Week 2 Flashcards
why do organisms die?
Organisms face fundamental trade-offs in their use of energy and time
Changes in life history are caused by changes in the allocation of energy
senescence
aka aging is a late life decline in an individual’s fertility and probability of survival. Age reduces an individual’s fitness
- Aging should be opposed by natural selection
rate of living theory of aging
Aging is a function of metabolic rate
An accumulation of irreparable damage to cells and tissues
They have reached the limit of repair
Telomeres are the reason we cannot keep replicating cells
Many populations harbour genetic variation for longer lives but this has not evolved - having a longer life might be a trade off against something else
rate of living theory of aging assumptions
Bc cell and tissue damage is the by product of metabolism, aging rate should be correlated with metabolic rate
Organisms have been selected to resist and repair damage to maximum extent possible so species should not be able to evolve longer lives when subjected to selection
evolutionary theory of aging
Under evolutionary theory of senescence the failure to completely repair damage is caused by either deleterious mutations or trade-offs between repair and reproduction
Mutation accumulation hypothesis - fail to repair damage and succumb to mutations
Natural selection is weak in late life (already reproduced before mutations kill them so not selected against) so alleles that cause aging are only mildly deleterious. They may persist in mutation-selection balance or rise to high frequency by drift
Trade-offs and aging: the antagonistic pleiotropy hypothesis - because natural selection is weaker in later life alleles that enhance early life reproduction may be favoured even if they also hasten death
Researchers have documented trade-offs between reproduction early in life and longevity
In populations where mortality rates are high, individuals tend to more heavily invest in early reproduction
an evolutionary explanation for menopause
Menopause could be a nonadaptive artefact of modern lifestyle
Or it could be a life-history adaptation associated with the contribution grandmother’s make to feeding their grandchildren
how many offspring should an individual produce a year?
The more offspring, the less time and resources can be invested into raising them
As clutch size increases, chance of survival of offspring increases until it decreases again - tradeoff between number of offspring and ability to raise them
how big should offspring be?
Trade-offs between size and number of offspring
Larger offspring can have a better chance of survival
Many low-quality offspring or few high-quality offspring
Selection on parents favours a compromise but selection on offspring favours high quantity
conflict between mates: genomic imprinting
When different males father offspring within the same litter or clutch the reproductive interests of the father and the mother conflict
Genomic imprinting occurs when male and female alleles contain distinct chemical markers and transcribed differently
physiological conflict between mates: sexual coevolution
When mates are not monogamous the life-history strategy that is optimal for one sex may be suboptimal for the other
how do life histories maintain genetic variation?
life-history traits are closely correlated with fitness and have relatively low heritabilities
semelparous vs iteroparous
Semelparous: reproduce once and die
Iteroparous: have at least three reproductive episodes before dying
life history and invasive species
Life histories of one species that is benign in its natural habitat can be incredibly invasive when introduced into new areas
life history and extinction
Life history also is involved in macro-evolution and the threat of extinction
trade-offs and constraints
It is impossible to build a perfect organism
Organismal design reflects a compromise among competing demands
Resources devoted to one body part or function may be resources stolen from another part or function
Traits or behaviours that would appear to be adaptive may be physiologically or mechanically problematic
Populations sometimes lack the genetic variation that would provide the raw material to evolve particular adaptations
alternative mating strategies
When there is intense competition among males over mates, alternative sneaky strategies sometimes evolve
what was the problem with the Seeley experiment?
Seeley did not test for heritability
Another study found snails were able to use phenotypic plasticity to grow thicker shells - this does not mean there is no evolution taking place and that the trait is not heritable
how can you test for evolution?
Start with one large population - enough variation and reduce genetic drift
Split into many populations
Expose half of populations to control and half to treatment
Follow over about 10 generations
Did the diverge?
Control - no crab
Treatment - crabs
Common garden conditions - let the progeny of 10 generations to develop under the same environmental conditions for two generation. Allows us to see that changes are not due to phenotypic plasticity and evolution was observed
Take next generation from both conditions and test some in no crab and some in crab water and see how much is due to phenotypic plasticity
All postulates can be tested with this experiment
Seychelles warbler: ecological constraints case study
Seychelle’s warbler
Archipelago in Indian Ocean
Tropical island
Once a very rare species - island was a coconut plantation until it was made a nature reserve - no longer threatened
Territorial species - aggressively defend their territory
Insectivorous - find insects in their territory
Sigmoidal growth curve
Number of territories reached carrying capacity but more birds kept being born - saturated habitat
Young birds were forced to stay in their families territory - cooperative breeding began to be seen. Children forgo dispersal because of ecological constraints and help raise their parents subsequent children. This helps indirect fitness
Places closer to the coast (affected by salt spray) are low quality territory. Centre of the island is the high quality
How do we test that ecological constraints led to evolution of cooperative breeding?
