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Define coevolution?

Change in the genetic composition of one species in response to a genetic change in another. The idea of reciprocal evolutionary change in interacting species. This reciprocal evolutionary change should result in reciprocal adaptations if the traits under selection are inheritable.


Types of interactions and their effects on species?


When can coevolution occur?

  1. Big fitness consequence or benefit from interaction 
  2. Genetic variation for heriable trait mediating interaction 
  3. Reciprocal natural selection
  4. Tight ecological relationship, with some degree of specialisation 
  5. Ecological relationships that may coevolve. 


Benefits of specialisation?

If there is a constant supply, the organism has a good food supply. 

However, if the prey goes extinct, the organism is a lot more vulnerable. 


Example of species specialisation?

Buchnera is a primary symbiont of aphids. Passed onto offspring. 

Does not occur in any other insect group, and aphids do poorly without it. 

Pair the different genotypes in a table of the 2 organisms. If there is variation in whether the host gets infected, there is potential for coevolution. Yes there is. 


Example of mutualistic coadaptation?

Lycaenid caterpillars secrete honeydew that ants drink. Ants defend caterpillars against parasitic wasps. 

Honeydew secretion and defense are co adaptations. 


Example of predative coadaptation?

Crossbills prefer smaller cones as easier to get seeds out. 

Cone width, mass and length have increased as a result as taking seeds out is a disadvantage for the trees. 

Crossbill beak size increases because of change in cone size, but only seen in areas where there is high specialisation between crossbills and cones.


When is it not coevolution?

Traits in one species that evolved before the association began. Circumstantial

Traits in one species evolve, but there is no investigation of the traits in other species.

Generalised adaptations.

New associations of species with mutually congruent “adaptations”

One species tracks another’s evolutionary changes, but not selecting for reciprocal changes.


Criteria for coevolution?

Fitness consequences


Genetic variation for heritable traits

Selection is reciprocal


Snail and trematode example for coevolution?

Glacial Lake on West coast of NZ. Snail is asexual and sexual.

Once they are infected by parasites, they lose all reproductive ability.

When snails are closer to shore (shallower water), they have higher infection. Theory is that coevolution occurs closer to the shore.

Are parasites adapted? Snails in the shallow were really susceptible to infection, as parasites were very good at infecting the snails in the shallows.

Trematodes are very good at infecting some snail genotypes.

The host genetic background is changing over time.

Parasites that can’t infect the host die, whilst infected snails die.


Bacteria and viral phage example for coevolution?

The viral phage is parasite, injects DNA into bacteria.

The ability of coevolved phage isolates to infect coevolved bacteria hosts varies across populations.

There is variation in the parasite to infect, and variation in the host’s ability to resist.

Test using a time shift assay. Uses bacteria from the past, present and future, exposes them to phage taken from a single transfer.

Bacteria evolved increasing resistance to phage infection over time. Found that future bacteria had a very high resistance. Demonstrates parasite evolutionary change.

Also look at phage’s ability to infect. Can see that pathogens evolved more rapidly during coevolution. Demonstrates parasite evolutionary change.

Can see that both evolve in time. Don’t know whether its reciprocal.


What does reciprocal selection favour?

Victim traits that decrease the efficacy or frequency of the interaction 

Exploiter traits that increase the efficacy or frequency of the interaction 


What are the two dynamics of coevolution?

Escalation (Arms race) - directional selection

Red Queen (Cyclical) - good to have rare traits 


Describe symmetric coevolution?

Fitness effects are equally good or bad to the host and parasite 

Red Queen and Cyclical dynamics 

Reciprocal selection favours increased phenotypic values in both victim and expoiter 

Alleles expressing those favoured phenotypes fix until a new allele arises. 


Describe asymmetric coevolution?

Life dinner principle, costs and benefits aren't equal (predator prey relationship) 

Arms race and escalatory dynamics. 

Reciprocal selection favours exploiters that match the victim phenotype and victims that mismatch the exploiter phenotype. 


Coevolution example with escalation/arms race?

Taricha granulosa newts have powerful anti-predator toxins (TTX). Thamnophis sirtalis snakes are major predator with evolved resistance to TTX

Snake resistance is predicted by newt toxicity. Some garter snake populations have dramatically increased TTX resistance.

Endless escalation of phenotypes
The winner is the species with the greatest response to selection

Future bacteria are the best at resisting contemporary parasites. Contemporary parasites are average at infecting.


Coevolution example with red queen/cyclical dynamics?

Daphnia life cycle - produce resting eggs every winter in the sediment. Resting eggs and bacterial spores (sterilising bacterial parasite Pasteuria) fall to the bottom of the lake, and are conserved in lake sediments. Can extract sediment core and see the isolated hosts & parasites over a 4 year period.

With red queen dynamics, expect current parasites to be bad at infecting past and future water fleas. True - contemporary parasites are better at infecting.

Alleles cycle endlessly - constantly running. Exploiter adapts to common victim genotypes. The average frequency of the allele remains constant. Should produce an advantage for rare victim genotype