eeob 400: lecture 15 coevolution. what is coevolution? two (or more) species: 1) exert selective...
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EEOB 400: Lecture 15
Coevolution
Coevolution
What is coevolution?
Two (or more) species: 1) exert selective pressures on each other, and2) evolve in response to each other
Because each species is evolving in response to the other, one important feature of coevolution is that the selective environment is constantly changing
When does coevolution occur?
Selective pressure will be strongest when there is a close ecological relationship
“Close” ecological relationship = usually specialists rather than generalists
Important ecological relationships that give rise to coevolution:
1) predators & prey 2) parasites & hosts 3) mutualists 4) competitors
predator
prey
parasite
host
+ - + -mutualist A
mutualist B
+ +competitor A
competitor B
- -
How do we study coevolution?
Like most evolutionary questions, it can be studied at various levels:
Adaptations of individuals Interactions between species Broad evolutionary patterns
Coevolution
Coadaptation Reciprocal adaptations of two species
Could refer to species, adaptations possessed by individuals, genotypes, etc.
Lycaenid caterpillars secrete “honeydew” that ants drink Ants defend caterpillars against parasitic waspsHoneydew secretion and defense are coadaptations
Does coadaptation demonstrate coevolution?
Biologists often have a strict definition of coevolution: evidence of parallel evolution between taxa is required
Fig-wasp mutualism
Fig trees (Ficus)
~750 tropical species, all of which depend entirely on wasps for pollination
Figs are not fruits – they are specialized inflorescences with hundreds of unisexual flowers
Fig-wasp mutualism
Fig wasps (Agaonidae)
Males: suited only for boring holes and mating
Females: adaptated for flying, burrowing into figs, and laying eggs in fig oocytes
Coadaptations
- Receptive figs produce scents that are specific to a particular pollinator species- Shape of ostiole specific to head shape of particular wasp species (lock-and-key)- Morphology of individual flowers specialized to a particular wasp species
Male Female
Female wasp enters via ostioleand oviposits infemale flowers
Male flowers
Female flowers
Flower styles aredifferent lengths –wasps only ovipositin ones w/ short styles
Pollen
Fig-wasp mutualism
Don’t worry…the wasps leave before the fruit is ripe to eat
Fig-wasp mutualism
Seed dispersal
Although pollination is very host-specific, seed dispersal is usually not
Over 1200 different vertebrate species are known to eat & disperse fig seeds
Accordingly, we would expect fig-disperser coevolution to be much weaker
Fig-wasp mutualism
A twist to the story…parasitism
In addition to pollinating wasps, figs are associated with parasitic wasps
Parasitic wasps do not enter the ostiole and do not pollinate the fig’s flowers
Instead, they use a long ovipositor to puncture the fig and lay eggs from outside
Parasites reduce fitness of figs and pollinator wasps
- By ovipositing in flowers that would otherwise produce pollinator wasps
- By directly predating pollinator wasps in some species
- By ovipositing in flowers that would otherwise produce seed for the fig
Cophylogeny
Congruent phylogenies due to cospeciation – strong evidence for coevolution
Fig-wasp mutualism
Cospeciation
Congruent
Host jumping Duplication “Missing the boat”
Incongruent
Figs Wasps
A statistical method known asphylogenetic reconciliation analysis tests the hypothesis that two phylogeniesare more different than expected by chance
Fig-wasp mutualism
Cospeciation
Figs and pollinator wasps show a very high degree of cospeciation
Despite pressure fromparasitic wasps, fig –pollinator specificityis maintained
Indicates a very tight ecological relationship
Weiblen & Bush (2002) Mol. Ecol. 11:1573-1578
Cospeciation
Figs and parasites do not show as strong evidence for cospeciation
Weiblen & Bush (2002) Mol. Ecol. 11:1573-1578
Host jumping
Host duplication
“Missing the boat”
Fig-wasp mutualism
Fig-wasp mutualism
Figs are coadapted to both pollinators and parasites
Figs must balance their own reproductive success against the need to maintainpollinator specificity and reduce impact of parasites
Some ancestral figs solve this problem byproducing flowers with styles of differentlengths so at least some will produce seed
Parasites oviposit through fig and intooutermost layers of oocytes
Pollinators oviposit from within the fig and into innermost layers of oocytes
Fig-wasp mutualism
Figs are coadapted to both pollinators and parasites
Figs must balance their own reproductive success against the need to maintainpollinator specificity and reduce impact of parasites
Functional dioecy
Some species produce figs with eitherall long or all short styled flowers
A will produce pollen and pollinator eggs,so it is functionally male (= no fig seed)
B will produce only seed (and parasiteeggs), but to do so it has to smell like Ato trick pollinator females into entering
This strategy doesn’t eliminate parasitism,but it guarantees that seed will be set
Note that mutualism is not all “warm and fuzzy”…mutualists will always try to maximize their benefit (pollination) and minimize their cost (loss of seed production)
Plant-insect coevolution
Cospeciation in a plant-herbivore system
Tetraopes beetles eat milkweed plants in the genus Asclepias cospeciation
Plant-insect coevolution
Cospeciation in another plant-herbivore system
Blepharida beetles eat Bursera plantsPlant Beetle
Becerra (1997) Science
Plant-insect coevolution
Cospeciation in another plant-herbivore system
Blepharida beetles eat Bursera plants
There is a high degree of host-specificity
Then why so much host-jumping?
