tradeoffs and the origin of diversity e3: lecture 4

Tradeoffs and the Origin of Diversity Tradeoffs and the Origin of Diversity

E3: Lecture 4

Exorcising Darwinian demons• From an evolutionary perspective, an

extremely fit organism would:– Live a very long time– Produce many offspring frequently– Reproduce asexually

• From an ecological perspective, an extremely successful organism would:

– Outcompete its neighbors– Colonize distant patches– Utilize all available resources– Resist predators and stress

• Why don’t we see such super-species?

• The cliché “a jack of all trades is a master of none” hints at part of the reason: organisms deal with physical constraints and tradeoffs. N P K S Ca Mg N P K S Ca Mg

Lecture Outline

• Introduction to tradeoffs

• Tradeoffs and diversity

• Tradeoffs and the origin of diversity

• Tradeoffs and maintaining diversity

• Summary


What are tradeoffs?

• A tradeoff occurs when improvement in performance (e.g., fitness) under one set of circumstances comes with a concomitant loss in performance in another set of circumstances.

• The inability to have the “best of all worlds” in every world is at the heart of a tradeoff.

• The existence of a tradeoff leads to ranking reversals and may produce specialists and generalists or different functional types.

performance inenvironment A

perf

orm

ance

in

envir

onm

ent

B 1

2

3

B specialist

A specialist

generalist

Rankingin env. A

Rankingin env. B

1

2

31

2

3best

intermediate

worst

What are the causes of tradeoffs?• Fundamental limitations in resource use (e.g.,

how an organisms spends its time, energy, etc.)

• Overlap between the functional domains of specific phenotypic traits.

• Inflexibility of the phenotype across different contexts, where the appropriateness of the phenotype varies.

• Flexibility of the phenotype across different contexts, where adaptive plasticity in one trait comes with non-adaptive plasticity in another trait.

Both bacteriophage and maltose bind to the LamB protein on the surface of E. coli cells. (Szmelcman & Hofnung, 1975)

Increases in gill filament lengthin response to hypoxia in P. multicolor victoriae comewith associated shortening ofmuscles affecting feeding. (Chapman et al. 2000)

Bold/aggressive “behavioralsyndrome” in female D. triton affecting foraging, courtship and anti-predator behavior. (Johnson & Sih, 2005)

There is some evidence that whilesmall-sized seeds are more numerous, large-sized seeds giverise to more competitive seedlings (Muller-Landau 2003)

Atoms and spandrels

• In a famous paper, Gould and Lewontin (1979) cautioned against unbridled application of the adaptationist programme to every component of an organism.

• One of the lines of criticism was that an organism cannot be atomized into a collection of independent parts—rather, different parts interact (non-independence) and single parts have multiple roles (pleiotropy).

• Tradeoffs are a natural consequence of the interconnectedness of parts and the multifarious functions of parts.

StephenJay Gould

RichardLewontin

Lecture Outline





• Summary


Implications of tradeoffs for diversity

Take 5 minutes to talk to your group about the following:

1. Are tradeoffs important to the origin and maintenance of biodiversity?

2. If so, what specific types of tradeoffs could be important?

3. How might you experimentally explore the role of tradeoffs in biodiversity?

Tradeoffs and biodiversity• Coexistence between multiple species is possible when there are tradeoffs:

- In efficiency using different resources:Tilman (1976) showed that two

species demonstrating a tradeoff between growth under limiting phosphate

and limiting silicate could coexist under certain concentrations of both

nutrients

- Between tolerence to stress and competitive ability:

Connell (1961) found that two species of barnacles demonstrating a tradeoff between stress tolerance and competitive ability were able to coexist in the rocky intertidal

- Between resistance to predators and competitive ability:

Bohannan & Lenski (1997) showed that two strains of E. coli (T4 resistant and T4 sensitive) could coexist due to a resistance/competitive tradeoff

growth whensilicate is limiting

grow

th w

hen

phos

phat

e is

lim

itin

g

Asterionellaformosa

Cyclotellameneghiniana

Res

ourc

e T

rad

eoff

competitive ability

tole

ranc

e to

dess

icat

ion

Chthamalusstellatus

Balanusbalanoides S

tres

s/C

omp

etit

ive

Tra

deo

ff

competitive ability

phag

ere

sist

ance

T4R

E. coli

T4S E. coliPre

dat

or R

esis

tan

ce/

Com

pet

itiv

e T

rad

eoff

Temporal and spatial heterogeneity• With spatial or temporal heterogeneity in the

environment, there can be a place or time for types on each end of a tradeoff curve to shine.