Translocate small number of birds to another island where there were no warblers
Tracked number of birds and territories
High quality territories increases and plateaus before medium quality territories fill
As territories fill, almost all high quality sites had children helpers and 80% of medium - saturation led to birds switching to cooperative breeding
collared flycatcher: evolutionary constraints case study
RV (reproductive value) = current reproduction + residual reproductive value
RV: expected contribution to the population through current and future reproduction
RRV: future reproductive capacity through its investment in growth and survivorship
Collared flycatcher on Gotland Island
Breeds in Europe
Nest in nest boxes so can be observed easily
Observed that female flycatchers can start breeding when they are 1 or when they are 2
Birds that delay have larger clutches than those that started at 1
Is there a cost to reproduction?
Brood manipulation experiment
Half females to control and half to treatment (add an extra egg to each female nest) - did this for birds that started breeding at 1
Control increase clutch size by 0.2 across three years but treatment (after first year which was larger than control) there was a fast reproductive decline
Females that put more energy into first year couldn’t invest as much in future years - cost of parental care
Support for cost of reproduction tradeoff
Insights from comparative studies:
Species that invest more in somatic cells, invest less in gonadal function
A species can’t be large and support a high gonadal investment
coho salmon alternative mating
Males are aggressive and fight over females
Semelparous species
After establishing dominance they form a cue for the female
Female lays eggs in a hole and closest male has the first chance to inseminate the eggs - best chance of passing on genetics
Smaller males sneak in
Two morphs of male - jacks sneak up and look more female. And there are hook nose males
Data suggests that jacks and hook nose have similar success
Fish choose jack pathway or hook nose pathway about year 2. They are the same size at the time of divergence but jacks do not grow as much after the divergence
Small males sneak, big ones fight
Intermediate males have least chance - disruptive selection
Some frequency dependence. If there are many hook noses, being a jack is helpful. Rarest phenotype has highest chance
Two types of selection: negative frequency dependent disruptive selection
dung beetle alternative mating
Males prepare a dung ball as a gift for meals to get them to mate with them and eggs are laid in the dung
Male morphs: long horns and short horns
Body size has a bell-shape curve
Horn length is bimodal
Horn length is size dependent - big beetles have big horns
Males of each horn length do equally well with reproduction - both maintained in reproduction; mixed evolutionary stable strategy
Long horns can fight
Short horn males can dig and sneak into the female
Lack’s hypothesis about how many offspring you should have in a year
selection will favour the clutch size that produces the most surviving offspring in a year
Probability if survival decreases with increasing clutch size
Experiment on starlings - intermediate clutch sizes seemed best
Assumed linear relationship
Assumed all birds would have equal access to resources
Did not consider the cost of reproduction - that residual reproduction would be smaller
Individuals from large clutches may have worse condition even if they survive - overlooked
Assumes all egg sizes are equal
Too simplistic
But it is a really useful null model
mathematic function for number of offspring
Assume a negative linear function - but we do not know it is this shape for sure
With each extra egg, survival of each offspring decreases linearly
CS x Probability of survival shows the predicted number of surviving offspring for each clutch size - this produces an optimal clutch size curve
case study on the great tit looking at number of offspring
The great tit studied in a forest in Oxford
Looked at nearly 5000 clutches over 22 years
Mean clutch size was between 8 and 9 eggs but most productive clutch size was actually 12 (very few lay this number of eggs)
lack’s hypothesis was not supported in this case
Mothers had either eggs added or removed (or no change)
Researchers then looked at reproductive success of female children
Strong linear relationship: manipulation of mother affected reproductive success of daughters
Smaller clutch daughters had larger clutches and vice versa
salmon case study for size of offspring
Salmon in hatcheries
Trade off between number of eggs and size of eggs
Bigger eggs had larger chance of survival
Maternal fitness is optimal at intermediate egg size
Optimal size in the wild is larger - hatcheries are safer and there are less selection pressures
These hatcheries restock wild populations (using small eggs) so unlikely to do well in natural populations
Egg size is evolving to be smaller in rivers that have large numbers of restocking from hatcheries
Artificial selection has occurred
Gene flow from hatcheries has driven evolution for maladaptive egg size