Why not cospeciation like in Tetraoptesbeetles and milkweed plants?
Becerra (1997) Science
Plant Beetle
Host specificity is determined by the chemical defenses of the plant
Four major chemical classes of plant defenses against herbivory(indicated by colors)
These chemical classes do notcorrespond to plant clades (top)
The bottom figure shows beetlephylogeny with branches codedfor the chemical type of the host
The phylogenies are incongruentbecause host switching can occur as long as the beetle switches to anew host with chemical defenses towhich it is already adapted
Plant-insect coevolution
Becerra (1997) Science
Cophylogenies
Congruent phylogenies can arise for more than one reason
Today we are discussing congruence as a result of cospeciation
But recall that congruence is also predicted by vicariance biogeography
A
B
C
D
E
IncongruentCongruentCongruent
Plant-pollinator coevolution
Flower and fly or moth pollinators
Many flies and moths have outlandish proboscises to extract nectar from similarly outlandish flowers
Darwin received a specimen of the orchid Angraecumsesquipedale and predicted from it that there mustexist a pollinator with a proboscis measuring 10-12”
This prediction was not confirmed until 1903 withXanthopan morgani moth
Host specificity drives coevolution
A flower “wants” its pollen spreadto other flowers of the same species
Flower evolvesFly responds
Extravagant traits cancoevolve in response
A coevolutionaryescalation
Plant-pollinator coevolution
Mutualisms can be exploited
Figs 2-4 = flower species that produces nectar
Figs 5-6 = mimic orchid that “cheats” Anderson et al. (2005) Am. J. Botany 92: 1342
Mutualisms can be exploited by “cheaters” that collect benefits but avoid costs, as in the case of the deceptive orchid Disa nivea
D. nivea mimics a nectar-producingflower to fool the flyProsoeca ganglbaueri
Plant-pollinator coevolution
Ant mutualisms
Ants and insects that produce “honeydew”
Ants participate in dozens of mutualisms and show coadaptations for each
Many different insects provide ants with “honeydew” – source of nutrition for theants that has no other function for the insect – specifically coevolved for ants
In return, ants defend insects from parasites and predators
Ants tending a lycaenid caterpillar Ant drinking honeydew from an aphid
Ant mutualisms
Ants and acacia trees
Pseudomyrmex ants protect acacia trees from herbivores – in return, the acaciafeeds the ant with nectar and protein rich Beltian bodies, and provides a place forthe ants to live in the acacia’s modified thorns
Ant-fungus mutualism
Attine ants (~210 species) have cultivated fungal gardens for over 50 million years
Benefits to the ant: Fungi produce nutritional “gongylidia” that areharvested by ants to feed their larvae
Fungi can digest cellulose, ants can not
Captive colony of Atta mexicana tending to a fungal garden
Atta cephalotes collecting leafcuttings for their fungal garden
Benefits to the fungi: Ants remove plants and otherfungi that compete for nutrientsand provision fungi with leaves
Ants cultivate actinomycetebacteria that produce antiboioticsagainst Escovopsis fungi, which would otherwise parasitize the mutualist fungi Ant with pockets
of bacteria
Ant mutualisms
Ant-fungus mutualism
To simplify the system in a diagram:
Ant Cultivar (gardened fungus)
Bacterium (actinomycete)
Parasite (Escovopsis fungi)
+ +
+ +
Parasite kills cultivar
Antibiotics kill parasite
-
-
Mutualism
Mutualism or commensalism
A four-way symbiosis – but do these species coevolve?
Ant mutualisms
Currie et al. (2003) Science 299: 386-388
Coevolution – Patterns of parallel evolution between ants and fungal cultivars
…and between these two groups and Escovopsis parasites !!