• With persistant spatial heterogeneity, each species (strain, variant, etc.) has a particular address where it does well– coexistence occurs at a regional level. performance in

environment A

perf

orm

ance

in

envir

onm

ent

B 1

2

1

2

B A

1 1 2 2

Year 1B A

1 1 2 2

Year 2B A

1 1 2 2

Year 3

B

1 1 11

1 2 1 2 1 2 1 2

Year 1

Chthamalus

Balanus

• With persistant temporal heterogeneity, as long as there is a method for persisting through bad times (seed bank, resting eggs, etc.), then each species will thrive at a certain time and there will be coexistence over prolonged periods. post-wildfire flowering

A

2 2

1 2 1 2 1 2 1 2

Year 2

2 2

B

1 1

1 2 1 2 1 2 1 2

Year 3

1 1

Biodiversity: Other distinctions• Conceptually, it is important to separate the emergence of diversity from the maintenance of diversity.

- Ultimately, mutation is the generator of diversity.- Physical separation and tradeoffs between various phenotypic characteristics (paired with spatial/temporal heterogeneity) can be important in the maintenance of variants.- Adaptive radiations involve both the emergence of diversity (new variants originate) and the maintenance of this diversity (new variants persist)

• It is also important to separate external from internal causes of coexistence of types.

- External causes of diversity include elements outside the biotic community of interest that contribute to persistence.- Internal causes of diversity attribute coexistence to properties of the biotic community itself.- Both external and internal causes of diversity can operate simultaneously.

nitrogen-fixation invetch

altitude gradients

Lecture Outline





• Summary


Studying adaptive radiation• Rapid speciation from a single or few species to a variety of adaptive forms is termed an “adaptive radiation”

• Natural experiments can be executed to determine:- Variation in various phenotypic attributes- The heritability of potentially key phenotypic traits- Putative selective pressures affecting the evolution of such phenotypic traits

• Another approach is to design experiments in which there is the potential for radiation in real-time.

• Desirable properties of such real-time systems:- The generation time of the organism should be short- The population size of the organism should be large- Evolving populations should be easy to replicate- A living fossil record would be ideal

• All of the above properties are achieved in microbial microcosms. Such microcosms provide one approach (in a set including natural experiments) to study radiation.

generation time 20 min.

population1010 cells

deep freezestorage

A system for a Rainey day• Paul Rainey and Michael Travisano (1998) used the bacterium Pseudomonas fluorescens.

• A few pieces of information on the microbe:- Colonizes plant surfaces and soil particles- Produces a green fluorescent pigment- It is a obligate aerobe- It is motile by means of multiple flagella

• These authors placed a single ancestral strain of this bacterium into two different types of environments:

- Unshaken microcosm (in which gradients of O2 could develop)- Shaken microcosm (in which such gradients were destroyed)

• By following the pattern of diversification between treatments, the authors studied the effect of structure and gradients on the origin of variants.

Paul Rainey Mike Travisano

O2O2

unshakentreatment

shakentreatment

A micro-radiation• When a single genotype is placed into an unshaken microcosm, several different colony morphs evolve, among them:

- smooth morph (SM; like the ancestor)- wrinkly spreader (WS)- fuzzy spreader (FS)

• Each morph is associated with a particular physical location within the tube:

- SM throughout the broth- WS at the air-broth interface- FS at the bottom of the tube

• The same morphs reliably appear across experimental replicates and the sequence of appearance was also maintained.

• This radiation occurred over the course of 7 days and diversity continued to remain high for as much as a month later.

SMWS FS

The effect of structure• Rainey and Travisano found that the shaken microcosm did not diversify, suggesting that the gradients (e.g., in O2) afforded by the heterogeneous treatment were important for diversification.

• They used a common measure of diversity, the Shannon-Weaver index:

where N is the number of individuals and ni is the number of individuals of the ith colony morphology. H peaks when morphs are evenly distributed.

