and/or MANY Generations A Trait That Can Evolve !!!!BACTERIA!!!!
Up to 4 TRILLION in a 1 Gal Milk Jug! Thousands of Generations in a Year! COMPLEX Traits! When they evolve, we can FIND THEM!
Each Transfer Produces 6.6 generations of evolution! 46 generations per week Almost 400 per month Over 2,400 in a year 24,000 in ten years! Inoculate flask 10 ml broth Grow overnight-> Transfer 0.1 ml to new flask You have just produced 6.6 generations of Evolution! We can produce thousands of generations in a year!
We can FIND evolution Because When the microbe EVOLVES it GROWS or GROWS BETTER!!!!
Evolution Before your very eyes!
MORE Evolution Before your very eyes! About 3 mm
My Question: Can evolution do two things at once? Can a microbe evolve when two mutational events BOTH have to happen for evolution to occur? Not just my pet question! Acknowledged by evolutionists The fundamental problem of irreducible complexity- 2 or more components, all required for a function, and all required for any function.
Why Should Requiring Two Changes Make Evolution Difficult? Mutation rates are typically one in 100 million; a teaspoonful of bacteria would have over about 50 mutants! If you need TWO mutations, then the chances of BOTH mutations occurring is 1 in 10,000 trillion! Now you would need a population of 10,000 liters to produce the mutant!
Is the Need for Two Independent Mutations REALLY an Evolution- Stopper? Studies with the trpA Gene of Escherichia coli
Testing the Two Mutation Rule Find a well-studied gene, with known mutations that inactivate it. Introduce 1,2,3, or four inactivating mutations Let the gene evolve under highly selective conditions
Source: Hyde, et. al, The Gene of Choice: trpA (tryptophan synthase A) Mutation 1 Mutation 2
Results So Far: If Evolution Requires Two or More Independent Mutations NOTHING HAPPENS
Testing Large Populations of RS202-5 (two mutations) for Evolution: Test in liquid culture: about 1 trillion cells tested without evidence of evolution. Test on agar plates: ~ trillion cells tested without evidence of evolution. RS201-2 (mutation one) routinely produced Trp + colonies/plate, 10 4 /ml Trp + cells/ml in liquid culture. No evidence of evolution of RS202-5
Results of Serial Transfer One culture lost its trpAB genes within 275 generations. Two additional cultures have been tested for ~870 transfers, or about 5,700 generations No Trp + evolution observed. The dead trpA gene after 2,000 generations is identical to the original, dead gene. HOWEVER….
The cultures have evolved to be able to grow better in the tryptophan-limited medium
CONCLUSION:TWO is an evolution stopper Important, but somewhat boring
Warning: This Next Part May be Hard!
Then the story got more interesting- and more complicated Dead TrpA
We thought that BOTH mutations inactivated trpA, but only Mutation 1 did; Mutation 2 only weakened the gene When we made a version with just Mutation two- the gene was not completely dead! weak TrpA
Why was this a big deal? It meant that our gene with two mutations should have evolved! M1, M2 Dead TrpA M1, M2 Weak TrpA Mutation Selection because of fitness advantage M1, M2 Strong TrpA Population of fully Trp + cells THIS DOES NOT HAPPEN!! Evolution not only can’t do two things at a time, in this case it can’t do one thing at a time!
So why didn’t our gene evolve?? Maybe, while it was evolving, the expression of the weak trpA gene was lowered, so that now the trpA gene with just M2 was now DEAD. This would mean that EVOLUTION MADE IT UNEVOLVEABLE
How can we find out if it “evolved to become unevolveable”? We did “part swapping” experiments! We took the large part (red) from our original plasmid, combined it with trpA-M2; the gene was weakly trpA. When we took the same piece was from a plasmid that had evolved, the trpA-M2 gene was DEAD! Original piece- trpA-M2 gene WEAK Evolved piece- trpA-M2 gene DEAD
What did this mean? Most likely, a mutation had occurred, outside the trpA-M2 gene, that had turned the weakly trpA gene OFF. Mutation somewhere in red, not in the trpA gene! Evolution in red switched off the trpA gene in blue. That somewhere was a single base change at
A case of evolution preventing evolution M1, M2 DeadTrpA M1, M2 Weak TrpA Mutation Selection because of fitness advantage M1, M2 Strong TrpA Population of fully Trp + cells M1, M2 Dead TrpA Mutation lowers trpA expression, increases fitness M1, M2 STILL Dead TrpA Reversion of M1 no longer selective; both must now revert for a Trp+ cell Change at 1584 ALWAYS happens This path NEVER happens
The Concept of a “Fitness Peak”
How is evolution like a guy? 1) It has trouble doing two things at once 2) Even when it only has one thing to do, it sometimes gets sidetracked
Acknowledgements Merck Foundation Biologic Institute UW-Superior A.C. Matin Lab and Stanford University NUMEROUS undergraduate students! –Pravien Abeywickrema,Kayo Sakaguchi –Robert Jennings, Ranjuna Weerasekera –Lynn Meyers, Sarah Rahn, Stephanie Ebnet, Benjamin Okemwa