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?
