coevolution

Question 1

definition of coevolution

Answer 1

• Change in the genetic composition of one species in response to a genetic change in another. More generally, the idea of RECIPROCAL evolutionary change in interacting species.
• Has to be reciprocal – changes occur as a result of the interaction
• Such reciprocal selection should result in reciprocal adaptations if the traits under selection are heritable.
• Term invented by: Paul Erlich & Peter Raven, 1964. Two scientists were studying noxious compound diversity in butterflies driving diversity in poisons – we now know its NOT a coevolutionary interaction

Species A evolves an adaption in response to species B and vise versa

Question 2

what’s required for evolutionary change to occur?

Answer 2

• Way of getting genetic variation in a trait
• Some sort of selective pressure
• A heritable trait that provides a fitness advantage

Question 3

When can coevolution occur?

Answer 3

• Big fitness consequence or benefit from interaction
• Genetic variation for heritable traits that are mediating the interaction
• Reciprocal natural selection
• Tight ecological relationship = specialisation

eg. predator/prey, parasite/host, mutualists, competitors

Question 4

Specialisation

• what’s it worth specialising on
• risk
• levels

Answer 4

• What is worth specialising on?
– Reliable resource (something that’s always there)
– Consistent resource
• The risk of specialisation is that it could go away and you’d be screwed. However if you’re a generalist you might not be very good at getting any of them
• Species-level specialisation (aphids and buchnera) and genetic-level (daphnia) specialisation

Question 5

Aphid and buchnera roles

Answer 5

Aphids and Buchnera are metabolically interdependent – mutualistic symbiosis

Aphid host role
- Supplies energy, carbon, and nitrogen, in the form of glutamine from phloem

Bacterial (Buchnera) symbiont role

• Production of amino acids (esp. tryptophan)
• The bacteria allow aphids to live on a v. poor diet

Question 6

adaptation

Answer 6

trait that confers a fitness advantage that is heritable

Question 7

co-adaptation definition

Answer 7

• Coadaptation: reciprocal adaptations of two interacting species
• Traits that one species and another species possess that mediate their interactions

Example 1 – Mutualism
o Lycaenid caterpillars secrete “honeydew” that ants drink
o Ants defend caterpillars against parasitic wasps
o Honeydew secretion and defense that the ants confer are potential coadaptations

Example 2 – Predation
o Adaptation = bill shape and cone width, mass and length
o Cones hide seeds inside them
o Hypothesis = bill shape is key to birds getting seeds
o Turns out that bill depth is v important, and higher bill depth gives higher fitness
o Crossbills prefer smaller cones - easier to get seeds out
o Cone width, cone mass, and cone length have increased as a result to prevent their seeds being stolen
o Crossbill beak size increased because of change in cone size
o You see specialisation in one population – this is where coevolutionary interactions take place
o The other pop has generalisation, there is not so much of a tight evolutionary interaction
o Divergence between crossbill populations as a result of these 2 pops

Question 8

When is it coevolution

CRITERIA

Answer 8

any interaction eg. mutualism is NOT synonymous with coevolution

Criteria:
• Fitness consequences
• Specialisation
• Genetic variation for heritable traits
• Selection is reciprocal

Question 9

When is it not coevolution

Answer 9

o Traits in one species evolved before the association began
– it might look like a plant is perfectly shaped for an insect to pollinate, but shape may have evolved much earlier than insect adaptation

o Traits in one species evolve, but there is no investigation of the traits in other species
– eg. genetic variation for resistance, you must look at the other species too (reciprocity!!)

o New associations of species with mutually congruent “adaptations” - phenotypic comparison has been made but no true investigation of reciprocal selection

o One species tracks another’s evolutionary changes, but not selecting for reciprocal changes – one species is a specialist and the other species is a generalist. NO reciprocal selection
o Generalised adaptations (e.g., broad-spectrum defence)

Question 10

Example of coevolution that WORKS

Answer 10

Bacteria and viral phages (red queen)
• Authors collected variety of bacterial and phage isolates
• There is variation in result of interaction. White = no infection (resistance in pop), grey = intermediate levels of infection
• Ie. The ability of coevolved phage isolates to infected coevolved bacteria hosts varies across populations

