Life History

Question 1

Define the term “antagonistic pleiotropy” in the context of life history tradeoffs

Answer 1

When a gene influences more than one trait with opposing effects on fitness
Net effect on fitness is positive
E.g. early reproduction coupled with early senescence

Question 2

What evidence is there that the evolution of menopause in some mammals could be connected to a life history tradeoff?

Answer 2

Also occurs in killer whales (orcas)

• Females can live >100 yrs but stop reproducing after age 45
• Older females more likely to lead whale pods, esp. during years when food is scarce
• Male orcas in their 30s are 14X more likely to die within a year if their menopausal mother dies
• Menopause “frees” females to assist in the survival of their own children and grandchildren (inclusive fitness, i.e. of self and related individuals that share DNA)

Question 3

Give examples of pairs of life history traits that tend to be associated with one another

Answer 3

• daughters at a time/age at reproduction those that breed earlier have more young per breeding bout
• mean number of seeds/mean weight of seeds
• clutch size/mortality rate
• female adult lifespan/age at maturity

Question 4

Define the term “adaptive suite”, and give an example

Answer 4

Sets of traits can occur together in an adaptive suite. E.g. the horned lizard and the thorny devil, diet of primarily ants necessitates an adaptive suite of certain traits, slow, fat, cryptic.

Question 5

Compare the performance curve of a generalist with that of a specialist

Answer 5

Generalists have wide, low curves, while specialist have narrow, high curves. The benefit of being a generalist (greater X-value) comes with the cost of reduction of maximum performance (lower Y-value)

Question 6

Describe the characteristics of r and K-selected species

Answer 6

r selected species:

• Colonizing ability is maximized
• Smaller body size
• Fast development
• Shorter life span
• Higher mortality rate
• Relatively more investment in reproduction
• Reproduce ASAP
• Usually semelparous
• Less investment per progeny (time, material)
• Relatively many progeny

k selected species:

• Competitive ability is maximized
• Slow maturation
• Larger body size
• Long life span
• Lower mortality rate
• Relatively more investment in growth and maintenance
• Usually iteroparous
• More investment per progeny (time, material)
• Relatively few progeny

Question 7

Determine whether r or K selection is predicted to operate under a given set of circumstances

Answer 7

Hazardous, short-lived or unpredictable environment > low competition > maximize reproduction in a hurry = r selection
Non-hazardous, stable environment > high competition > Maximize reproductive success under crowded conditions = K selection

Question 8

What are some objections to the r and K model of life history theory?

Answer 8

• Focuses too heavily on competition (or lack of) as an explanation for life history traits
• Difficult to apply in practice, especially when comparing different species. Some species fit aspects of both.

Question 9

Explain why the relationship between body volume and other traits often has a ¾ slope on a log-log graph

Answer 9

Living things are sustained by the transport of materials through branched networks. The terminal branches of these networks are roughly the same size, regardless of body size - they need to be small enough to allow diffusion of materials to and from cells. The summed cross-sectional areas of the daughter branches (e.g. capillaries) are equal to the parent branch (e.g. the aorta). - the 3/4 rule is a property of organisms that display this.

Question 10

Describe the mathematical relationship between plant mass and density in a self self-thinning population

Answer 10

Smallest individuals have high mortality > Survivors increase in size as resources are freed up = self-thinning.

𝜔= CN-4/3
Where 𝜔= mean weight per plant
N = density
C = constant final yield of plant biomass per unit area

Question 11

Define “life history strategy”

Answer 11

Life History: The set of rules or strategies an organism uses to allocate its energy, materials, and time.
We operate in a finite world: there is not infinite energy, materials, or time
Trade-offs among traits that affect fitness

Life history traits are traits that influence the schedule of birth, growth, reproduction and death

Question 12

Give examples of life history traits

Answer 12

• Growth rates of organisms
• Age at maturity
• Size at maturity
• Reproductive investment
• Number of offspring
• Mortality rates
• Size at birth
• Lifespan

Question 13

Explain why tradeoffs exist between different life history traits, and give examples of those tradeoffs

Answer 13

Increased investment in survival often means reduced investment in reproduction (or vice versa) eg. foxgloves flowing, optic gland of octopus

the existence of trade-offs allows tons of strategies and promotes diversity (Darwinian demon).

Question 14

Give examples of phenotypic plasticity in life history traits, and explain how such plasticity can benefit fitness

Answer 14

The ability to alter a trait in response to changing environmental conditions, e.g. birds who are indeterminate layers

Question 15

Explain how within-species variation in life history traits can lead to polymorphism, using examples

Answer 15

Negative frequency-dependent selection > polymorphism

e.g. hooknoses and jacks in chinook salmon

Question 16

Explain how the term “allocation decisions” is used in life history biology, with reference to what is being allocated, and how it is allocated

Answer 16

What finite things do organisms budget? (matter (nutrients), energy, time, etc.) What interests do organisms allocate these 3 finite things among? (foraging (acquisition of nutrients/energy), growth, maintenance, reproduction)

Question 17

Give examples of context-dependency in allocation decisions

Answer 17

Can depend on the conditions throughout the year. Consider a female migratory songbird:
April: high energy allocation to migration
June: high energy allocation to egg production
August: high energy allocation to moulting feathers
January: high energy allocation to foraging

Question 18

What the different types of parity?

Answer 18

Semelparity: single reproduction before death, typically large number of offspring
Iteroparity: numerous reproductions before death, typicaly lower number of offspring

Question 19

Describe “Cole’s paradox” and the solution to it

Answer 19

Minimal (in this case,1%) increase in early reproductive effort (even at the cost of early death and reduced lifetime fecundity)  greater long-term reproductive output
Cole’s paradox: why be iteroparous?

Usually, offspring survival to adulthood is not 100%, juveniles tend to have higher mortality than adults

Question 20

Explain how the evolution of life history traits can account for the decrease in size of commercial and sport fish in recent decades, referring to observational and experimental evidence

Answer 20

Bigger fish get caught more, smaller fish therefore have higher fitness > there’s a shift in average size to be smaller and increase their fitness.

Question 21

Define the term “Lack optimum”

Answer 21

= the number of offspring that yields the greatest fitness

Question 22

What experimental approaches have been used to test the hypothesis that life history traits in birds have been optimized under natural selection?

Answer 22

Birds: brood size manipulations

• Remove eggs from some nests (reduced clutch size > reduces brood size)
• Add eggs to other nests (increased clutch size > increases brood size)
• Leave others as controls

Removal of chicks has only a small beneficial effect on adult survival (but reduces number of offspring)
Adding chicks has a larger harmful effect on survival > Suggests unmanipulated (control) nests have optimal number of chicks to maximize overall fitness