Lecture 8 THE EVOLUTION OF LIFE HISTORIES–I Flashcards
what is Life history?
The pattern of investment an organism makes in growth and reproduction
such as:
size at birth
growth rate
age, size at maturity
reproductive investment
mortality rates
lifespan
Life history trait
A trait of an individual is a life history trait if, holding all other traits constant, a correlation remains between that trait and fitness
Fast-slow continuum:
A continuum of life history variation; patterns we see in species when we look at how they differ in fertility, mortality and offspringsize (fitness-related traits).
Fast end continuum
High mortality
*Short lifespan
*Mature early
*Small offspring (fast rate)
*High fertility
slow end continuum
Low mortality
*Long lifespan
*Mature late
*Large offspring (slow rate)
*Low fertility
what is Life History Evolution
Life history theory explained differences in:
*Size
*Maturation
*Offspring number
*Lifespan
*Analyses the evolution of all the components of fitness
*The organism is an evolutionary solution to an ecological problem
Life History Evolution is the interface between evolution and ecology
What causes life history to evolve?
Life histories result from the interaction of extrinsic (outside of the organism) and intrinsic (inside the organism) factors
*Extrinsic factors influence:
*Age-specific rates of mortality and reproduction
*Here ecology plays a role (also a lot of phylogenetic effects)
*Intrinsic factors are:
*Trade-offs among traits
*Here phylogeny, development, genetics and physiology plays a role
what are trade offs?
Trade-offs constrain and optimize the range of possible life-history strategies that can evolve across the Tree of life
Age and size at maturity
Optimal when the positive difference between the benefits and costs is maximized
*Mature at age and size where the payoff in fitness is greatest
*Can make some prediction on the age and size at maturity (one way with 4 assumptions)
*Older first-time parents have offspring with higher survival rates than younger first-time parents
*Older first-time parents have more offspring than younger first-time parents (take longer to grow before they reproduce because they are bigger)
*Advantages of delaying maturity counter-balanced by advantages of having a shorter generation time (can only get by maturing earlier)
*In a population at evolutionary equilibrium the advantages and disadvantages of at what age to mature will come into balance
Size and number of offspring
Once matured how many offspring?Billions of small ones (e.g., Orchid)
*One big one (e.g., Kiwi)
- The idea goes back to David Lack (1947)
*If nestling survival decreases as clutch size increases, then the intermediate number of eggs produces most fledglings
*Fitness is often maximized at intermediate reproductive investments
*Particularly in organisms that reproduce more than once in their lifetime
Lifespan and aging
*Reproductive lifespan
*Balance between selection to increase number of reproductive events per lifetime and effects that increase the intrinsic sources of mortality with age
*Selection pressures:
*Lengthen life –gives more reproductive opportunities
*Shorten life –by products that cause intrinsic increase in mortality rate
*Senescence –the deterioration in the biological functions of an organism as it ages
juvenile vs adult
*Selection pressures that lengthen life
*Decreases relative contribution to the fitness of offspring and increases that of adult
juvenile (high mortality)- Offspring in that environment have low survival probability to get same age and size as parent
Adult (low mortality)- Fitness advantage in investing in the preservation of that adult (unlikely to get another individual to that state)
Selection pressures that shorten life
*If adult mortality increases, then organisms should evolve more rapid aging
*Why invest in maintaining a body that will be dead anyway?
*No sense in investing additional energy in avoiding predators in high predation in the environment
*Better to concentrate on reproducing while you can
Causes of aging
*A byproduct of selection for reproductive performance
*Arises through accumulation of genes that have:
*Positive or neutral effects on fitness components early in life
*Negative effects on fitness components later in life
Positive effects early and negative effects late in life –antagonistic pleiotropy
what is the p53 gene
p53 tumour-suppressing protein: effective at defending against cancer in humans and other vertebrates
p53 force cells into early retirement
can Kill or stunt so many cells that tissue renewal is no longer possible
