Lecture 13 Coevolution Flashcards
Defining coevolution
The process of reciprocal evolutionary change that occurs between two (or more) species as they interact with one another
Red Queen Effect
each member of a host-parasite or predator-prey relationship must evolve as fast as is necessary to overtake the other in an evolutionary ‘arms race’
Predator-prey dynamics
as prey population increases, predator population increases just after, as the prey population decreases the predator population decreases just after
Predator-prey dynamics- Garter snake and the rough-skinned newt
*Newt produces tetrodotoxin (TTX) which inhibits nerves, causing paralysis and asphyxiation.
*Natural selection predicts newts should produce only just enough to defend themselves: too much TTX = less energy for reproduction
*Snake has evolved resistance to the newt’s tetrodotoxin (high binding affinity for voltage-gated sodium channels).
*Too much resistance = slower speed.
*Strong phenotype matching implies that resistance and toxicity must evolve together.
Bats and moths
*Echolocation: high frequency calls between 12-210kHZ for most bats.
*Moths have evolved the ability to hear the echolocation so they can evade the bats.
*Some bats have then changed echolocation frequency/intensity to counter the auditory defences.
*One moth species has even evolved wings with tiny scales designed to absorb bat echolocation!
*Another new strategy: some moths have evolved ultrasonic clicks to use defensively when under bat attack. *Startle and alert the bat to unpalatability.
*Confuse bats by interfering with the ability to process returning echoes.
*Evidence of an arms race: frequencies that bats emit, and which moths can hear are closely matched in different locations
Snowshoe hare and Canadian Lynx
as prey population increases, predator population increases just after, as the prey population decreases the predator population decreases just after
*Frequently cited example in North America.
*Based on collection on animal pelts by trappers over.
*Cycle repeats approximately every 10 years
The Lotka-Volterra model
*Developed independently by Lotka(1925) and Volterra (1926).
*Simple model of predator-prey dynamics.
*Commonly used in ecology to explain interspecific interactions
Assumptions:
1.Prey always finds food.
2.Rate of change of a population is proportional to size.
3.Predators have a limitless appetite.
4.Predator food supply is just this prey population.
5.Environment is stable
The Lotka-Volterra model
*Realism is often sacrificed for simplicity.
*A better model would incorporate terms that represent:
1.Carrying capacity for the prey population;
2.Realistic functional responses for the predator population;
3.Realistic risks of extinction;
4.Complexity in the environment;
5.A more accurate, direct measure of population size
Host-parasite dynamics. Daphnia magna and its microparasite, Pasteuriaramonsa
*Archived gene pools used to reconstruct evolutionary dynamics.
*Exposed eggs to a parasitic ‘time-shift’. *Each core of sediment is a different generation, providing a snapshot of arms race as it occurred in the past
*Negative frequency-dependent selection (the more common the phenotype, the lower its fitness).
*Gradual increase in virulence and consistent infections rates over time *Daphnia adapts according to its contemporary parasite = Red Queen dynamics
Brood parasitism through egg mimicry
*Lay eggs which mimic host colours/shapes in the nests of other birds to avoid costs of parenting.
*Hosts counter-adapt through improving egg and/or chick recognition
Cuckoo brood parasitism through egg mimicr
*40% cuckoo species -mostly host specialists with various strategies:
*Adapting egg colourto host nest-types, e.g., “cryptic” eggs.
*Eggs hatch 24 hours earlier due to longer internal incubation period. *Thicker, stronger shells.
*Innate chick behaviour.
The European Cuckoo
*Most well-known species, with unique ‘host-switching’:
*Multiple hosts -maternal lines ‘pick’ one and pass down genes for regulating egg colour.
*Speciation prevented by males maintaining gene flow
*How have these strategies evolved? How do cuckoos make their eggs mimic the hosts’ eggs?
*Genetic inheritance
*Natal philopatry
*Nest-or habitat-site selection
Host defences -egg recognition
*Changes in the number of eggs or in egg appearance.
