LECTURE 08 CH 10 LIFE HISTORIES AND EVOLUTION pg 199-202.5; 206.5-216
MAJOR CONCEPTS
Life history is the lifetime scheduling of resources and time to maximize fitness.
A set of ‘rules’ explains traits affecting survival and reproduction and favored by
natural selection
Life history traits are adaptations molded by natural selection. They evolve.
Trade-offs in the allocation of resources provide a basis for understanding life
histories.
All organisms have access to limited energy and other resources.
A life history represents the best resolution of conflicting demands on the organism.
A trade-off exists between number and size of offspring (few larger offspring vs.
many smaller offspring).
If adult survival is lower, organisms begin reproducing at an earlier age and invest a
greater proportion of their resources into reproduction; if adult survival is higher,
organisms delay reproduction and allocate a smaller proportion of their resources to
reproduction.
The great diversity of life histories may be classified on the basis of a few traits:
Survival, age of reproductive maturity, fecundity (# offspring/reproductive bout),
offspring size, parity (# times reproduce per lifetime), rate of aging, lifespan).
Life histories vary along a slow-fast continuum.
Life history traits vary among individuals and are subject to natural selection
Life history traits influence lifetime reproductive success.
Natural selection results in evolution of this traits and leads to adaptations.
Resources and time are limited.
Allocation decisions (and compromises) must be made.
Alternative pathways:
Immediate reproduction
Delayed reproduction: resources into growth – enhance competitive ability
resources into maintenance – enhance survival
Tradeoffs
Resolution of conflicting demands for limited resources
Principle: limited time and resources are allocated among competing functions so as
to maximize lifetime reproductive success.
Costs: Allocation to current reproduction affects survival, growth, and future
reproduction.
1) Age of Maturation (when first reproduce?)
Benefit of early reproduction = immediate fitness gain; short generation time
Benefit of delay = age-related gains in fecundity from greater growth or experience
Cost of delay
risk of mortality before reproduce
tradeoff of survival and reproduction
reduced fecundity at later ages
Species with high adult survival mature later than those with low adult survival
2) Fecundity (How many offspring per reproductive bout?)
Fecundity vs. offspring survival
Fecundity vs. parental investment per offspring
Seed size vs. seed number tradeoff
Clutch size vs. number of offspring parent can feed
Variation in fecundity among species is huge
Growth vs. fecundity
If indeterminate growth, fecundity is related to body size
Increased fecundity reduces growth and thus fecundity in future
Short-lived species emphasize fecundity over growth
High extrinsic adult mortality favors increased reproduction now
at expense of adult survival and future reproduction
Long-lived species emphasize growth over immediate fecundity
3) Parity (How many times reproduce per lifetime?)
Semelparous (monocarpic): once
Iteroparous (polycarpic): repeated
If semelparous, at what year? Annual, biennial, long-lived
Hypotheses about what favors semelparity?
Pay-off for reproduction highly variable but favorable conditions predictable from
environmental cues
Pollinators attracted to massive display
Seed predators satiated by many seeds
Preparation for reproduction extremely costly
What favors iteroparity?
Low current reproduction results in maintaining high future reproduction
4) Aging and lifespan
Senescence: decline in physiological function with age
: causes decline in fecundity and survival
Strength of selection on senescence varies with mortality rate
If high mortality and few reach old agelittle selection for mechanisms to
prolong life; they age faster
Why does aging vary?
Subject to natural selection and evolutionary modification
Strength of selection is less on traits expressed at progressively later ages
Life histories vary along a slow-fast continuum
Slow: (K-selected species) in persistent habitats; near carrying capacity
slow development
delayed maturity
large adult size
low reproductive rate
high parental investment/offspring low mortality
long life
low dispersal
Fast (r-selected species) in temporary habitats; much pop. growth potential
Opposite of slow