Life history 1: semelparous/ iteroparous Flashcards
Life history traits
Fundamental characteristics of an organism’s life cycle and reproductive strategy
Example: age of mature, reproductive window
Life history strategies
Strategies as a result of an organisms life history traits.
Example: Annual semelparity, Perennial iteroparity.
Trade offs and Darwinian demons
Trade-offs between life history traits due to limited resources, physiological limitations and microevolutionary processes stop the formation of darwinian demons.
What does life history theory study.
Life history is concerned with the strategic decisions over an organisms life time.
How are there strategies adapted to their environment.
Longitudinal cohort studies
Tracking a specific cohort. ime to examine various factors, behaviours and outcomes (life history traits) to gain an insight over their life history strategies.
Example of longitudinal cohort study
Yellow-bellied marmots
- They hibernate for 7-8 months a year which has resulted in extended life times as hibernation pauses ageing.
- Study followed the lives of 73 females looking at the variation in epigenetic age (Accumulation of methyl groups on DNA)
- Epigenetic age increased during active periods but did not during hibernating periods
Cross-sectional cohort studies
It is not always possible to do longitudinal studies so instead a snap short of time is observed and then life history traits are inferred.
Example of cross sectional studies
Greenland sharks:
- They live for up to 450 years so it is not practical to observe their entire life.
- Study carried out between 2010 and 2013, radiocarbon dating the stable isotopes inside the eye nuclei.
- Isotopes have been present since birth.
- Radioactive pulse following the 1960s when bombs released radiocarbons into the oceans (useful timestamp)
Bristlecone:
- Growth rings
- Tends to be a new node per year.
Cole’s paradox
‘For an annual species, the absolute gain in population growth achieved by switching to a perrenial species can be matched by increasing their average litter size by one’
Cole reasoned that selection should favour semelparity but he did not consider the mortality of semelparous species.
Iteroparous
Organisms reproduce multiple times throughout life in smaller more frequent reproductive events.
Perrenial species.
Exponential growth.
Semelparous
Organisms reproduce once in their life cycle in a ‘bing bang’ where they invest a lots in reproduction.
Annual species.
Exponential growth if they increase the litter size by one compared to iteroparous species (to replace themselves)
Cole’s assumptions
- No costs of reproduction
- Juveniles are not fragile
- Lack of stage-specific density-dependence
The evolution of semelparity
**The reproductive hypothesis: **
Semelparity will evolve when reproductive success is dependent on reproductive efforts (low reproductive effort= low reproductive success).
Fitness benefits of reproduction increase disproportionately at high levels of reproductive effort so individuals must invest highely which can have negative impact on future reproductive efforts.
Explains male on semelparity
- Intense sexual selection can explain male semelparity
- Intense sexual competition between males in the form of post-copulatory competition driven by sexual conflict. (investment of sperm)
- There is also loss of future reproductive capacity before breeding stage which can take away from future reproductive effort -> (e.g. amputation of genitals to improve agility during sexual competition in comb footed spider, using their body as a mating plug, allowing themselves to be eaten)
- females can shorten mating period to increase competition further and lead to greater sperm competition.
- Study found that species with suicidal reproduction have shorter mating seasons and larger testes relative to body size
Semelparity not seen in male plants due to less sexual selection
So although semelparity normally occurs in stochastic environments, it can also occur in stable environments where mate competition is high and sexual selection is high -> males must invest a lot in order to reproduce
**Demographic hypothesis: **
Semelparity will evolve in species that have a small reproductive window due to increasing mortality rate through life.
When the chance of future reproduction is low enough due to high risk of death, natural selection will favour strategies where an organism stops withholding resources for future reproduction and instead uses all of its available resources in a single reproductive event at the expense of its own future survival
Occurs when adult chance of death post breeding is higher than juvenilles.
Example: Increasing aridity can select for semelparity in short-lived plants through a mechanism of adult-biased mortality due to heat and water stress
Example: migrating salmon
Most species follow prediction from the demographic hypothesis
Predicting age of reproduction example
Carduus nutans
- The age of reproduction for this flowering plant was predicted by looking at the vital rates as a function of size.
- From predictions, the age of reproduction was later than predicted.
- They re-did the analyses to take into account the stochastic nature of the environment and the prediction matched the observations (ESS)
Perennial semelparity
Organisms may have a mixture of both, reproducing once and investing a lot in reproduction but living for long periods.
Trade-off between age of reproduction and risk of death.
plants: >30 Families
Plants grow for multiple season before reproducing -> allows greater reproductive event-> possible in stable environments where future survival chance is high.
Or intense investment in parental care -> parents invest so much that they cannot produce any more offspring