Test: Time Shift Assay. Took parasites from middle time point and infected hosts from all 3 timepoints.
• Bacteria from the past, present, and future (1 transfer difference) were exposed to phage taken from a single transfer
• In every case, future host is really good at resisting (have higher resistance level). This suggests Bacteria evolved increasing resistance to phage infection over time

Test: Coevolve bacteria and bacteriophage parasites, monitor phage molecular evolution
o Coevolution predicted to drive rapid evolution from ancestor strain - continual natural selection for adaptation and counter-adaptation
o Observation: Coevolution drives faster and greater genetic divergence of pathogen from ancestor (ref)

Question 11

Time shift assay

Answer 11

reveal type of coevolutionary dynamics

red queen - contemporary best with contemporary eg. daphnia

arms race - future always better than contemporary eg. bacteria and phage, newts and garter snakes

Question 12

how does the bacteria phage example meet all the requirements of coevolution?

Answer 12

Fitness consequences? YES for both

• if phage doesn’t infect it dies
• if bacteria is infected it dies

Specialisation? Genetic level but not species level?
- diff bacterial and phage isolates have diff levels of infection

Genetic variation for heritable traits? YES
– spread of resistance of host increased over time (shown using time-shift assay) so it’s heritable
- Parasite also shown to see rapid evolutionary change from ancestor when you coevolve in lab
- However these data are whole genome data – you need to be able to show in infectivity gene that there is variation

Selection is reciprocal? YES

Question 13

What must you use to determine if coevolution is happening

Answer 13

must determine by

1) observation
2) experiments
3) phylogenetic analysis

Question 14

what are the two types of coevolutionary dynamics?

Answer 14

Escalation (arms race)
Red Queen (cyclical)

Question 15

antagonism on a phenotypic level

Answer 15

reciprocal selection favours

• victim traits that decrease the efficacy or frequency of interaction
• exploiter traits that increase the efficacy or frequency of the interaction

victim doesn’t want exploiter to overlap entirely with it

Question 16

Escalation (arms race)

- what is it

Answer 16

o Mutation in victim gives new beneficial phenotype
o Allele increases in frequency and FIXES in the population, same then happens in enemy, until new allele arises
o NOVELTY & RECIPROCAL DIRECTIONAL SELECTION make it arms race
o More likely to get extreme phenotypic values as reciprocal selection favours this - endless escalation of phenotypes in victim and exploiter
o The ‘winner’ is the species with greatest response to selection (‘runs away’ or ‘catches up’ the fastest)

o Reliance on mutation for new phenotype.

Question 17

Example of escalation (arms race)

Answer 17

o Newts have powerful, anti-predator toxins (TTX)
o Garter snakes are major predator – evolved resistance to TTX
o TTX is 100X more powerful than necessary to kill any other predator but snakes can be resistant to even v v high levels (this is an example of extreme phenotype)
o You can map the TTX resistance levels of snakes
o Red regions are where snakes have no other prey items so have to be very good at resisting toxins (strong reciprocal selection)
o Snake resistance is predicted by newt toxicity, as predicted. Very remarkably positively correlated newt toxicity and snake resistance to it
o Some Garter snake populations have dramatically increased TTX resistance

Question 18

Evidence for escalation (arms race)

Answer 18

Evidence for arms race
o Future bacteria are the best at resisting contemporary parasites, much better than past hosts
o Contemporary parasites are average at infecting
o NO matching, host just keeps getting better and better

Question 19

Red Queen (cyclical)

Answer 19

o Victim allele is really common in population
o Exploiter adapts to common victim genotypes
o Should produce an advantage for rare victim genotypes
o As soon as victim becomes common, more exploiters, constant cycling
o Allele frequencies cycle endlessly

o No fixation or directional selection
o Both parasite and host have to run, just to remain in the population. Average frequency stays the same over time
o Lack of fixation means no additive effects, not likely to see extreme phenotypes
o Negative frequency dep selection/ rare advantage – where a common genotype is bad and a rare genotype is good.
o Maintenance of diversity NOT novelty

