Life history Flashcards
Life history definition
the way organisms maximise fitness through trade-off between diff bio functions – survivorship and fecundity e.g.rabbits focus on reproduction and have short gestation + investing very little time in their offspring whereas elephants reproduce less frequently, long gestation and invest more in each individual offspring
1938: The first Life history model was introduced by Ronald Fisher
He combined Darwinian and Mendelian theory to produce fisherian fitness theory that the fittest organisms tended to have the most reproductive events ( Notably Fisher was part of the eugenics movement and suggested that humans with the most favourable traits should reproduce the most)
1967: MacArthur and Wilson introduce the concept of R and K strategists
R-reproduce more short-lived
K –reproduce less long lived
They produced together a book called : The theory of island biogeography
1970’s: Stephen Stearns proposed phenotypic plasticity
Introduced a more quantitative method
Proposed the influence of environment in addition to genetic influence on phenotypes
Wrote the book a new view of life history evolution published 1980
Methods of study
-longterm field study
-demographic modelling
-lab based studies: example Stearns 2000:
one fly group exposed to HAM – high adult mortality (90% flies killed by researchers every 2 weeks)
the other fly group LAM – low adult mortality (10% killed by researchers every 2 weeks)
Flies in the first group matured earlier, have more reproductive events and live shorter than LAM group
-phylogenetic/genetic studies
Why are trade-offs necessary?
Each individual has finite amount of resources available for life
The proportion of resources allocated to each aspect of life history differs between species, populations and morphs
Example of differential resource allocation: polymorphism in female crickets (Zera et al 2006) :
Flightless short wing morph invests more in reproduction – less energy allocated for flight
Long-winged focuses on dispersal to move far but produce less eggs
Different morphs allows species longevity
Growth and maturity: advantages and disadvantages of being an R or a K strategist
R strategists: Short-lived animals reproduce young – usually have smaller adult body size
Fast maturation, many offspring, little parental care, small at birth.
Advantages:
Increased likeliness of reaching reproductive age to pass on genes before being predated or dying of disease
Faster rate of allele turnover – able to adapt quickly due to short generations
Disadvantages:
Decreased future fecundity
Decreased ability to defend territory and compete for mates e.g. in flying fox males (Todd et al 2018)
K strategists: Long-lived animals Put energy into producing and maintaining extremely few fit individuals. Large at birth, low mortality rates.
Advantages:
More time to develop
More reproductive episodes possible in lifetime
Disadvantages:
More impacted by disease – less able to recover due to slow rate of allele turnover
The growth curve of R strategists has troughs and peaks due to regular boom and bust whereas K strategists tend to increase in number until they reach carrying capacity and maintain at that level.
Reproductive value model
RV= current reproduction + residual reproductive value (RRV)
The sum of investments in current reproduction
combined with RRV – the investment in future reproduction
e.g. female burying beetles Craighton et al 2009
Beetles that invested heavily in current reproduction had lower RRV shorter lives and overall fecundity.
Those that invested less energy in current reproduction had longer lives, higher RRV and higher fecundity overall
Older females invested more in current reproduction as the probability of surviving to have more offspring in the future declines with age supporting terminal investment hypothesis
RVM only useful in organisms that can be separated by age
Other models use size or life stage instead
Terminal investment hypothesis
trade off declines with age
e.g in a study of crocodile fecundity: no. Of eggs in lifetime
No. of eggs produced reduced as the crocodile aged
reduction in fecundity and allocation of resources to eggs
Waste water management
a combination of R and K bacteria are more effective in combination
This is currently at the theoretical stage (Yin, 2022)
Summarising survivorship and reproduction: Life history tables
Covers each year an organism lives
How many died or survived and reproduced at each age
A cohort life history table could use pelage, mark and recapture tagging e.g. to follow monkeys over their lifetime
The other technique is snapshot which takes a sample at one time
From this we can calculate net reproductive rate by multiplying amount that survive by amount of offspring for each age – used to calculate population growth
(see notes for eaquation)
Survivorship curves
concave curve - type 3 (usually R e.g. burying beetle)
Straight line (mx=c) downwards (mix of R and K e.g chimp)
convex curve - type 1 (usually K e.g. elephants)
Phenotypic plasticity
-ability of a genotype to produce different phenotypes on an environmental gradient
-genotype-by-environment interactions (G × E) can be reflected by phenotypic plasticity, visualised using reaction norms.
- Predictive Adaptive Response (PAR) hypothesis – how life history traits adapt to environmental cues
-Environmental factors such as diet affect life history patterns
Phenotypic expression of a life history
P = G + E + (G x E)Phenotype (P) is a function of:
Genotype (G)
Environment (E)
Ways in which the genotype interacts with the environment (G × E)
High plasticity and high yield – selectively breeding choosing one genotype is effective
Increase long term study on natural populations
Analyse variation at phenotypic and genetic levels for endemic and rare species
Understand mechanisms of plasticity