O2 O2

unshakentreatment

shakentreatment

dive

rsit

yab

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nce

time

dive

rsit

yab

unda

nce

time

time time

NnnNNH ii /)loglog(

• Then the authors experimented on diversified microcosms by letting them sit or shake– this provided more evidence suggesting gradients were important

The role of tradeoffs

Morph A Morph B

• Rainey & Travisano placed the bacteria producing two different colony morphologies in a tube together in 1:100 ratio

• By measuring the density of both morphs at the beginning and end of a growth cycle, the authors could calculate relative fitness:

• With one exception (FS invading a population of WS), every morph could increase when rare.

• The WS produces an extracellular cellulosic polymer allowing large numbers of this type to form a biofilm at the air-broth interface. Such production is costly and in competition for well-oxygenated broth, the WS is selected against by the SM that eschews the cost.

• Thus, we have a competition/cooperation tradeoff!

0

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AA

BAw

SM

WS

FS

What processes fuel this radiation?

SMWS FS

abun

danc

e

timeab

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time

Take 3 minutes to talk to your group about the following:

1. Name some potential internal and externalfactors in this system.

2. What evidence do you have for internal versus external causes of diversity emergence and maintenance in this system?

3. How would you experimentally test for thesehypothetical causes?

Lecture Outline





• Summary


Competition/colonization tradeoff• Species may differ with their

ability to disperse, produce offspring, colonize new areas, etc.

• A superior colonization strategy may allow competitive inferiors to establish in less favorable areas as “fugitives”.

• Tilman (1994) suggests that there is a tradeoff between colonization ability and competition for nitrogen with the plants at Cedar Creek. Schizachyrium scoparium

(little bluestem)

Agrostis scabra(tickle grass)

B

B A

1 1 2 22

2

11 1 22

21

1

1

A taxing exercise• We need a volunteer from the class.

• This volunteer will be blindfolded and then asked to find a hanging bag of goodies.

• This volunteer moves with very small steps in very straight lines.

• His/her sensory system (the rest of the class) can detect how far the individual is away from the goodies (the class will say “you’re getting warmer” or “you’re getting colder”)

• If the volunteer hears you’re getting colder, after taking a few steps he/she is to spin around a few times and continue plodding ahead.

Experimental system to explore c-c tradeoff• Flagella production and operation can be

costly in Escherichia coli – Resources invested in building the flagellum– Energy expended by turning the flagella

• When grown in well-mixed environments (chemostats, flasks, etc.) E. coli mutants that delete part of their flagella operon are selected.

• However, in structured habitats, flagella allow a cell (and its clones) to colonize empty areas and utilize unexploited resources.

• In previous years, we used an experimental set up not far from Rainey & Travisano’s to show a tradeoff between competition and colonization in one of our labs.

– A spatially structured environment (an agar plate with super soft agar)

– A spatially unstructured environment (a shaken tube).

Strain BK35:unflagellated

E. Coli

Strain BK32:flagellated

E. Coli

structured habitat

unstructured habitat

An analogy• Imagine two people out in a lake, each

in their own canoes.

• There are several anchored booeys in the lake each with an apple on top.

• Each of the people would like to get to each of these apples, but expend as little energy doing so.

• One of the people has a paddle (pink canoer) and the other does not (orange canoer).

• On a still day, the pink canoer cleans up getting many apples to compensate for the extra effort of paddling.

• On a tremendously windy rainy day, the orange canoer gets about the same number of apples and saves energy.

Lecture Outline





• Summary


Summary • No organism has been able to attain “Darwinian demon” status– as organisms face inherent tradeoffs.

• A tradeoff is defined as compromised function under one set of circumstances that comes with improved function in another.

• Tradeoffs may play important roles in the origin and maintenance of biodiversity; especially in spatially and temporally heterogeneous environments.

• Rainey and Travisano demonstrated that spatial heterogeneity could be crucial to the emergence of a diverse system de novo.

• The adaptive radiation found by these authors was partly supported by tradeoffs (between biofilm formation and competition) in a heterogeneous world.

• Even without spatial heterogeneity, tradeoffs between competition and colonization are predicted to contribute to biodiversity maintenance.

tradeoffs and the origin of diversity e3: lecture 4

Documents

tradeoffs tradeoffs

origin of diversity

tradeoffs important

causes of tradeoffs

role of tradeoffs

specific types of tradeoffs

diversity summarytradeoffs

origin of diversity