Ant mutualisms
Host-parasite coevolution
Coevolution – Thus far we have seen examples from mutualism interactions
Pocket gophers (Geomyidae) are are parasitized by lice (Mallophaga)
Clear pattern of cospeciation – this example also shows how rates of evolution can be compared (b) to provide further evidence for coevolution (letters in b = branches in a)
Coevolutionary arms races
“Arms race”
Coevolving species have to constantly “improve” to meet each new adaptation with a “better” adaptation of their own
Escalation
Coadaptations become increasingly powerful, yet species are not any betteradapted because the selective landscape is constantly changing
This may sound familiar: it is Van Valen’s Red Queen Hypothesis: - running as fast as possible just to stay in the same place
An inherent feature of coevolution
We often think of “arms races” as occurring between predators and prey, or between parasites and hosts – this makes intuitive sense
But it is not really that different in mutualists – each mutualist will be best adaptedwhen it receives the maximum benefit while paying the minimal cost
Coevolutionary arms races
An arms race in a predator-prey interaction
Taricha granulosa newts have powerful tetrodotoxins(TTX) that are secreted as protection from predators
Thamnophis sirtalis garter snakes are the only major predator of this newt – they have evolved resistance to TTX
Escalation
Toxins produced by newts are hundreds of times more powerful that necessaryto kill any other predator (including humans), but snakes are resistant
Can we find evidence for coevolution?
Brodie et al. (2002) Evolution 56:2067-2082
Coevolutionary arms racesSnake populationsoutside of newt’s rangehave low resistance
Brodie et al. (2002) Evolution 56:2067-2082
Snake populationsvary in resistanceto newt toxins
A geographic mosaicwith two coevolutionary“hotspots”
Coevolutionary arms races
Brodie et al. (2002) Evolution 56:2067-2082
Snake resistance ispredicted by newttoxicity, as expectedif these species arecoevolving
An arms race in a predator-prey interaction
The extremely high toxicity of Taricha granulosa, which is hundreds of times more toxic than necessary for most predators, is a result of an escalating armsrace with one species, Thamnophis sirtalis
Evidence for coevolution
Local coadaptation
Snakes and newts are locally coadapted: - snakes have not evolved resistance in populations outside of the newt’s range - populations with high newt toxicity have high snake resistance
Snails and their castrating trematode parasites
In three separate studies, parasites were better able to infect snails from their ownpopulation than hosts from other populations – parasites are locally coadapted
Curt Lively’s research: http://www.indiana.edu/~curtweb/Research/local_adaptation.html
Inferring an arms race from fossils
Shells of fossil gastropods
Difficult to infer coadaptation from fossils because we can’t observe interactions
But we can use characteristics that reflect predator-prey interactions
When a shell is repairedfollowing a failed predationattempt, it leaves a clearpattern evident in fossils
The incidence of shell repair increases through time, suggesting predation isbecoming more intense
Gastropods “cement” themselves to the substrateas an adaptation againstpredators
The incidence of mobilegastropods that lack ameans of attachmentdecreases over time
Gastropods with thickenedor narrowed apertures are better able to survive predation events
The incidence of thickenedor narrowed aperturesincreases over time
Fossils and the Red Queen
Probability of extinction
The fossil record also supports another important theoretical point:
Probability of extinction is constant through the course of evolution
Why is this important?
It shows that evolution is not progressive – taxa that have been around longerhave not become “better adapted” and thus better able to avoid extinction
Supports the Red Queen model and implicates coevolution as a major force: Organisms have to keep running (evolving) just to stay in place (avoid extinction)
Coevolution and radiation
Haldane’s reply: “An inordinate fondness for beetles"
Biologist JBS Haldane was once asked by theologians: “What could one conclude about the Creator from a study of His creation?”
Why are beetles so speciose?
Over half of all beetles are phytophagous (feed on plants), and a large number ofthese herbivorous beetles feed on angiosperms (flowering plants)
Farrell (1998) hypothesized that specialization on different angiosperm species led to the radiation of beetle species
Farrell (1998) Science 281: 555-559
Coevolution and radiation
The increase in herbivorous beetle genera correlates with the exponential increase of angiosperms beginning in the Cretaceous
Radiation of angiosperms
Why are beetles so speciose?
Phytophagous beetles are a monophyletic group, but specialized feeding on angiosperms has evolved multiple times within phytophagous beetles
Beetles feed on:
CycadsConifersAngiosperms (dicots)Angiosperms (monocots)
(A) Curculionoidea (B) Chrysomeloidea
Farrell (1998) Science 281: 555-559
Many more beetle genera occur in clades that feed on angiosperms
Coevolution and radiation
Why are beetles so speciose?
Specialization on angiosperms leads to rapid beetle speciation via coevolution
Radiation of angiosperms
Evolutionary changes in plant host lead to incredible beetle radiations
It is important to note that this samepattern is observed in five different clades, indicating that the change inhost type is driving this pattern
Coevolution and radiation
Farrell (1998) Science 281: 555-559
Coevolution
Why is coevolution important?
We can simplify ecology as consisting of 2 types of interactions
1) Abiotic – interactions with temperature, light, nutrients, humidity, etc.
2) Biotic – interactions with other organisms
Coevolution occurs only as a direct result of biotic interactions
A simple question about the importance of coevolution:
If change in the physical environment ceased, would evolution come to a stop?
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