Ch. 4 What is life history?

What is life history?

A description of the major characteristics of an organism from its birth to its death.

Sets of coevolved traits that affect an individual’s survival and reproduction success (fitness)

Limited resources need to be allocated to:

1. Growth

2. Somatic maintenance

3. Reproduction

Life history traits?

1. Size and age at maturity

2. # and size of offspring

3. State of development at birth

4. Longevity

5. Mating system

6. Num of reproductive episodes

7. Amount of parental care

Why has variation in life history evolved?

Life histories represent trade offs resulting in complex sets of traits that maximize fitness

Example of a tradeoff.

Offspring number and size = large and few, many and small

Fecundity

How many offspring and organism can have in one cycle

Explain the graph and label the axes

Y axis: Number of offspring

X axes: Size of offspring

Number of offspring decrease as size increases

Explain graph and label axes.

Y axis: Survivorship of offspring

X axis: Size of offspring

Offspring survival increases with increase in size. Environment 1 is better because organisms are larger.

Explain graph and label axes.

Y axis: Maternal fitness

X axis: Size of offspring

Fitness is higher at immediate offspring size

Optimum in environment 2 is higher because more chance to reproduce

Why are seed beetles excellent subjects for life history studies?

They have short generation times and easy to maintain in a lab

What two plants do seed beetles lay their eggs on and what are the differences between the trees?

They lay their eggs on

1. paloverde - well defended seeds with > 50% survivability

2. Acacia - >95% survivability

What egg sizes are likely to be set for each spp of tree (seed beetles)?

Large eggs - Paloverde

Small eggs - Acacia

What have seed beetles evolved to have in regards to their eggs?

Egg plasticity - they can adjust the size of the egg based on the host available

How are seed beetles able to have egg plasticity?

Delayed oviposition - they can finish maturing the egg after it is placed on the host by adjusting the egg size in an adaptive way.

Fitness consequences for seed beetles placing their offspring on the different plants?

Paloverde: directional selection for larger eggs because they are the only ones who can bust through the protective seed coat

Acacia: no selection for larger eggs because the seed coat is easy for the organism in the egg to break through

Optimality models

Models used to help us better understand LH traits

Generally need to make assumptions about:

1. Trade-offs involved (constraints)

2. Consequences of each trait on fitness (currency)

Goal of optimality models

to identify the combination of traits that exceeds all others in fitness

(used to predict how natural selection shapes life history)

what does the goal of the optimality model assume?

that natural selection will drive an organism to that particular set of traits/strategy

How to generate an optimality model?

1. Designate the parameter to be optimized (# of offspring?)

2. Construct a set of rules that defines the life history pattern of an organism (will be a variety of possible life history, optimal = the one that maximizes fitness)

Confounding factors/limitations for optimality models (things to ignore)

1. Fluctuating environmental condition

2. Spatial variability

3. Underlying genetics

4. Choice of currency used

Semelparity vs Iteroparity Example for Optimality Models

Semelparity: high annual fecundity, high adult mortality

Iteroparity: lower annual fecundity, high adult survival

Annuals vs. Perennials predictions in optimality models

Annuals (semelparous):

• Fitness is determined by:

  • its own survival to maturity

  • fecundity once matured

• Selection for high annual fecundity

Perennials (iteroparous):

• Fitness is determined by:

  • its juvenile survival

  • annual fecundity

  • adult survival year to year

• Selection for high adult survival

Most apparent between close relatives inhabiting adjacent environments
Ex: Lobelia spp in Mount Kenya in Africa

One spps is semelparous (Lobelia telekii)

• Grow on dry, rocky slopes

• High annual fecundity

• High adult mortality

Other spps is iteroparous (Lobelia deckenii)

• Grow on moister valley bottoms

• Lower annual fecundity

• High adult survival

Fast continuum in mammals

Slow (K-selected)

Fast (R-selected)

Slow (K-selected)

Humans, elephants, buffalos

• Long lived

• Delayed maturity

• Large bodies

  • Small litters with large offspring

Fast (R-selected)

Rats and shrews

• Small bodied

• Short lifespans

• Mature at an early age

• Large litters and small offspring

In mammals many LH traits are related to ________

body size

Allometric exponent

Have a characteristic power function

Allometry

biological study of how characteristics of organisms change in relation to body size

Equation for allometry

Y=aM^b

Y=aM^b

Y = life history trait

a = y-intercept

M = body mass

b = slope of regression line (exponent)

Explain the relationship between body size and LH trait, include how to get a straight line.

Often need to take the log of body size and LH trait to get a straight line (relationships are curvilinear)

this is because growth starts to slow down and then it stops

Birth rates in mammals

Birth rates decrease and body size increase

Values of exponent in Y=aM^b

b = 0 trait is not related to body size

b = >1 trait increased with body mass

b = <1 trait decreased with body mass

All allometric exponents for mammalian life history traits are close to what multiplier?

some multiplier of 0.25

Invariants

life history traits, when multiplied together, have values that are unrelated to body size, but may be relatively constant across spps

provide a unified approach to the study of the evolution of life histories, rather than a mixed basket of species-specific models

Metabolic scaling

How energy use changes with body size

Hypometrically

the mathematical relationship between the two can be defined by a power function with an exponent less than one (M^0.75)

Metabolic rates in large vs small animals

Larger animals have lower mass-specific metabolic rates (energy used per gram/tissue) than smaller animals

• Ex: a mouse has a higher mass-specific metabolic rate than an elephant