More about MATCHING
o Exploiters wanna match, victims don’t
o Reciprocal selection favours exploiters that match the victim phenotype, and victims that mismatch the exploiter phenotype

Question 20

Example of Red Queen (cyclical)

Answer 20

Example of MATCHING – Snails & trematodes
o Trematodes can only infect snails with specific “matching” genotypes
• Rare snail genotypes are less frequently infected than common snail genotypes

Example - Daphnia
▪ Daphnia are infected by a sterilising bacterial parasite, Pasteuria
▪ life-cycle: in sexual stage produce resting eggs every winter
▪ Resting eggs (host) and bacterial spores (parasites) fall to bottom of lake, and are conserved in lake sediments for 10s of years
▪ Extract ‘sediment core’ and isolate hosts and parasites from over 4-year period.

time shift assay
- Gives you a vertical archive of generations (past hosts and parasites, future, contemporary hosts and parasites) = resurrection ecology!
o Do past and future parasites match to contemporary hosts?
o Red queen dynamics predicts contemporary parasites being best at infecting contemporary hosts as they are adapting to them. We find this!
o Past and future parasites don’t match as well
o This indicates we have red queen dynamics (triangle shape graph, up at contemp, down at past and future)

NOTE (if this was arms race future would be better than contemporary)

Question 21

Symmetry or asymmetry alters selection strength and type of dynamics between two antagonists

Answer 21

Symmetric (RED QUEEN)
• fitness effects are equally good or bad (hosts and parasites)
• survival and death
• cyclical dynamics

Asymmetric (ARMS RACE)
• ‘life-dinner principle’ (predators trying to get dinner and prey is running for life)
• A more asymmetric relationship

Question 22

How does coevolution affect evolutionary change?

• test with red queen example

Answer 22

Rapid evolutionary change

Test: Coevolve bacteria and bacteriophage parasites, monitor phage molecular evolution

o Coevolution predicted to drive rapid evolution from ancestor strain - continual natural selection for adaptation and counter-adaptation

o Observation: Coevolution drives faster and greater genetic divergence of pathogen from ancestor (ref)

Question 23

Coevolution and competition

• classic example

Answer 23

Reciprocal selection favours:
o Divergence in traits mediating the interaction
o ‘Character displacement’ from average intermediate level to two extremes. Neither one wants to interact with the other.

Classic example of character displacement
Sticklebacks in glacial lakes
• Coexistence leads to reciprocal selection for divergence in body size
• Two diff forms with v different ecological characteristics. Same species
• LIMNETIC = smaller, jaw hangs higher, longer gill rakes
• Interactions thought to be ~10,000 years old. Second wave of invasion caused split

o Paxton Lake: limnetics (longer gill rakers) and benthic
o Cranby Lake: Intermediates (intermediate gill rakes)
• Measured fitness effect of competition between ‘ancestral’ intermediates with Paxton limnetics
• Stabilising selection without competition ie. when only intermediates present
• When in competition with limnetics, directional selection towards benthic form, higher fitness of fish with small gill rakes (the ones that looked more benthic)
• Growth rate used as surrogate measure for fitness

Question 24

Coevolution and macroevolution - phylogenies

Answer 24

• Many coevolutionary interactions are short-lived, but very close ones can be long-term
• Molecular phylogenies of interacting species - historical context for the study of co-adaptation and coevolution
• Look for concordance – cospeciation is the extreme end-point of coevolution
• Use DNA sequences of two species, build gene trees
• Strong concordance = evidence for co-adaptation with co-speciation

Phylogenetic Associations
Eg. Doves and feather lice
• Lice can’t live without the dove
• Dove gets groomed by lice
• Many instances of cospeciation between different species
• Can also happen where speciation only happens in one (it’s a bit messy…)

Aphids and buchnera
• One of the tightest levels of concordance that we have
• Species level specialisation of buchnera and aphid are v tight