Week 2: Testing adaptationist hypotheses Flashcards
The black-tailed gull
The most common gull in Korea
Found in mudflats, coasts, shores, open seas
Breeding occurs in a few remote islands between March and June
Ground-nesting sea gulls
Hongdo has the largest breeding colony of the black-tailed gull in Korea
They build their nests on the ground
1-3 eggs per nest
<1m between nests. Very close
Mobbing behavior
Potential predators: humans, hawks, crows, mammals
When a potential predator approaches to the colony, many gulls
- swoop at the intruder
- dive at the intruder
- defecate on the intruder
- call loudly
Adaptationists approach to understanding mobbing behavior
Does mobbing by gulls increase their reproductive success? –> seems likely
Hypothesis:
- mobbing behavior distracts certain predators, reducing the chance that they will find the mobbers’s offspring, which boosts the fitness of mobbing parent gulls
Testable predictions:
- mobbers’ offspring are more likely to survive
Estimation of fitness
Fitness is the number of offspring produced by an individual
Implied in this is lifetime reproductive success
It is very difficult to measure lifetime reproductive success.
Difficulties testing the adaptationists approach
- Costs are involved in mobbing behaviors
- time
- energy
- risk of injury or death - difficult to measure reproductive success or fitness
- difficult to measure fitness over a long time
- difficult to measure fitness itself
Alternative methods to measuring fitness
- cost-benefit analysis
- comparative method
Proxies for fitness measurement
An indicator or correlates of reproductive success
Only approximate actual reproductive success
Examples:
- egg survival
- proportion of young that survive to fledgling
- number of mates inseminated
- the quantity of food ingested per unit time
- the ability to acquire a breeding territory
- the attractiveness to the other sex
Cost benefit analysis
Fitness benefit (B):
- the positive effect of the trait that tends to raise the fitness of the individual
- more eggs and chicks saved bc of mobbing
Fitness cost (C):
- the damaging effect of a trait that tends to reduce the fitness of individuals
- the time and energy that mobbers expend when they are screaming, diving, and flabing
- mobbers can also lose their lives
The minimum requirement for an adaptation is that B > C
New testable predictions
Previous predictions:
- Mobbing responses enhance individual success in passing on genes to the next generation.
New predictions:
- Mobbing gulls force nest-robbing predators to expend more effort than they would otherwise
- The closer predators approach to the breeding colony, the more attacks they receive from mobbing gulls.
- The more intense mobbing occurs, the safer eggs are.
Kittiwake gulls
Kittiwakes nesting on extremely narrow ledges of cliffs
It is hard for small mammalian predators to scale cliffs in search of prey
Predatory birds have a difficult time manoeuvring near cliffs in turbulence
Predation pressure on kittiwake eggs and young has been greatly reduced
Cliff-nesting kittiwake gulls have relatively few nest predators
comparative methods
Assumption 1:
related species –> different selection pressures –> differences in behavior
Assumption 2:
Unrelated species –> similar selection pressures –> the same adaptive responses
Ex: lions, langurs, giant water bugs –> immediate reproductive success after having access to other sex –> infanticide
Gull phylogeny
Phylogeny: an evolutionary genealogy of the relationship among a number of species
50 or so species of gulls living today
The large majority nest on the ground and exhibit communal mobbing behavior against predators –> the ancestral gull was probably a ground-nesting species
Mobbing behavior in California ground squirrels
They live in groups and dig burrows in the ground
They react to a hunting rattlesnake by gathering around and kicking sand in its face
Convergent evolution
Natural selection of similar characteristics in two or more unrelated species
Ex: both ground-nesting gulls and California ground squirrels exhibit mobbing behavior
Divergent evolution
Natural selection for differences among closely related species that live in different environment and are therefore subject to different selection pressures
Ex: Ground-nesting gulls exhibit mobbing behavior, whereas cliff-nesting gulls don’t
