LECTURE 06 CH 7 LIFE HISTORIES AND EVOLUTION
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.
Life histories vary along a slow-fast continuum.
The great diversity of life histories may be classified on the basis of a few traits: p 132, 7.4
Survival, age of reproductive maturity, fecundity (# offspring/reproductive bout),
offspring size, parity (# times reproduce per lifetime), rate of aging, lifespan.
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 involves tradeoffs with survival, growth, and
future reproduction. 7.5, 7.6
Life histories vary along a fast-slow continuum
Traits vary consistently with respect to habitat or environmental conditions
Variation in one trait is often correlated with variation in other traits
Slow (long life, slow development, delayed maturity, low fecundity, high parental investment)
Fast (opposite of slow)
Larger organisms: longer life spans and lower reproductive rates 7.1
1) Age of Maturation (when first reproduce?) Table 2
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?) 7.2, 7.3
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 Table 3, 7.8
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 7.9, 7.11
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
Preparation for reproduction extremely costly p 132 salmon
Hypotheses about what favors iteroparity?
Low current reproduction results in maintaining high future reproduction
Bet hedging: spread reproduction over both good and bad years
4) Aging and lifespan
Senescence: decline in physiological function with age decreased fecundity/survival 7.2
: causes decline in fecundity and survival
Strength of selection on senescence varies with mortality rate 7.13
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
Individual life histories are sensitive to environmental influences 7.15, 7.16
Change feeding and growth rates
Shift between life history stages at different times or sizes 7.17
Summary: 1-3, 5-10, 13
Review Questions 1, 3-6