*Two main hypotheses:
*True recognition of parasitic eggs (costly and imperfect)
*Discordancy hypothesis (‘odd ones out’, risky)
*Combination of both (one compensates for the limits of the other)
Host defences -chick recognition
*Nestling discrimination rare –only evolves when parasitism rates are high enough to outweigh cost of recognition errors.
*The best defense is often prevention, e.g., the Fairy Wren strategy.
Host defences
*Diversify egg colour–negative frequency-dependent selection.
*The less common host colour, the easier it is to recognize imposter eggs, resulting in higher fitness.
*Make cavity nests instead of open nests.
*Increases the risks of unsuccessful eviction of host offspring by the cuckoo nestling.
*Do nothing!
*Sometimes acceptance is more adaptive than resistance e.g., the avian mafia hypothesis and the brown-headed cowbird.
*Evolutionary lag?
Cuckoo counter-adaptations
*Increasing egg resemblance.
*Batesian mimicry of other harmless birds, so host is less likely to reject an egg when recognizing another bird near its nest.
*Host-switching to take advantage of naïve species (more applicable in generalists)
Why is sexual reproduction advantageous, despite its cost?
*New gene combinations advantageous in dynamic environments
*Sexual reproduction = genetic diversity and new genetic defences
*Co-evolutionary interactions may select for sexual reproduction in hosts to reduce the risk of infection by parasites, for example:
*‘Matching alleles’ –an important assumption in RQD
*Sexual reproduction in hosts reduces risk of infection
*Parasites continually mutate to ‘fit’ host
*Negative frequency-dependent selection favours sexual forms
Freshwater snail, Potamopyrgusantipodarum, and its trematode parasite, Microphallus
*Two-host life cycle.
*Polymorphic reproductive strategies:
*obligately parthenogenetic (exclusively asexual).
*sexual and dioecious.
*Strong selection pressures because parasites sterilise hosts.
*Occur across multiple glacial lakes in New Zealand where interactions vary, creating ‘coevolutionary hot and cold spots’:
*Infection highest in shallow water where competition between sexual and asexual snails is highest and where ducks (final parasite host) commonly forage.
*Infection rarest in deeper regions where asexual snails dominate.
*Independent evolutionary trajectories shown by local adaptation
The Red Queen Hypothesis assumes that…
*Genetic variation for resistance exists in the host population;
*Each parasite genotype can only infect a small subset of the possible host genotypes;
*Each parasite population is “chasing” host genotypes in the same location;
*More genetic diversity and more sexual reproduction will be found in locations where the risk of exposure to virulent parasites was greater;
*Negative frequency dependent selection acting on the host
Antimicrobial resistance (AMR)
A very current and important topic
*~700,000 lives lost worldwide per year (2019) due to AMR, with an estimation of 10 million per year by 2050.
*An escalating and highly complex problem, one of the greatest threats to human existence.
*Billions of pounds/dollars wasted through unnecessary and improper use, which drives the spread of drug-resistant bacteria.
*Hundreds of studiesand dozens of lab groups around the world dedicated to finding ways to prevent the spread.
Our arms race with AMR The origins of MRSA
*Began in the ‘golden age’ of antibiotics (1950s-70s).
*High reproduction rate of bacteria means that they evolve fast, resulting in a constant battle within our lifetimes.
*Evolution of bacteria into ‘superbugs’ like MRSA(methicillin-resistant Staphylococcus aureus).
Bacterial defences
*Bacteria have evolved ways to defend themselves against modern medicine.
Passing on resistance
*They have evolved several ways to pass on their resistance:
*inherited genetically.
*passed between bacteria through mobile genetic elements
Take-away messages
*A coevolutionary arms race describes the ______________of adaptations and counter-adaptations in response toan ongoing struggle between competing sets of _____________, as well as ___________ and ______________.
*Mathematical models are useful tools for understanding how _________________________can lead to coevolutionary arms races, but these have a variety of limitations.
*_____________dynamics, ___________dynamics, the evolution of _________________, and _______________are common systems involving coevolutionary arms races
